Orthodontic treatment for posterior crossbites

Alessandro Ugolini; Paola Agostino; Armando Silvestrini-Biavati; Jayne E Harrison; Klaus BSL Batista

doi:10.1002/14651858.CD000979.pub3

. 2021 Dec 24;2021(12):CD000979. doi: 10.1002/14651858.CD000979.pub3

Orthodontic treatment for posterior crossbites

Alessandro Ugolini ^1,^✉, Paola Agostino ¹, Armando Silvestrini-Biavati ¹, Jayne E Harrison ², Klaus BSL Batista ³

Editor: Cochrane Oral Health Group

PMCID: PMC8709729 PMID: 34951927

Abstract

Background

A posterior crossbite occurs when the top back teeth bite inside the bottom back teeth. The prevalence of posterior crossbite is around 4% and 17% of children and adolescents in Europe and America, respectively. Several treatments have been recommended to correct this problem, which is related to such dental issues as tooth attrition, abnormal development of the jaws, joint problems, and imbalanced facial appearance. Treatments involve expanding the upper jaw with an orthodontic appliance, which can be fixed (e.g. quad‐helix) or removable (e.g. expansion plate). This is the third update of a Cochrane review first published in 2001.

Objectives

To assess the effects of different orthodontic treatments for posterior crossbites.

Search methods

Cochrane Oral Health's Information Specialist searched four bibliographic databases up to 8 April 2021 and used additional search methods to identify published, unpublished and ongoing studies.

Selection criteria

Randomised controlled trials (RCTs) of orthodontic treatment for posterior crossbites in children and adults.

Data collection and analysis

Two review authors, independently and in duplicate, screened the results of the electronic searches, extracted data, and assessed the risk of bias of the included studies. A third review author participated to resolve disagreements. We used risk ratios (RR) and 95% confidence intervals (CIs) to summarise dichotomous data (event), unless there were zero values in trial arms, in which case we used odds ratios (ORs). We used mean differences (MD) with 95% CIs to summarise continuous data. We performed meta‐analyses using fixed‐effect models. We used the GRADE approach to assess the certainty of the evidence for the main outcomes.

Main results

We included 31 studies that randomised approximately 1410 participants. Eight studies were at low risk of bias, 15 were at high risk of bias, and eight were unclear.

Intervention versus observation

For children (age 7 to 11 years), quad‐helix was beneficial for posterior crossbite correction compared to observation (OR 50.59, 95% CI 26.77 to 95.60; 3 studies, 149 participants; high‐certainty evidence) and resulted in higher final inter‐molar distances (MD 4.71 mm, 95% CI 4.31 to 5.10; 3 studies, 146 participants; moderate‐certainty evidence).

For children, expansion plates were also beneficial for posterior crossbite correction compared to observation (OR 25.26, 95% CI 13.08 to 48.77; 3 studies, 148 participants; high‐certainty evidence) and resulted in higher final inter‐molar distances (MD 3.30 mm, 95% CI 2.88 to 3.73; 3 studies, 145 participants, 3 studies; moderate‐certainty evidence). In addition, expansion plates resulted in higher inter‐canine distances (MD 2.59 mm, 95% CI 2.18 to 3.01; 3 studies, 145 participants; moderate‐certainty evidence).

The use of Hyrax is probably effective for correcting posterior crossbite compared to observation (OR 48.02, 95% CI 21.58 to 106.87; 93 participants, 3 studies; moderate‐certainty evidence). Two of the studies focused on adolescents (age 12 to 16 years) and found that Hyrax increased the inter‐molar distance compared with observation (MD 5.80, 95% CI 5.15 to 6.45; 2 studies, 72 participants; moderate‐certainty evidence).

Intervention A versus intervention B

When comparing quad‐helix with expansion plates in children, quad‐helix was more effective for posterior crossbite correction (RR 1.29, 95% CI 1.13 to 1.46; 3 studies, 151 participants; moderate‐certainty evidence), final inter‐molar distance (MD 1.48 mm, 95% CI 0.91 mm to 2.04 mm; 3 studies, 151 participants; high‐certainty evidence), inter‐canine distance (0.59 mm higher (95% CI 0.09 mm to 1.08 mm; 3 studies, 151 participants; low‐certainty evidence) and length of treatment (MD −3.15 months, 95% CI −4.04 to −2.25; 3 studies, 148 participants; moderate‐certainty evidence).

There was no evidence of a difference between Hyrax and Haas for posterior crossbite correction (RR 1.05, 95% CI 0.94 to 1.18; 3 studies, 83 participants; moderate‐certainty evidence) or inter‐molar distance (MD −0.15 mm, 95% CI −0.86 mm to 0.56 mm; 2 studies of adolescents, 46 participants; moderate‐certainty evidence).

There was no evidence of a difference between Hyrax and tooth‐bone‐borne expansion for crossbite correction (RR 1.02, 95% CI 0.92 to 1.12; I² = 0%; 3 studies, 120 participants; low‐certainty evidence) or inter‐molar distance (MD −0.66 mm, 95% CI −1.36 mm to 0.04 mm; I² = 0%; 2 studies, 65 participants; low‐certainty evidence).

There was no evidence of a difference between Hyrax with bone‐borne expansion for posterior crossbite correction (RR 1.00, 95% CI 0.94 to 1.07; I² = 0%; 2 studies of adolescents, 81 participants; low‐certainty evidence) or inter‐molar distance (MD −0.14 mm, 95% CI −0.85 mm to 0.57 mm; I² = 0%; 2 studies, 81 participants; low‐certainty evidence).

Authors' conclusions

For children in the early mixed dentition stage (age 7 to 11 years old), quad‐helix and expansion plates are more beneficial than no treatment for correcting posterior crossbites. Expansion plates also increase the inter‐canine distance. Quad‐helix is more effective than expansion plates for correcting posterior crossbite and increasing inter‐molar distance. Treatment duration is shorter with quad‐helix than expansion plates.

For adolescents in permanent dentition (age 12 to 16 years old), Hyrax and Haas are similar for posterior crossbite correction and increasing the inter‐molar distance.

The remaining evidence was insufficient to draw any robust conclusions for the efficacy of posterior crossbite correction.

Keywords: Adolescent; Child; Humans; Bias; Dental Care; Dentition, Permanent; Europe; Malocclusion; Malocclusion/therapy

Plain language summary

Are braces effective for treating crossbite (top back teeth biting down inside the bottom back teeth)?

Key messages

Orthodontic treatment with quad‐helix (fixed) or expansion plates (removeable) is effective for correcting posterior crossbite correction in children. Quad‐helix is probably even more effective than expansion plates. For adolescents, there is probably no difference between Hyrax and Haas for posterior crossbite correction.

What is the problem?

Posterior crossbite occurs when the top teeth or jaw are narrower than the bottom teeth. It can happen on one side or both sides of the dental arches. This condition may increase the likelihood of dental problems (e.g. tooth wear), abnormal development of the jaws, joint problems, and unbalanced facial appearance. Posterior crossbites affect around 4% and 17% of children and adolescents in Europe and America.

Different treatment approaches have been proposed, resulting in many different braces being produced. The basic treatment to correct crossbite correction treatment involves using an orthodontic device on the palate (roof of the mouth) to expand the upper jaw by exerting pressure on both sides of the jaw. The devices can be fixed (e.g. quad‐helix, Haas, Hyrax expander) or removable (e.g. expansion plate). Fixed appliances are bonded to the teeth, while removeable devices can be taken out of the mouth by patients.

What did we want to know?

We wanted to know the effects of different braces for posterior crossbite correction.

What did we do?

We searched for studies that assessed the effectiveness of braces used to correct posterior crossbites.

What did we find?

We found 31 studies with 1410 children and adolescents who were randomly assigned to a treatment or no treatment group. Thirteen studies included children (7 to 11 years old), 12 included adolescents (12 to 16 years old), and six included both. Eight studies were conducted in Turkey, four in Brazil, four in Sweden, three in the USA, three in Italy, two in Canada, and one each in Germany, UK, Switzerland, Iran, Spain, India, and Australia. Twenty‐seven studies were carried out in universities and clinical centres, one study in private practice, and three did not state the location.

What were the main results?

For children, expansion of the upper arch with fixed or removable braces can correct posterior crossbites.

When testing fixed and removable braces against each other, the studies found that quad‐helix (fixed appliance) was more successful than expansion plate (removable appliance) and that treatment with quad‐helix took less time.

For other comparisons between different types of treatments, there was no evidence to show that one worked better than another, but we had moderate to low certainty in the results, so future research may change them.

How reliable are these results?

Our confidence in the results is high to moderate for the main results. For the other comparisons, our confidence in the results is low.

How up‐to‐date is this review?

This review is an update. The evidence is current to April 2021.

Summary of findings

Summary of findings 1. Removable tooth‐borne expansion (expansion plate) versus observation for posterior crossbite.

Removable tooth‐borne expansion (expansion plate) versus observation for posterior crossbite
Population: children (7 to 11 years old) with posterior crossbite Setting: public dental health service and university orthodontic/dental clinics Intervention: expansion plate Comparison: observation
Outcomes	*Illustrative comparative risks (95% CI)**		Relative effect (95% CI)	№ of participants (studies)	Certainty of the evidence (GRADE)	Comments
	Assumed risk	Corresponding risk
	Observation	Expansion plate
Crossbite correction	14 per 1000	260 per 1000 (154 to 404)	OR 25.26 (13.08 to 48.77)	148 (3 RCTs)	⊕⊕⊕⊕ High	Decreases the incidence of posterior crossbite
Molar expansion measured by inter‐molar distance	The mean expansion ranged across the observation group from 0.15 mm to 0.5 mm	The mean expansion in the expansion plate group was 3.3 mm higher (2.88 higher to 3.73 higher)	‐	145 (3 RCTs)	⊕⊕⊕⊝ Moderate^a	Probably increases inter‐molar distance
Canine expansion measured by inter‐canine distance	The mean expansion ranged across the observation group from −0.17 mm to 0.3 mm	The mean expansion in the expansion plate group was 2.59 mm higher (2.18 higher to 3.01 higher)	‐	145 (3 RCTs)	⊕⊕⊕⊝ Moderate^b	Probably increases inter‐canine distance
*The risk in the intervention group (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). For the assumed risk, we used a conservative low risk for cross‐bite correction in the control group. CI: confidence interval; RCT: randomised controlled trial; RR: risk ratio
GRADE Working Group grades of evidence High certainty: we are very confident that the true effect lies close to that of the estimate of the effect. Moderate certainty: we are moderately confident in the effect estimate: the true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different. Low certainty: our confidence in the effect estimate is limited: the true effect may be substantially different from the estimate of the effect. Very low certainty: we have very little confidence in the effect estimate: the true effect is likely to be substantially different from the estimate of effect.

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Summary of findings 2. Fixed tooth‐borne expansion (quad‐helix) compared with observation for posterior crossbite.

Quad‐helix compared with observation for posterior crossbite
Population: children (7 to 11 years old) with posterior crossbite Setting: public dental health service and university orthodontic/dental clinics Intervention: quad‐helix Comparison: observation
Outcomes	*Illustrative comparative risks (95% CI)**		Relative effect (95% CI)	No of participants (studies)	Certainty of the evidence (GRADE)	Comments
	Assumed risk	Corresponding risk
	Observation	Quad‐helix
Crossbite correction	14 per 1000	413 per 1000 (271 to 570)	OR 50.59 (26.77 to 95.60)	149 (3 RCTs)	⊕⊕⊕⊕ High	Decreases the incidence of posterior crossbite
Molar expansion measured by inter‐molar distance	The mean expansion ranged across the observation group from 0.15 mm to 0.5 mm	The mean expansion in the quad‐helix group was 4.71 mm higher (4.31 higher to 5.10 higher)	‐	146 (3 RCTs)	⊕⊕⊕⊝ Moderate^a	Probably increases inter‐molar distance
Canine expansion measured by inter‐canine distance	The mean expansion ranged across the observation group from −0.17 mm to 0.3 mm	The mean expansion in the quad‐helix group was 3.15 mm higher (2.77 higher to 3.53 higher)	‐	146 (3 RCTs)	⊕⊕⊝⊝ Low^b	May increase inter‐canine distance
*The risk in the intervention group (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). For the assumed risk, we used a conservative low risk for cross‐bite correction in the control group. CI: confidence interval; RCT: randomised controlled trial; RR: risk ratio
GRADE Working Group grades of evidence High certainty: we are very confident that the true effect lies close to that of the estimate of the effect. Moderate certainty: we are moderately confident in the effect estimate: the true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different. Low certainty: our confidence in the effect estimate is limited: the true effect may be substantially different from the estimate of the effect. Very low certainty: we have very little confidence in the effect estimate: the true effect is likely to be substantially different from the estimate of effect.

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Summary of findings 3. Fixed tooth‐borne expansion (Hyrax) compared with observation for posterior crossbite.

Hyrax compared with observation for posterior crossbite
Population: children and adolescents with posterior crossbite Settings: public dental health service and university orthodontic/dental clinics Intervention: Hyrax Comparison: observation
Outcomes	*Illustrative comparative risks (95% CI)**		Relative effect (95% CI)	No of participants (studies)	Certainty of the evidence (GRADE)	Comments
	Assumed risk	Corresponding risk
	Observation	Hyrax
Crossbite correction	21 per 1000	511 per 1000 (319 to 699)	OR 48.02 (21.58 to 106.87)	93 (3 RCTs)	⊕⊕⊕⊝ Moderate^a	Probably decreases the incidence of posterior crossbite
Molar expansion measured by inter‐molar distance	The mean expansion ranged across the observation group from −0.07 mm to 0.91 mm	The mean expansion in the Hyrax group was 5.80 mm higher (5.15 higher to 6.45 higher)	‐	71 (2 RCTs)	⊕⊕⊕⊝ Moderate^b	Probably increases inter‐molar distance
Canine expansion measured by inter‐canine distance	Not measured	‐	‐	‐	‐
*The risk in the intervention group (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). For the assumed risk, we used a conservative low risk for cross‐bite correction in the control group. CI: confidence interval; RCT: randomised controlled trial; RR: risk ratio
GRADE Working Group grades of evidence High certainty: we are very confident that the true effect lies close to that of the estimate of the effect. Moderate certainty: we are moderately confident in the effect estimate: the true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different. Low certainty: our confidence in the effect estimate is limited: the true effect may be substantially different from the estimate of the effect. Very low certainty: we have very little confidence in the effect estimate: the true effect is likely to be substantially different from the estimate of effect.

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^aDowngraded (1 level) for high risk of bias in 1 domain in one study and unclear risk of bias in at least 2 domains in one study. ^bDowngraded (1 level) for unclear risk of bias in at least 2 domains in one study.

Summary of findings 4. Fixed tooth‐borne expansion (quad‐helix) compared with removable tooth‐borne expansion (expansion plate) for posterior crossbite.

Quad‐helix compared with expansion plate for posterior crossbite
Population: children (7 to 11 years old) with posterior crossbite Settings: public dental health service and university orthodontic/dental clinics Intervention: quad‐helix Comparison: expansion plate
Outcomes	*Illustrative comparative risks (95% CI)**		Relative effect (95% CI)	No of participants (studies)	Certainty of the evidence (GRADE)	Comments
	Assumed risk	Corresponding risk
	Expansion plate	Quad‐helix
Crossbite correction	763 per 1000	984 per 1000 (862 to 1000)	RR 1.29 (1.13 to 1.46)	151 (3 RCTs)	⊕⊕⊕⊝ Moderate^a	Probably decreases the incidence of posterior crossbite
Molar expansion measured by inter‐molar distance	The mean expansion ranged across the expansion plate group from 3.09 mm to 3.5 mm	The mean expansion in the quad‐helix group was 1.48 mm higher (0.91 higher to 2.04 higher)		151 (3 RCTs)	⊕⊕⊕⊕ High	Increases inter‐molar distance
Canine expansion measured by inter‐canine distance	The mean expansion ranged across the expansion plate group from 1.43 mm to 3.3 mm	The mean expansion in the quad‐helix group was 0.59 mm higher (0.09 higher to 1.08 higher)		151 (3 RCTs)	⊕⊝⊝⊝ Low^b	May increase inter‐canine distance
Length of treatment measured in months	The mean length of treatment ranged across the expansion plate group from 6.12 months to 11.4 months	The mean length of treatment in quad‐helix group was 3.15 months lower (4.04 to 2.25 lower)		148 (3 RCTs)	⊕⊕⊕⊝ Moderate^c	Probably decreases the length of treatment
The basis for the assumed risk* (e.g. the median control group risk across studies) is provided in footnotes. The corresponding risk (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). CI: confidence interval; RCT: randomised controlled trial; RR: risk ratio
GRADE Working Group grades of evidence High certainty: we are very confident that the true effect lies close to that of the estimate of the effect. Moderate certainty: we are moderately confident in the effect estimate: the true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different. Low certainty: our confidence in the effect estimate is limited: the true effect may be substantially different from the estimate of the effect. Very low certainty: we have very little confidence in the effect estimate: the true effect is likely to be substantially different from the estimate of effect.

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^aDowngraded (1 level) because there was unexplained inconsistency that was supported by substantial I² statistic values and statistically significant heterogeneity of effect estimates (Chi² = 8.24, df = 2 (P = 0.02); I² = 76%. ^bDowngraded (2 levels) because there was unexplained inconsistency that was supported by non‐overlapping confidence intervals with considerable I² statistic values and statistically significant heterogeneity of effect estimates (Chi² = 13.71, df = 2 (P = 0.001); I² = 85%. ^cDowngraded (1 level) because there was unexplained inconsistency that was supported by substantial I² statistic values and statistically significant heterogeneity of effect estimates (Chi² = 6.37, df = 2 (P = 0.04); I² = 69%).

Summary of findings 5. Fixed tooth‐tissue‐borne expansion (Haas) versus fixed tooth‐borne expansion (Hyrax) for posterior crossbite.

Hyrax compared with Haas for posterior crossbite
Population: children with posterior crossbite Settings: public dental health service and university orthodontic/dental clinics Intervention: Hyrax Comparison: Haas
Outcomes	*Illustrative comparative risks (95% CI)**		Relative effect (95% CI)	No of participants (studies)	Certainty of the evidence (GRADE)	Comments
	Assumed risk	Corresponding risk
	Haas	Hyrax
Crossbite correction	976 per 1000	1000 per 1000 (918 to 1000)	RR 1.05 (0.94 to 1.18)	83 (3 RCTs)	⊕⊕⊕⊝ Moderate^a	There is probably no difference in the incidence of posterior crossbite
Molar expansion measured by inter‐molar distance	The mean expansion ranged across the Haas group from 5.49 mm to 8.49 mm	The mean expansion in the Hyrax group was 0.15 mm lower (0.86 lower to 0.56 higher)		46 (2 RCTs)	⊕⊕⊕⊝ Moderate^a	There is probably no difference in the inter‐molar distance
Canine expansion measured by inter‐canine distance	Not measured
The basis for the assumed risk* (e.g. the median control group risk across studies) is provided in footnotes. The corresponding risk (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). CI: confidence interval; RCT: randomised controlled trial; RR: risk ratio
GRADE Working Group grades of evidence High certainty: we are very confident that the true effect lies close to that of the estimate of the effect. Moderate certainty: we are moderately confident in the effect estimate: the true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different. Low certainty: our confidence in the effect estimate is limited: the true effect may be substantially different from the estimate of the effect. Very low certainty: we have very little confidence in the effect estimate: the true effect is likely to be substantially different from the estimate of effect.

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^aDowngraded 1 level for unclear risk of bias in at least 2 domains in one study.

Summary of findings 6. Fixed tooth‐borne expansion (Hyrax) versus tooth‐bone‐borne expansion for posterior crossbite.

Tooth‐bone borne expansion compared with Hyrax for posterior crossbite
Population: children with posterior crossbite Setting: public dental health service and university orthodontic/dental clinics Intervention: tooth‐bone‐borne expansion Comparison: Hyrax
Outcomes	*Illustrative comparative risks (95% CI)**		Relative effect (95% CI)	No of participants (studies)	Certainty of the evidence (GRADE)	Comments
	Assumed risk	Corresponding risk
	Hyrax	Tooth‐bone‐borne expansion
Crossbite correction	950 per 1000	969 per 1000 (874 to 1000)	RR 1.02 (0.92 to 1.12)	120 (3 RCTs)	⊕⊝⊝⊝ Low ^a	There may be no difference in the incidence of posterior crossbite
Molar expansion measured by inter‐molar distance	The mean expansion ranged across the Hyrax group from 4.8 mm to 8.6 mm	The mean expansion in the tooth‐bone‐borne group was 0.66 mm higher (0.04 lower to 1.36 higher)		65 (2 RCTs)	⊕⊝⊝⊝ Low ^a	There may be no difference in the inter‐molar distance
Canine expansion measured by inter‐canine distance	Not measured
The basis for the assumed risk* (e.g. the median control group risk across studies) is provided in footnotes. The corresponding risk (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). CI: confidence interval; RCT: randomised controlled trial; RR: risk ratio
GRADE Working Group grades of evidence High certainty: we are very confident that the true effect lies close to that of the estimate of the effect. Moderate certainty: we are moderately confident in the effect estimate: the true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different. Low certainty: our confidence in the effect estimate is limited: the true effect may be substantially different from the estimate of the effect. Very low certainty: we have very little confidence in the effect estimate: the true effect is likely to be substantially different from the estimate of effect.

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^aDowngraded 1 level for high risk of bias in 1 domain in one study and 1 level for unclear risk of bias in at least 2 domains in another study.

Summary of findings 7. Fixed tooth‐borne expansion (Hyrax) versus bone‐borne expansion for posterior crossbite.

Tooth‐borne expansion compared with bone‐borne expansion for posterior crossbite
Population: children with posterior crossbite Setting: public dental health service and university orthodontic/dental clinics Intervention: bone‐borne expansion Comparison: tooth‐borne expansion
Outcomes	*Illustrative comparative risks (95% CI)**		Relative effect (95% CI)	No of participants (studies)	Certainty of the evidence (GRADE)	Comments
	Assumed risk	Corresponding risk
	Hyrax	Bone‐borne expansion
Crossbite correction	1000 per 1000	1000 per 1000 (940 to 1000)	1.00 (0.94 to 1.07)	81 (2 RCTs)	⊕⊝⊝⊝ Low^a	There may be no difference in the incidence of posterior crossbite
Molar expansion measured by inter‐molar distance	The mean expansion ranged across the Hyrax group from 4.2 mm to 5.83 mm	The mean expansion in the tooth‐bone‐borne group was 0.14 mm lower (0.85 lower to 0.57 higher)		81 (2 RCTs)	⊕⊝⊝⊝ Low^a	There may be no difference in the inter‐molar distance
Canine expansion measured by inter‐canine distance	Not measured
The basis for the assumed risk* (e.g. the median control group risk across studies) is provided in footnotes. The corresponding risk (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). CI: confidence interval; RCT: randomised controlled trial; RR: risk ratio
GRADE Working Group grades of evidence High certainty: we are very confident that the true effect lies close to that of the estimate of the effect. Moderate certainty: we are moderately confident in the effect estimate: the true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different. Low certainty: our confidence in the effect estimate is limited: the true effect may be substantially different from the estimate of the effect. Very low certainty: we have very little confidence in the effect estimate: the true effect is likely to be substantially different from the estimate of effect.

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^aDowngraded 1 level for high risk of bias in 1 domain in one study, 1 level for unclear risk of bias in at least 2 domains in one study.

Background

Description of the condition

Posterior crossbite is the term used to describe when the top back teeth bite inside the bottom back teeth. A posterior crossbite occurs when the top teeth or jaw are narrower than the bottom teeth and can happen on one (unilateral) or both (bilateral) sides of the mouth. The prevalence of posterior crossbites in children and adolescents is around 4% in Europe and 17% in America (Lombardo 2020). It is likely to be higher in white populations compared with children of African or Asian ethnicity (Malandris 2004). Most posterior crossbites (50% to 90%) persist when the permanent teeth erupt. However, in a minority of children, the malocclusion self‐corrects after the change from deciduous to permanent dentition (Thilander 1984).

A posterior crossbite may develop or improve at any time from when the deciduous (baby) teeth come into the mouth to when the permanent (adult) teeth come through (Heikinheimo 1987; Kurol 1992; Leighton 1966; Thilander 1984). It is unclear what causes posterior crossbites but they may be due to skeletal, soft tissue, dental or respiratory factors, or develop as the result of a habit (e.g. thumb sucking; Boronat‐Catalá 2017; Bresolin 1983; Cheng 1988; D'Onofrio 2019; Modeer 1982; Ogaard 1994; Schmid 2018; Subtelny 1980).

A functional posterior crossbite occurs when there is interference between two or more teeth when an individual closes their jaw. To obtain a more comfortable bite, the mandible (lower jaw) shifts to one side into a position that allows more teeth to come into contact. However, while it is unlikely that young children with a posterior crossbite will experience any pain or have problems with chewing (Iodice 2016), this displacement may increase the likelihood of the individual developing bruxism (tooth grinding), which may lead to other dental issues, including the tooth surface being worn away, abnormal growth and development of the teeth and jaws, and jaw joint problems (Andrade 2009; Malandris 2004; Talapaneni 2012; Tsanidis 2016). The strain on the jaw muscles and joints due to the unusual movement and position of the lower jaw may lead to skeletal facial asymmetries. This asymmetry can be corrected only in adulthood with a combination of orthodontic treatment and maxillofacial surgery.

Temporomandibular joint problems, such as pain, clicking, or locking of the jaw joints have many causes, but studies of teenagers and adults have shown that some people with a crossbite may have an increased risk of developing jaw joint problems and show more signs and symptoms of these problems (Egermark 1990; McNamara 1997; Ninou 1994; O'Bryn 1995; Pullinger 1993). However, more recent studies have been less conclusive regarding an association between posterior crossbites and jaw joint problems (Ellabban 2018; Gesch 2004; Iodice 2013; Thilander 2002; Thilander 2012).

Description of the intervention

As self‐correction occurs in only a very small minority of cases, treatment of malocclusion can be recommended. Treatment of a posterior crossbite usually involves expansion of the maxillary (upper jaw) arch, removal of any occlusal interferences, and elimination of the functional displacement of the mandible.

Maxillary expansion treatment initiated during the early mixed dentition stage (children younger than eight years old) may require the use of lower forces to achieve expansion of the upper arch because the mid‐line bony suture has not yet fused. However, obtaining co‐operation with treatment may be more difficult in younger children and treatment may be complicated by the loss of deciduous teeth. In older children (aged 12 years and above) who are in the early permanent dentition stage, greater forces may be required to achieve maxillary expansion. Maxillary expansion can be achieved using either fixed or removable appliances and expansion can be either rapid or slow. Expansion appliances can be anchored only in the tooth (tooth‐borne), tooth and oral tissues (tooth‐tissue‐borne), tooth and bone (tooth‐bone‐borne), or only in bone (bone‐borne). Expansion of the top jaw is termed 'rapid' when expansion takes place at a rate of 0.5 mm per day, and 'slow' when expansion takes place at a rate of 0.5 mm per week. Braces that can bring about this expansion are fixed to the back teeth by either metal bands around the individual teeth (banded rapid or slow maxillary expansion) or acrylic splints over several teeth (bonded rapid or slow maxillary expansion) joined by a wire framework attached to a screw in the mid‐line that can be opened to expand the top jaw.

Removal of occlusal interferences involves grinding of teeth (usually deciduous teeth) to enable the jaws to bite together in a more normal position. The reported success rate of this approach varies widely between 27% and 64% (Kennedy 2005; Lindner 1989). Alternatively, composite onlays can be placed to prevent the jaw from shifting when interferences exist.

How the intervention might work

Expansion of the maxillary arch can be achieved using either fixed or removable appliances (see Appendix 1 for a description of appliances). These appliances have expansion screws that are adjusted during treatment to increase the width of the maxillary arch. The duration of the expansion may be either over two to six weeks (when rapid) or over six to 12 weeks (when slow). Overexpansion may be appropriate to allow for some relapse after the appliance removal. The use of maxillary expansion during the early mixed dentition stage may also reduce crowding in the permanent dentition.

Orthodontic appliances may also correct the sideways displacement of the mandible, which may decrease the pressure on the temporomandibular joint.

Where functional displacement of the mandible is associated with reversible causes such as non‐nutritive sucking habits (e.g. sucking on a dummy, thumb, fingers), eliminating the causal behaviour is a desirable adjunct to orthodontic treatment (Borrie 2015).

Why it is important to do this review

A crossbite is a common transverse feature seen in malocclusions in the posterior region of the dental arch. There is still debate about the optimum timing of orthodontic treatment, the use of either fixed or removable appliances, and the type of appliance and activation protocol that leads to the best outcomes for the patients. This updated systematic review will summarise the evidence available from randomised controlled trials to inform treatment decision making. This is the second update of a Cochrane review first published in 1998 (the first update was published in 2014).

Objectives

To assess the effects of different orthodontic treatments for posterior crossbites.

Methods

Criteria for considering studies for this review

Types of studies

We included randomised controlled trials (RCTs) of parallel design that assessed orthodontic treatments to correct a posterior crossbite. We included studies irrespective of language or publication status.

Types of participants

We included RCTs of children and adults with a posterior crossbite, without a Class III skeletal relationship, cleft lip or palate (or both), or other syndrome associated with craniofacial anomalies.

Types of interventions

We included studies of any orthodontic or dentofacial orthopedic (not surgical) treatment used to correct posterior crossbites or expand the top back teeth, or both when compared against another such treatment or no treatment.

Types of outcome measures

Primary outcomes

Correction of the posterior crossbite, measured as a dichotomous outcome.

Secondary outcomes

Expansion of the upper jaw or teeth measured as changes in the width between the molars or canines, or both
Stability of crossbite correction
Signs and symptoms of temporomandibular joint dysfunction (e.g. pain, clicking, locking of the jaw joints, problems eating)
Signs and symptoms of respiratory issues (e.g. mouth breathing, nasal airway resistance)
Quality of life (using any validated measurement tool)
Length of treatment
Costs of treatment

Search methods for identification of studies

Electronic searches

Cochrane Oral Health’s Information Specialist conducted systematic searches in the following databases for RCTs and controlled clinical trials. There were no language, publication year, or publication status restrictions.

Cochrane Oral Health’s Trials Register (searched 8 April 2021; Appendix 2)
Cochrane Central Register of Controlled Trials (CENTRAL; 2021, Issue 3) in the Cochrane Library (searched 8 April 2021; Appendix 3)
MEDLINE Ovid (1946 to 8 April 2021; Appendix 4)
Embase Ovid (1980 to 8 April 2021; Appendix 5)

Subject strategies were modelled on the search strategy designed for MEDLINE Ovid. Where appropriate, they were combined with subject strategy adaptations of the highly sensitive search strategies designed by Cochrane for identifying RCTs and controlled clinical trials (as described in the Cochrane Handbook for Systematic Reviews of Interventions, Version 6.2 (Lefebvre 2021)).

Searching other resources

The following trial registries were searched for ongoing studies (see Appendix 6 for details of the search strategy):

US National Institutes of Health Ongoing Trials Register ClinicalTrials.gov (clinicaltrials.gov; searched 8 April 2021);
World Health Organization International Clinical Trials Registry Platform (apps.who.int/trialsearch; searched 8 April 2021).

We handsearched the following journals from 1970 to 2020:

European Journal of Orthodontics;
American Journal of Orthodontics and Dentofacial Orthopedics;
Orthodontics and Craniofacial Research;
The Angle Orthodontist;
British Journal of Orthodontics (from 1973 to 1999) and Journal of Orthodontics (from 2000 to 2020);
Progress in Orthodontics.

We also searched for personal references. We contacted the authors of the included studies by email to ask for additional details of their trials and for any information they may have about any further published and unpublished research.

We searched the reference lists of included studies and relevant systematic reviews for further studies.

We checked that none of the included studies in this review were retracted due to error or fraud.

We did not perform a separate search for adverse effects of interventions used. We considered adverse effects described in included studies only.

Data collection and analysis

Selection of studies

Two review authors screened the titles and abstracts of the results of the searches independently and in duplicate. The search was designed to be sensitive and include controlled clinical trials. These were filtered out early in the selection process if they were not randomised. We obtained full‐text copies of all studies that appeared to meet the inclusion criteria or were unclear due to insufficient information in the title or abstract, or both. When necessary, we contacted study authors for clarification. We solved any disagreements by discussion including a third author as a referee when necessary. Cochrane volunteers translated any non‐English language studies. We recorded the studies that did not meet the inclusion criteria with the reasons in Characteristics of excluded studies.

Data extraction and management

Two review authors (AU and KB) extracted data from the included studies using a piloted data extraction form. We resolved any disagreements through discussion. We involved a third review author (ASB) when necessary.

We entered the following extracted data into the Characteristics of included studies tables.

Study design, location, number of centres, recruitment period, funding, and the experience level of the clinician
Inclusion and exclusion criteria, age, sex, number of participants randomised to each group, number of participants evaluated
Details of the type of intervention/comparator, timing, duration
Details of the outcomes reported, including method of assessment, and time(s) assessed
Sample size calculations, any other notable details

Assessment of risk of bias in included studies

Two review authors assessed the risk of bias of each included study, independently and in duplicate, following the domain‐based two‐part tool described in the Cochrane Handbook for Systematic Reviews of Interventions (Higgins 2017).

We assessed the following six domains of risk of bias for each included study.

Random sequence generation (selection bias)
Allocation concealment (selection bias)
Blinding of outcome assessment (detection bias)
Incomplete outcome data (attrition bias)
Selective reporting (reporting bias)
Other bias

We did not consider the blinding of participants and personnel (performance bias) as the all interventions compared were different. Therefore, a risk of performance bias is possible in all of the included studies. However, it should be possible for outcome assessors to be blinded when measuring casts or models, or radiographs, or all these if the appliances had been removed. Therefore, we focused on assessing the risk of detection bias. We completed a risk of bias table for each included study. For each of the above domains, we described what was reported, and this formed the rationale for our corresponding judgement of low risk of bias, high risk of bias, or unclear risk of bias for each domain. We categorised the overall risk of bias in any included study according to the following.

Low risk of bias (plausible bias unlikely to seriously alter the results) if all key domains were assessed as at low risk of bias.
Unclear risk of bias (plausible bias that raises some doubt about the results) if one or more key domains were assessed as at unclear risk of bias.
High risk of bias (plausible bias that seriously weakens confidence in the results) if one or more key domains were assessed as at high risk of bias.

We have presented the results of the risk of bias assessments in the text and graphically.

Measures of treatment effect

For dichotomous outcomes (posterior crossbite corrected or not), we expressed the estimate of treatment effect as risk ratios (RR) with 95% confidence intervals (CI), unless there were zero values in trial arms, in which case we used odds ratios (ORs). For continuous outcomes (width between the molars), we used means and standard deviations (SD) to calculate mean differences (MD) with 95% CIs.

Unit of analysis issues

The statistical unit of analysis for inter‐molar width was the distance in millimetres from the right to the left upper molar. We excluded outcomes that measured the inter‐molar distance from molars to the midline twice as we could not assess the sum and standard deviation of the entire width.

For studies with more than two groups, we selected the one we considered most appropriate for comparison.

Dealing with missing data

Where possible, we attempted to contact the author(s) of studies to obtain missing data or for clarification. We performed the analyses using only the available data (ignoring missing data); however, we intended to use methods for estimating missing SDs as described in chapter 10 of the Cochrane Handbook for Systematic Reviews of Interventions (Deeks 2021), if appropriate. This was not necessary. We did not use any other statistical methods or carry out imputations to account for missing data.

Assessment of heterogeneity

Where we performed meta‐analyses, we assessed heterogeneity by visual inspection of the forest plots. If there was no or little overlap of the CIs, we considered heterogeneity to be present. We assessed heterogeneity statistically using a Chi² test, where a P‐value less than 0.1 indicated statistically significant heterogeneity. We quantified heterogeneity using the I² statistic (Higgins 2003). An approximate guide to the interpretation of the I² statistic given in the Cochrane Handbook for Systematic Reviews of Interventions is:

0% to 40% might not be important;
30% to 60% may represent moderate heterogeneity;
50% to 90% may represent substantial heterogeneity;
75% to 100% may represent considerable heterogeneity (Deeks 2021).

Assessment of reporting biases

In future updates, when more than 10 studies have been pooled in a meta‐analysis, we will create a funnel plot to check for asymmetry (Deeks 2021; Egger 1997), which can highlight publication bias. We will interpret this carefully as there are other possible reasons for funnel plot asymmetry.

Data synthesis

We carried out a meta‐analysis when studies of similar comparisons reported the same outcomes. We combined MDs for continuous outcomes, and RRs for dichotomous outcomes, using a fixed‐effect model when there were fewer than four studies, or a random‐effects model when there were four or more studies. We summarised single‐study comparisons in additional tables.

Subgroup analysis and investigation of heterogeneity

We did not conduct subgroup analyses. In future updates, if sufficient data are available for each comparison and outcome, we will conduct subgroup analyses according to age, sex, degree of severity of the maxillary transversal deficiency, type of expander, expansion protocol, length of follow‐up, and harms.

Sensitivity analysis

We did not conduct sensitivity analyses. In future updates, if sufficient data are available for each comparison and outcome, we will assess the robustness of the results by excluding studies at high and unclear risk of bias from the meta‐analyses.

Summary of findings and assessment of the certainty of the evidence

We produced summary of findings tables following GRADE methods (GRADE 2004), and using GRADEpro GDT software. The comparisons included were removable tooth‐borne expansion (expansion plate) versus observation; fixed tooth‐borne expansion (quad‐helix) versus observation; fixed tooth‐borne expansion (hyrax) versus observation; fixed tooth‐borne expansion (quad‐helix) versus removable tooth‐borne expansion (expansion plate); fixed tooth‐tissue‐borne expansion (Haas) versus fixed tooth‐borne expansion (Hyrax); fixed tooth‐borne expansion (hyrax) versus tooth‐bone‐borne expansion (Haas); fixed tooth‐borne expansion (hyrax) versus bone‐borne expansion. The main outcomes included were crossbite correction, inter‐molar distance, inter‐canine distance and length of treatment.

We presented the number of participants and studies for each comparison and outcome, along with effect estimates and their 95% CIs for each main comparison and outcomes, illustrated by assumed and corresponding risks for the control and intervention groups, respectively. Although none of the included studies showed an event in the observation groups for our primary outcome of crossbite correction, as there is evidence that crossbites can right themselves (Thilander 1984), we assumed a conservative, low risk for cross‐bite correction in the observation groups. The assumed risk for other outcomes and for head‐to‐head trials was based on the mean score in the included control groups.

We assessed the certainty of the body of evidence for each comparison and outcome, considering the overall risk of bias of the included studies, directness of the evidence, consistency of the results, precision of the estimates, and risk of publication bias. We categorised the certainty of the body of evidence of each of the main outcomes for each comparison as high, moderate, low, or very low.

Results

Description of studies

Results of the search

The previous version of the review included 15 studies (Agostino 2014). The electronic searches for this update retrieved 1326 records. After removing duplicates, this was reduced to 715. After examination of the titles and abstracts of these references, we discarded 379 records as irrelevant. We attempted to retrieve 336 reports, but could not retrieve 275 of them. We obtained full‐text copies of potentially relevant studies where possible and we excluded 46 of them with reasons described in the Characteristics of excluded studies tables. Therefore, we identified 16 new studies for inclusion in the review, making 31 studies in total. The study selection process is presented as a flow chart in Figure 1.

Included studies

Characteristics of trial setting

We identified 31 parallel‐group RCTs that met the inclusion criteria (see Characteristics of included studies tables).

Location

Twenty‐seven studies were carried out in university orthodontics departments and clinical centres (Thilander 1984, Sweden; Mossaz‐Joelson 1989, Switzerland; Asanza 1997, USA; Oliveira 2004, USA; Garib 2005, Brazil; McNally 2005, UK; Kilic 2008, Turkey; Petrén 2008, Sweden; Alberta 2010, Canada; Ramoglu 2010, Turkey; Godoy 2011, Brazil; Martina 2012, Italy; Oshagh 2012, Iran; Lippold 2013, Germany; Halicioglu 2014, Turkey; Venancio 2014, Spain; Toklu 2015, Turkey; Ugolini 2015, Italy; Baysal 2016, Turkey; Sweden 2017, Sweden; Gopalakrishnan 2017, India; Ottaviano 2018, Italy; Araujo 2020, Brazil; Cheung 2020, Australia; Massaro 2020, Brazil; Nam 2020, Canada; Sollenius 2020, Sweden). One study was carried out in private practice (Lamparski 2003, USA). Three studies did not report details of setting (Dindaroglu 2016, Turkey; Celenk‐Koca 2018, Turkey; Erhamza 2018, Turkey).

Details of studies that reported location

Eight studies were conducted in Turkey. Two studies were conducted at the Department of Orthodontics, Faculty of Dentistry, Atatürk University, Erzurum, Turkey (Halicioglu 2014; Kilic 2008), one study was conducted at the Department of Orthodontics of Selçuk University (Ramoglu 2010), one study was conducted at the Orthodontic clinic of Yeditepe University, Istanbul, Turkey (Toklu 2015), and one study was carried out at Izmir Katip Celebi University, Izmir, Turkey (Baysal 2016). Three studies conducted in Turkey did not state the place (Celenk‐Koca 2018; Dindaroglu 2016; Erhamza 2018).

Four studies were conducted in Brazil. Two studies were conducted at Orthodontics Department, Bauru School of Dentistry, University of Sao Paulo (Garib 2005; Massaro 2020), one at a small Dental unit of the Santo Amaro area by the University of Pernambuco (Godoy 2011), and one study was conducted at the Dental Clinic of the University of Northern Paraná, Londrina (Araujo 2020).

Four studies were conducted in Sweden. One study was conducted at a Public Dental Health Service, Enköping (Thilander 1984), one study was conducted at the Public Dental Health Service, Skane County Council, and at the Department of Orthodontics, Faculty of Odontology, Malmö University, Malmö (Petrén 2008), one study was conducted at the Postgraduate Dental Education Centre, Department of Orthodontics, Region Örebro County (Sweden 2017), and one study was conducted at three orthodontic specialist clinics and 10 general dentistry clinics (Sollenius 2020).

Three studies were conducted in the USA. One of them at the Orthodontics Department, Albert Einstein College of Medicine and Montefiore Medical Center, New York, USA (Asanza 1997), one at a private practice, Natrona Heights, PA (Lamparski 2003), and the other at the Department of Orthodontics and the Craniofacial Center, the University of Illinois in Chicago (Oliveira 2004).

Three studies were reported in Italy, one by the Department of Oral Science, University of Naples Federico II, Italy, (Martina 2012), another study was conducted at three different locations in Italy at Universities of Genova, Siena, and Insubria (Ugolini 2015), the third study at two different centres Unità Operativa Complessa di Chirurgia Maxillo‐Facciale e di Odontostomatologia, Fondazione IRCCS Ca’ Granda, Ospedale Maggiore Policlinico, and one study was conducted at ENT Clinic, Department of Neurosciences DNS of Padua University (Ottaviano 2018).

Two studies were conducted at the Orthodontic Clinic at the University of Alberta in Edmonton, Alberta, Canada (Alberta 2010; Nam 2020).

One study was conducted at Sydney Dental Hospital, Sydney, Australia (Cheung 2020).

One study was conducted in Germany at the Orthodontics Department, Münster University's Medical Sciences Division and University Hospital Münster (Lippold 2013).

One study was conducted at the Department of Orthodontics, Tamil Nadu Government Dental College and Hospital, Chennai, Tamil Nadu, India (Gopalakrishnan 2017).

One study was conducted in Iran by the Orthodontic Department, School of Dentistry, Shiraz University of Medical Sciences, Iran (Oshagh 2012).

One study was conducted at the Pediatric Clinic at the School of Dentistry of the University Complutense of Madrid, Spain (Venancio 2014).

One study was conducted at the Department of Orthodontics, Dental School, University of Geneva, Switzerland (Mossaz‐Joelson 1989).

One study was conducted in the UK at Queen's Hospital, Burton on Trent, and The University of Birmingham, School of Dentistry, Birmingham (McNally 2005).

Number of centres

There were five multicentre studies.

Oliveira 2004 was carried out in three university orthodontic clinics and one private practice in the USA, McNally 2005 in a general hospital and a university dental hospital in the UK by experienced orthodontists, Petrén 2008 in two public dental health service clinics and one university orthodontic department in Sweden by experienced general practitioners under the supervision of specialist orthodontists, Ugolini 2015 in three Italian university orthodontic clinics (Universities of Genova, Siena, and Insubria, Italy), and Sollenius 2020 in 10 general dental health clinics at the Public Dental Health Service, Halland County Council, Sweden by two orthodontic specialists and 17 general dentists.

The remaining 26 studies were single‐centre.

Clinician experience

Five studies provided details of clinician experience (Araujo 2020; Godoy 2011; Lamparski 2003; Petrén 2008; Sollenius 2020). Three studies stated that the clinicians were specialist orthodontists (Lippold 2013; Sweden 2017; Thilander 1984). One study reported the clinician experience with the device tested (McNally 2005). In the remaining 22 studies, the experience or qualifications of the clinicians were not stated (Alberta 2010; Asanza 1997; Baysal 2016; Celenk‐Koca 2018; Cheung 2020; Dindaroglu 2016; Erhamza 2018; Garib 2005; Gopalakrishnan 2017; Halicioglu 2014; Kilic 2008; Martina 2012; Massaro 2020; Mossaz‐Joelson 1989; Nam 2020; Oliveira 2004; Oshagh 2012; Ottaviano 2018; Ramoglu 2010; Toklu 2015; Ugolini 2015; Venancio 2014).

Details of studies that reported clinical experience

One study was carried out by a board‐certified orthodontist with 27 years of experience (Lamparski 2003), one study by the authors that had used expansion arches of the type tested for several years (McNally 2005), one by five experienced general practitioners under the supervision of specialist orthodontists (Petrén 2008), one study by one specialist orthodontist with over 10 years of experience (Godoy 2011), one by two orthodontist residents and supervised by a faculty member (Araujo 2020), and one study by two orthodontists with 15 years of relevant experience (Sollenius 2020).

Funding

Fourteen studies provided details on funding (Araujo 2020; Baysal 2016; Cheung 2020; Erhamza 2018; Godoy 2011; Martina 2012; Massaro 2020; Oliveira 2004; Petrén 2008; Ramoglu 2010; Sollenius 2020; Sweden 2017; Thilander 1984; Venancio 2014). In one study, 3M Unitek supplied the quad‐helix arches (McNally 2005). Two study declared not receiving any funding (Ottaviano 2018; Nam 2020). The remaining 14 studies did not state if they had funding (Alberta 2010; Asanza 1997; Celenk‐Koca 2018; Dindaroglu 2016; Garib 2005; Gopalakrishnan 2017; Halicioglu 2014; Kilic 2008; Lamparski 2003; Lippold 2013; Mossaz‐Joelson 1989; Oshagh 2012; Toklu 2015; Ugolini 2015).

Details of studies that reported funding

Studies were funded by a range of government, clinical specialty, and university organisations.

Araujo 2020 was supported by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) ‐ Finance Code 001.
Baysal 2016 was supported by research grants from The Scientific and Technological Research Council of Turkey (Project112R033) and Izmir Katip Celebi University, Scientific Research Projects Unit (Project 2013‐3‐TSBP‐32).
Cheung 2020 was supported by the Australian Society of Orthodontists Foundation for Research and Education.
Erhamza 2018 was supported by the Coordinatorship of Scientific Research Projects of Kirikkale University financially supported the study.
Godoy 2011 was funded by the University of Pernambuco and a research grant from the Ministry of Education of Brazil (CAPES).
Martina 2012 was supported by a grant from the Italian Ministry of University and Research.
Massaro 2020 was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior ‐ Brasil (CAPES) ‐ Finance Code 001, and by the São Paulo Research Foundation (FAPESP) ‐ Grant numbers 2017/12911‐9 and 2017/24115‐2.
Oliveira 2004 was partially funded by a grant from the American Association of Orthodontists Foundation.
Petrén 2008 was funded and supported by the Swedish Dental Society, Skane County Council, and the Faculty of Odontology, Malmö University, Sweden.
Ramoglu 2010 was supported by Selçuk University Research Projects (2003‐2004).
Sollenius 2020 was funded and supported by the European Orthodontic Society Research grant (2015), Region Halland, Sweden.
Sweden 2017 was supported by the Uppsala‐Örebro Regional Research Council, Sweden, (grant number RFR‐72021).
Thilander 1984 was funded by the Grant of the Swedish Medical Research Council.
Venancio 2014 was funded and supported by the Fundación Investigación Médica Mutua Madrileña.

Recruitment dates

Nine studies provided recruitment dates: Thilander 1984 recruited from 1965 to 1967; Petrén 2008 recruited from 2001 to 2005; Martina 2012 recruited from May 2006 to October 2007; Nam 2020 recruited from May 2010 to May 2016; Sweden 2017 recruited from September 2010 to December 2015; Sollenius 2020 recruited from December 2013 to November 2018; Ottaviano 2018 recruited from June 2015 to January 2016; Cheung 2020 recruited from January 2017 to July 2017; and Massaro 2020 recruited from January 2017 to June 2018.

Two studies were unclear about recruitment dates: Alberta 2010 reported an 18‐month recruitment period and Toklu 2015 reported a two‐year recruitment period.

Twenty studies did not provide details on the period of recruitment (Araujo 2020; Asanza 1997; Baysal 2016; Celenk‐Koca 2018; Dindaroglu 2016; Erhamza 2018; Garib 2005; Godoy 2011; Gopalakrishnan 2017; Halicioglu 2014; Kilic 2008; Lamparski 2003; Lippold 2013; McNally 2005; Mossaz‐Joelson 1989; Oliveira 2004; Oshagh 2012; Ramoglu 2010; Ugolini 2015; Venancio 2014).

Characteristics of the participants

Age

The age range of participants was from 5 to 17 years old, with variations between studies. Twelve studies included only children, from 5 to 11 years old (Araujo 2020; Godoy 2011; Lippold 2013; Massaro 2020; Ottaviano 2018; Petrén 2008; Ramoglu 2010; Sollenius 2020; Sweden 2017; Thilander 1984; Ugolini 2015; Venancio 2014). Thirteen studies included only adolescents, from 11 to 16 years old (Alberta 2010; Baysal 2016; Celenk‐Koca 2018; Cheung 2020; Dindaroglu 2016; Erhamza 2018; Garib 2005; Gopalakrishnan 2017; Halicioglu 2014; Kilic 2008; McNally 2005; Nam 2020; Toklu 2015) and six studies included both children and adolescents (Asanza 1997; Lamparski 2003; Martina 2012; Mossaz‐Joelson 1989; Oliveira 2004; Oshagh 2012).

Dentition

Three studies enrolled only children in the permanent dentition stage (Dindaroglu 2016; Garib 2005; Toklu 2015), five studies enrolled children in the mixed dentition stage (Godoy 2011; Petrén 2008; Sollenius 2020; Sweden 2017; Ugolini 2015), seven studies enrolled children in either the mixed or permanent dentition stages (Erhamza 2018; Gopalakrishnan 2017; Lamparski 2003; Martina 2012; Oliveira 2004; Ottaviano 2018; Ramoglu 2010), and one study enrolled children in either the late deciduous or early mixed dentition stages (Lippold 2013). Fifteen studies did not state the stage of dental development of the children at study entry (Alberta 2010; Araujo 2020; Asanza 1997; Baysal 2016; Celenk‐Koca 2018; Cheung 2020; Halicioglu 2014; Kilic 2008; Massaro 2020; McNally 2005; Mossaz‐Joelson 1989; Nam 2020; Oshagh 2012; Thilander 1984; Venancio 2014).

Malocclusion

Fourteen studies enrolled participants with either a unilateral or bilateral crossbite malocclusion (Araujo 2020; Cheung 2020; Dindaroglu 2016; Garib 2005; Gopalakrishnan 2017; Martina 2012; McNally 2005; Mossaz‐Joelson 1989; Nam 2020; Oliveira 2004; Ottaviano 2018; Ramoglu 2010; Sweden 2017; Toklu 2015); six studies included only participants with a unilateral crossbite (Godoy 2011; Lippold 2013; Petrén 2008; Sollenius 2020; Ugolini 2015; Venancio 2014); four studies included only participants with a bilateral crossbite (Erhamza 2018; Halicioglu 2014; Kilic 2008; Oshagh 2012); the remaining seven studies did not specify the type of crossbite that participants presented with at study entry (Alberta 2010; Asanza 1997; Baysal 2016; Celenk‐Koca 2018; Lamparski 2003; Massaro 2020; Thilander 1984).

Characteristics of the interventions and comparisons

All included studies reported a clear description of the treatment protocols.

We classified the interventions for crossbite treatment as follows:

Removable tooth‐borne expansion (expansion plate) versus observation
1. Three studies compared expansion plate treatment with no treatment (Godoy 2011; Petrén 2008; Sollenius 2020).
Fixed tooth‐borne expansion (quad‐helix) versus observation
1. Three studies compared quad‐helix treatment with no treatment (Godoy 2011; Petrén 2008; Sollenius 2020).
3. Fixed tooth‐borne expansion (Hyrax) versus observation
1. Four studies compared Hyrax treatment with no treatment (Alberta 2010; Erhamza 2018; Ottaviano 2018).
Fixed tooth‐borne expansion (quad‐helix) versus removable tooth‐borne expansion (expansion plate)
1. Three studies compared quad‐helix treatment with expansion plate (Godoy 2011; Petrén 2008; Sollenius 2020).
Fixed tooth‐tissue‐borne expansion (Haas) versus fixed tooth‐borne expansion (Hyrax):
1. Three studies compared Haas treatment with Hyrax treatment (Araujo 2020; Dindaroglu 2016; Garib 2005).
Fixed tooth‐borne expansion (Hyrax) versus tooth‐bone‐borne expansion (Haas)
1. Three studies compared Hyrax treatment with tooth‐bone‐borne expansion treatment (Cheung 2020; Sweden 2017; Toklu 2015).
Fixed tooth‐borne expansion (Hyrax) versus bone‐borne expansion
1. Two studies compared Hyrax treatment with bone‐borne expansion treatment (Alberta 2010; Celenk‐Koca 2018).

Single study comparisons

Removable tooth‐borne expansion (expansion plate) + grinding or grinding versus no treatment
- One study compared grinding or grinding + expansion plate with no treatment (Thilander 1984).
Fixed tooth‐borne expansion (Hyrax) + U‐bow activator versus no treatment
- One study compared Hyrax + U‐bow activator with no treatment (Lippold 2013).
Fixed tooth‐borne expansion (bonded acrylic splint) versus no treatment
- One study compared bonded acrylic splint expander with no treatment (Baysal 2016).
Fixed tooth‐borne expansion (Hyrax) with different anchorages
- One study compared Hyrax with two different anchorages (banded versus bonded; Asanza 1997).
- One study compared four‐point expansion appliances with two‐point expansion appliances (Lamparski 2003).
- One study compared Haas anchored on deciduous or permanent molars (Ugolini 2015).
Fixed tooth‐borne expansion with different activation protocol
- One study compared semi‐rapid maxillary expansion with rapid maxillary expansion using acrylic bonded appliances (Ramoglu 2010).
- One study compared two‐band palatal expander with slow expansion versus two‐band palatal expander with rapid expansion (Martina 2012).
Removable tooth‐borne expansion (expansion plate) with different screws
- One study compared expansion plates with two different screws (conventional versus spring‐loaded; Oshagh 2012).
Fixed tooth‐borne expansion (Hyrax) with different screws
- One study compared Hyrax using a conventional screw with Hyrax using NiTi memory) screw (Halicioglu 2014).
- One study compared Hyrax with fan‐type maxillary expansion (Gopalakrishnan 2017).
- One study compared Hyrax using two screws with differential opening expansion (DOE) with Hyrax using fan‐type expansion (FE) (Massaro 2020).
Studies comparing different fixed tooth‐borne appliances
- One study compared bonded Minne appliance with banded Minne appliance (Mossaz‐Joelson 1989).
- One study compared quad‐helix + multi‐bracket with expansion arch + multi‐bracket (McNally 2005).
- One study compared Hyrax with a bonded expander plate (Kilic 2008).
- One study compared self‐ligating brackets (Damon System) with Hyrax (Nam 2020).

See Appendix 1 for a description of appliances.

Characteristics of the outcomes

Eight studies reported the primary outcome of this review, correction of the posterior crossbite (Celenk‐Koca 2018; Erhamza 2018; Godoy 2011; Massaro 2020; Petrén 2008; Sollenius 2020; Thilander 1984; Ugolini 2015). In 22 studies, the correction of crossbite was taken for granted because it was the endpoint of the treatment and the point at which activation of the appliances ceased (Alberta 2010; Araujo 2020; Asanza 1997; Baysal 2016; Cheung 2020; Dindaroglu 2016; Garib 2005; Gopalakrishnan 2017; Halicioglu 2014; Kilic 2008; Lamparski 2003; Lippold 2013; Martina 2012; McNally 2005; Mossaz‐Joelson 1989; Nam 2020; Oliveira 2004; Oshagh 2012; Ramoglu 2010; Sweden 2017; Toklu 2015; Venancio 2014). In one study, we confirmed correction of posterior crossbite by contacting the authors (Ottaviano 2018).

Twenty‐five of the included studies (the exceptions being Baysal 2016; Cheung 2020; Dindaroglu 2016; Ottaviano 2018; Thilander 1984; Venancio 2014), reported expansion of the upper jaw or teeth measured as changes in the width between the molars or canines, or both.

Two studies assessed the stability of crossbite correction (Godoy 2011; Mossaz‐Joelson 1989).

Excluded studies

We excluded 45 studies after reading the full paper. The main reasons for exclusion were as follows. See Characteristics of excluded studies table for further details.

21 had not considered posterior crossbite as an inclusion criteria (Altieri 2020; Altindis 2016; Alves 2020; Baldini 2018; Baratieri 2014; Canan 2017; Cunha 2019; Davami 2020; Jia 2021; Kabalan 2015; Lione 2018; Malkoc 2021; Matos 2020; Melgaco 2014; Nagrik 2020; Nieri 2020; Rinaldi 2018; Ring 2020; Ugolini 2020; Weissheimer 2011; Yagci 2016).
16 were not RCTs (Akim 2021; Alghamdi 2017; Annarumma 2021; Barone 2020; Biondi 2017; Calil 2020; Caprioglio 2020; Digregorio 2019; Erdur 2020; Farret 2015; Fastuca 2017; Kilic 2016; Lee 2021; Michelotti 2019; Mohan 2016; Taner 2018);
2 did not report if there was crossbite correction as they stopped expansion at 8 mm. Moreover, the sample is the same (Jacob 2019; Ribeiro 2020).
1 was randomised after posterior crossbite correction (Ferreira 2016).
1 included the same appliance in both groups (Garcia 2016).
1 Included Class III patients (Garib 2021).
1 included participants with no crossbites in both groups (Gianoni‐Capenakas 2021).
1 had participants admitted to surgery Kayalar 2019).
1 involved a subset of participants from one of the included studies (Petrén 2008), plus other participants and matched control (Petrén 2011).
1 the primary outcome was medicine effects on expanders (Cossellu 2019).

Risk of bias in included studies

Overall risk of bias

In summary, we judged 15 studies to be at high risk of bias in at least one domain and therefore assessed them as being at high risk of bias overall (Asanza 1997; Baysal 2016; Celenk‐Koca 2018; Cheung 2020; Gopalakrishnan 2017; Halicioglu 2014; Lippold 2013; Martina 2012; McNally 2005; Mossaz‐Joelson 1989; Oliveira 2004; Oshagh 2012; Ottaviano 2018; Sweden 2017; Thilander 1984). We assessed eight studies to be at low risk of bias (Araujo 2020; Erhamza 2018; Godoy 2011; Massaro 2020; Nam 2020; Petrén 2008; Sollenius 2020; Ugolini 2015), and eight studies as unclear (Alberta 2010; Dindaroglu 2016; Garib 2005; Kilic 2008; Lamparski 2003; Ramoglu 2010; Toklu 2015; Venancio 2014). See Figure 2.

Risk of bias summary: review authors' judgements about each risk of bias item for each included study

Allocation

Sequence generation

Twenty studies described adequate methods of sequence generation and we judged them to be at low risk of bias for this domain: four studies used a table of random numbers (Alberta 2010; Oshagh 2012; Baysal 2016; Sweden 2017); 14 used block randomisation (Araujo 2020; Celenk‐Koca 2018; Cheung 2020; Erhamza 2018; Lippold 2013; Martina 2012; Massaro 2020; Nam 2020; Oliveira 2004; Sollenius 2020; Toklu 2015; Ugolini 2015; Venancio 2014); one study used a method of drawing lots (Godoy 2011); and two used a similar method involving opaque sealed envelopes (Gopalakrishnan 2017; Petrén 2008).

We judged one study to be at high risk of bias as this study did not provide any information about the sequence generation method (McNally 2005). This study used a random numbers table to allocate the first half of the participants and then to allocate the remaining half to receive the alternative treatment to those in the initial allocation. Therefore, only half the participants were truly randomly allocated, and we judged this study to be at high risk of bias for this domain.

The remaining 10 studies stated that participants were randomly allocated and we were unable to obtain further details from the authors, so we judged these studies to be at unclear risk of bias for this domain (Asanza 1997; Dindaroglu 2016; Garib 2005; Halicioglu 2014; Kilic 2008; Lamparski 2003; Mossaz‐Joelson 1989; Ottaviano 2018; Ramoglu 2010; Thilander 1984).

Allocation concealment

Fourteen studies described adequate methods of allocation concealment and we judged them to be at low risk of bias for this domain (Araujo 2020; Baysal 2016; Celenk‐Koca 2018; Cheung 2020; Erhamza 2018; Godoy 2011; Martina 2012; Massaro 2020; Nam 2020; Oliveira 2004; Petrén 2008; Sollenius 2020; Sweden 2017; Ugolini 2015).

Several methods of allocation concealment were used. One study stated that one study author evaluated the participants and the other study author did the enrolling (Baysal 2016). One study used SPSS software for allocation concealment (Sweden 2017). Five studies stated that opaque and sealed envelopes containing the allocation number method were used for allocation concealment (Celenk‐Koca 2018; Erhamza 2018; Sollenius 2020). Two studies reported that randomisation was done using an Excel worksheet for the allocation concealment (Araujo 2020; Cheung 2020; Massaro 2020; Nam 2020; Ugolini 2015). Two studies stated that the envelope was in the care of one of the study authors, who was contacted and randomly extracted a note and informed the dentist which treatment strategy to use (Oliveira 2004; Petrén 2008). One study stated that the method used above would not allow anybody involved in the study to know their treatment allocation until they had been allocated (Godoy 2011). The remaining study stated that a single operator allocated the participants using a custom‐made Javascript and was responsible for the allocation concealment, that is, the allocation was disclosed only when a new participant was enrolled in the trial (Martina 2012).

The method of sequence generation carried out in one study would allow for the prediction of the allocation of half of the participants, and therefore we judged this study to be at high risk of bias for this domain (McNally 2005). Another study did not state the method for allocation concealment (Halicioglu 2014), so we judged it at high bias risk of bias for this domain.

The remaining 15 studies did not mention any method of allocation concealment, so we judged these studies to be at unclear risk of bias for this domain (Alberta 2010; Asanza 1997; Dindaroglu 2016; Garib 2005; Gopalakrishnan 2017; Kilic 2008; Lamparski 2003; Lippold 2013; Mossaz‐Joelson 1989; Oshagh 2012; Ottaviano 2018; Ramoglu 2010; Thilander 1984; Toklu 2015; Venancio 2014).

Blinding

Blinding of participants and personnel (performance bias)

We did not consider the blinding of participants and personnel (performance bias) as the interventions being compared in all studies were different and it would not have been possible to blind them as to which groups they were allocated. Therefore, a risk of performance bias is possible in all of the included studies.

Blinding of outcome assessment (detection bias)

It would have been possible for an independent blinded person to perform the clinical, model, and radiographic assessments. Therefore, we focused on whether the included studies minimised the possible effects of detection bias.

Sixteen studies described adequate methods for the blinding of outcome assessment and we judged them to be at low risk of bias for this domain (Araujo 2020; Baysal 2016; Celenk‐Koca 2018; Cheung 2020; Dindaroglu 2016; Erhamza 2018; Godoy 2011; Martina 2012; Massaro 2020; Nam 2020; Petrén 2008; Sollenius 2020; Sweden 2017; Toklu 2015; Ugolini 2015; Venancio 2014).

Fifteen studies did not state any information on the blinding of outcome assessment so we judged them to be at unclear risk of bias for this domain (Alberta 2010; Asanza 1997; Garib 2005; Gopalakrishnan 2017; Halicioglu 2014; Kilic 2008; Lamparski 2003; Lippold 2013; McNally 2005; Mossaz‐Joelson 1989; Oliveira 2004; Oshagh 2012; Ottaviano 2018; Ramoglu 2010; Thilander 1984).

Incomplete outcome data

Twenty‐two studies described an adequate method for accounting for attrition bias and we judged them to be at low risk of bias for this domain (Araujo 2020; Asanza 1997; Baysal 2016; Celenk‐Koca 2018; Dindaroglu 2016; Erhamza 2018; Godoy 2011; Gopalakrishnan 2017; Halicioglu 2014; Kilic 2008; Lippold 2013; Massaro 2020; McNally 2005; Nam 2020; Oliveira 2004; Ottaviano 2018; Petrén 2008; Ramoglu 2010; Sollenius 2020; Toklu 2015; Ugolini 2015; Venancio 2014). Ten studies were judged at low risk of attrition bias as they either clearly described dropouts (which were for similar reasons and in similar numbers per group), or the numbers of participants stated in the analyses were the same as those randomised, or an intention‐to‐treat analysis included all randomised participants (Asanza 1997; Oliveira 2004; McNally 2005; Kilic 2008; Petrén 2008; Ramoglu 2010; Godoy 2011; Lippold 2013; Araujo 2020; Sollenius 2020). One of the studies reported that one participant who lost the palatal miniscrews 2 days after insertion of the expander because of consuming hard foods was excluded from the study and we judged this study to be at a low risk of bias for this domain (Toklu 2015). We judged another 11 studies at low risk of bias for this domain and these studies also stated that participant treatment and observation were completed without dropouts from either group (Baysal 2016; Celenk‐Koca 2018; Dindaroglu 2016; Erhamza 2018; Gopalakrishnan 2017; Halicioglu 2014; Massaro 2020; Nam 2020; Ottaviano 2018; Ugolini 2015; Venancio 2014).

Four studies were judjed to be at a high risk of bias for this domain and these studies stated that there was a high degree of variation in the dropouts of treated and untreated groups (Cheung 2020; Martina 2012; Sweden 2017; Thilander 1984). One study clearly described the numbers and reasons for dropout but the high attrition rate (48% per group) meant that, if the missing participants had higher mean scores in one group than the other, as the attrition rate increased, so would over/understatement of the mean difference (Martina 2012). In another study, the attrition rate was very different between the treated group (3%) and the untreated group (18%) (Thilander 1984). Therefore, we judged these two studies to be at high risk of attrition bias. Two of the studies were judged at a high risk of bias for this domain due to the dropouts details reported (Sweden 2017; Cheung 2020).

The remaining five studies did not state any adequate methods of attrition bias and we judged them to be at unclear risk of bias for this domain (Alberta 2010; Garib 2005; Lamparski 2003; Mossaz‐Joelson 1989; Oshagh 2012). Five studies did not report whether or not there were any dropouts and it was unclear how many participants were included in the outcome assessment. Therefore, we judged these studies to be at unclear risk of attrition bias (Mossaz‐Joelson 1989; Lamparski 2003; Garib 2005; Alberta 2010; Oshagh 2012).

Selective reporting

Twenty‐five studies reported all outcome measures described in the corresponding methods sections in full and we assessed them as being at low risk of reporting bias (Alberta 2010; Araujo 2020; Baysal 2016; Cheung 2020; Dindaroglu 2016; Erhamza 2018; Garib 2005; Godoy 2011; Gopalakrishnan 2017; Halicioglu 2014; Kilic 2008; Martina 2012; Massaro 2020; McNally 2005; Mossaz‐Joelson 1989; Nam 2020; Oliveira 2004; Petrén 2008; Ramoglu 2010; Sollenius 2020; Sweden 2017; Thilander 1984; Toklu 2015; Ugolini 2015; Venancio 2014).

We judged four studies to be at high risk of reporting bias. Two studies did not report any measure of variance (Asanza 1997; Lippold 2013); in one study the results were so inadequately reported that we were unable to use any data (Oshagh 2012); and the remaining study stated ithat bone‐borne expansion resulted in uprighting of the maxillary posterior teeth with significant benefit to the buccal alveolar bone support (Celenk‐Koca 2018).

The remaining two studies we judged to be at an unclear risk of bias for this domain. These studies reported results poorly (outcomes not reported or reported without a measure of variance or only graphs and figures without tables) (Lamparski 2003; Ottaviano 2018).

Other potential sources of bias

Twenty‐three studies did not show any potential risk of bias for this domain, so we judged them to be at low risk of bias (Alberta 2010; Araujo 2020; Asanza 1997; Celenk‐Koca 2018; Cheung 2020; Dindaroglu 2016; Erhamza 2018; Garib 2005; Godoy 2011; Kilic 2008; Lamparski 2003; Lippold 2013; Martina 2012; Massaro 2020; McNally 2005; Nam 2020; Petrén 2008; Ramoglu 2010; Sollenius 2020; Thilander 1984; Toklu 2015; Ugolini 2015; Venancio 2014).

Eight studies were at high risk of bias for this domain (Baysal 2016; Gopalakrishnan 2017; Halicioglu 2014; Mossaz‐Joelson 1989; Oliveira 2004; Oshagh 2012; Ottaviano 2018; Sweden 2017). One of these eight studies reported that participants with different malocclusion (and growth pattern) were included (Class I = 8, Class II = 20, Class III = 6; Baysal 2016). Two studies reported including a large range of ages in the sample (Halicioglu 2014; Sweden 2017). One study included only 12 participants (Gopalakrishnan 2017). Three studies did not report the sample size calculation (Mossaz‐Joelson 1989; Oliveira 2004; Ottaviano 2018). In another study there were imbalances between the treatment groups in terms of numbers of participants and gender, possibly indicating a problem with the randomisation (Oshagh 2012).

Effects of interventions

See: Table 1; Table 2; Table 3; Table 4; Table 5; Table 6; Table 7

We divided the studies into seven comparisons for maxillary expansion treatment for posterior crossbite.

Removable tooth‐borne expansion (expansion plate) versus observation (Analysis 1.1; Analysis 1.2)
Fixed tooth‐borne expansion (quad‐helix) versus observation (Analysis 2.1; Analysis 2.2)
Fixed tooth‐borne expansion (Hyrax) versus observation (Analysis 3.2; Analysis 3.1)
Fixed tooth‐borne expansion (quad‐helix) versus removable tooth‐borne expansion (expansion plate; Analysis 4.1; Analysis 4.2; Analysis 4.3);
Fixed tooth‐tissue‐borne expansion (Haas) versus fixed tooth‐borne expansion (Hyrax; Analysis 5.1; Analysis 5.2)
Fixed tooth‐borne expansion (Hyrax) versus tooth‐bone‐borne expansion (Haas; Analysis 6.1; Analysis 6.2)
Fixed tooth‐borne expansion (Hyrax) versus bone‐borne expansion (Analysis 7.1; Analysis 7.2)

1.1 — Comparison 1: Removable tooth‐borne expansion (expansion plate) versus observation, Outcome 1: Outcomes at appliance removal: expansion plate versus observation

1.2 — Comparison 1: Removable tooth‐borne expansion (expansion plate) versus observation, Outcome 2: Outcomes at appliance removal: expansion plate versus observation

2.1 — Comparison 2: Fixed tooth‐borne expansion (quad‐helix) versus observation, Outcome 1: Outcomes at appliance removal: quad‐helix versus observation

2.2 — Comparison 2: Fixed tooth‐borne expansion (quad‐helix) versus observation, Outcome 2: Outcomes at appliance removal: quad‐helix versus observation

3.2 — Comparison 3: Fixed tooth‐borne expansion (Hyrax) versus observation, Outcome 2: Outcomes at appliance removal: Hyrax versus observation

3.1 — Comparison 3: Fixed tooth‐borne expansion (Hyrax) versus observation, Outcome 1: Outcomes at appliance removal: Hyrax versus observation

4.1 — Comparison 4: Fixed tooth‐borne expansion (quad‐helix) versus removable tooth‐borne expansion (expansion plate), Outcome 1: Outcomes at appliance removal: quad‐helix versus expansion plate

4.2 — Comparison 4: Fixed tooth‐borne expansion (quad‐helix) versus removable tooth‐borne expansion (expansion plate), Outcome 2: Outcomes at appliance removal: quad‐helix versus expansion plate

4.3 — Comparison 4: Fixed tooth‐borne expansion (quad‐helix) versus removable tooth‐borne expansion (expansion plate), Outcome 3: Outcomes at appliance removal: quad‐helix versus expansion plate (months)

5.1 — Comparison 5: Fixed tooth‐borne expansion (Hyrax) versus fixed tooth‐tissue‐borne expansion (Haas), Outcome 1: Outcomes at appliance removal: Hyrax versus Haas

5.2 — Comparison 5: Fixed tooth‐borne expansion (Hyrax) versus fixed tooth‐tissue‐borne expansion (Haas), Outcome 2: Outcomes at appliance removal: Haas versus Hyrax

6.1 — Comparison 6: Fixed tooth‐borne expansion (Hyrax) versus fixed tooth‐bone‐borne expansion, Outcome 1: Outcomes at appliance removal: Hyrax versus tooth‐bone‐borne expansion

6.2 — Comparison 6: Fixed tooth‐borne expansion (Hyrax) versus fixed tooth‐bone‐borne expansion, Outcome 2: Outcomes at appliance removal: Hyrax versus tooth‐bone‐borne expansion

7.1 — Comparison 7: Fixed tooth‐borne expansion (Hyrax) versus fixed bone‐borne expansion, Outcome 1: Outcomes at appliance removal: Hyrax versus bone‐borne expansion

7.2 — Comparison 7: Fixed tooth‐borne expansion (Hyrax) versus fixed bone‐borne expansion, Outcome 2: Outcomes at appliance removal: Hyrax versus bone‐borne expansion

We could not include 18 studies in meta‐analyses because they were single studies (Asanza 1997; Baysal 2016; Cheung 2020; Gopalakrishnan 2017; Halicioglu 2014; Kilic 2008; Lamparski 2003; Lippold 2013; Martina 2012; Massaro 2020; McNally 2005; Mossaz‐Joelson 1989; Nam 2020; Oshagh 2012; Ramoglu 2010; Thilander 1984; Ugolini 2015; Venancio 2014). One study was not included in meta‐analyses because it included children and adolescents in one group and only adolescents in the other group; group 1 were aged 10.7 to 13.4 years, group 2 were aged 7.3 to 14.6 years (Oliveira 2004). Also, there was a relevant imbalance between boys and girls in the groups (group1: 5 boys, 6 girls; group 2: 1 boy, 9 girls), which could be responsible for the difference in outcomes. We were able to include 13 studies in at least one meta‐analysis (Alberta 2010; Araujo 2020; Celenk‐Koca 2018; Cheung 2020; Dindaroglu 2016; Erhamza 2018; Garib 2005; Godoy 2011; Ottaviano 2018; Petrén 2008; Sollenius 2020; Sweden 2017; Toklu 2015). See Table 8, Table 9, Table 10 and Table 11.

1. Fixed versus fixed appliances (single study comparisons).

Study	Outcome	Group A	Group B	Results
Asanza 1997	Molar expansion (3 months after completion of expansion phase)	Hyrax banded mean 6 mm (min 4.3 mm‐max 7.2 mm)	Hyrax bonded mean 5.9 mm (min 3.2 mm‐max 7.7 mm)	P value = NS (as reported in the study ‐ no SD reported)
Kilic 2008	Molar expansion (1 week after completion of expansion phase)	Hyrax banded mean 7.67 mm (SD 1.99)	Acrylic bonded mean 7.31 mm (SD 1.45)	MD 0.36 (95% CI −0.72 to 1.44) P value = 0.51
Lamparski 2003	Molar expansion (3 months after completion of expansion phase)	4‐point expansion appliance (Hyrax): not reported by group	2‐point expansion appliance: not reported by group	P value = "no significant differences were found"
Lamparski 2003	Canine expansion (3 months after completion of expansion phase)	4‐point expansion appliance (Hyrax) mean 3.034 mm	2‐point expansion appliance mean 1.7 mm	P value = 0.331 (as reported in the study ‐ no SD reported)
Mossaz‐Joelson 1989	Molar expansion (7 to 15 weeks after start of treatment)	Bonded Minne mean 7.9 mm (SD 1.5)	Banded Minne mean 8.3^a mm (SD 1.1)	P value = NS (as reported in the study ‐ number of participants included in analysis not reported)
	Canine expansion (7‐15 weeks after start of treatment)	Bonded Minne mean 6.4 mm (SD 1.1)	Banded Minne mean 5.3 mm (SD 1.9)	P value = NS (as reported in the study ‐ number of participants included in analysis not reported)
	Stability: relapse of molar expansion (24 weeks after completion of expansion phase)	Bonded Minne mean 2.3 mm (SD 0.8)	Banded Minne mean 2.5 mm (SD 0.6)	P value = NS (as reported in the study ‐ number of participants included in analysis not reported)
	Stability: relapse of canine expansion (24 weeks after completion of expansion phase)	Bonded Minne mean 1.6 mm (SD 0.5)	Banded Minne mean 1.2 mm (SD 0.3)	P value = NS (as reported in the study ‐ number of participants included in analysis not reported)
Ramoglu 2010	Molar expansion	Semi‐RME mean 5.71 mm (SD 1.66)	RME mean 5.11 mm (SD 1.81)	MD 0.60 (95% CI ‐0.55 to 1.75) P value = 0.31
Ramoglu 2010	Canine expansion	Semi‐RME mean 5.13 mm (SD 1.47)	RME mean 4.77 mm (SD 1.53)	MD 0.36 (95% CI ‐0.64 to 1.36) P value = 0.48
Halicioglu 2014	Molar expansion (after retention period of 6 months)	Fixed conventional Hyrax 8.41 + 2.09 mm	Fixed NiTi (memory) Hyrax 8.94 + 1.73 mm	Not reported
Gopalakrishnan 2017	Molar expansion (post‐treatment 3 weeks)	Fixed Hyrax maxillary expander 8.00 ± 2.19 mm	Fixed fan‐type maxillary expander 1.83 ± 0.41 mm	P value < 0.001
Gopalakrishnan 2017	Canine expansion (post‐treatment 3 weeks)	Fixed Hyrax maxillary expander 6.00 ± 0.63 mm	Fixed fan‐type maxillary expander 7.33 ± 2.5 mm	P value = 0.235 (NS)
Ugolini 2015	Molar expansion (after a retention period of 5 months)	Fixed Haas maxillary expander anchored on deciduous molars mean 4.3 mm	Fixed Haas expander anchored on permanent molars mean 5.8 mm	P value < 0.01
Ugolini 2015	Canine expansion (after a retention period of 5 months)	Fixed Haas maxillary expander anchored on deciduous molars mean 4.2 mm	Fixed Haas expander anchored on permanent molars mean 3.3 mm	P value < 0.01
Martina 2012	Crossbite	2‐band palatal expander with slow expansion 12/12	2‐band palatal expander with rapid expansion 14/14	NS
Martina 2012	Molar expansion at molar cusp	2‐band palatal expander with slow expansion mean 6.3 ± 2.1	2‐band palatal expander with rapid expansion mean 5.7 ± 1.6	P value < 0.001
Massaro 2020	Crossbite correction	Expander with differential opening (two 10‐mm screws activated 2 quarter turns in the morning and in the evening for 6 days followed by 4 days activating only the anterior screw) 24/24	Fan‐type expander (for 10 consecutive days the 11‐mm expander screw was activated 2 quarter turns in the morning and in the evening) 18/24
	Molar expansion	Expander with differential opening mean 5.10 ± 1.17	Fan‐type expander mean 2.23 ± 0.75	P value < 0.001
	Canine expansion	Expander with differential opening mean 7.76 ± 1.23	Fan‐type expander mean 8.80 ± 1.33	P value = 0.008
Nam 2020	Crossbite correction	Damon system (fixed appliance with self‐ligating brackets) 41/41	Fixed Hyrax expander attached to the upper first premolars and first permanent molars 41/41	NS
CI: confidence interval; MD: mean difference; min: minimum; max: maximum; NS: not significant; RME: rapid maxillary expansion; RR: risk ratio; SD: standard deviation

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^aThere is a discrepancy between the mean reported in Table 1 (5.3) and the text (8.3). The latter must be the actual mean as the text reports that there is no significant difference, whereas if the mean were 5.3, the P value is < 0.05 (based on assumption of no dropouts).

2. Fixed versus removable (single study comparisons).

Study	Outcome	Group A	Group B	Results
McNally 2005	Molar expansion (12 weeks after start of treatment)	Quad‐helix + multi‐bracket mean 4.54 mm (SD 1.27)	Expansion arch + multi‐bracket mean 5.09 mm (SD 1.67)	MD −0.55 (95% CI −1.34 to 0.24) P value = 0.17
McNally 2005	Canine expansion (12 weeks after start of treatment)	Quad‐helix + multi‐bracket mean 1.4 mm (SD 1.75)	Expansion arch + multi‐bracket mean 2.12 mm (SD 1.11)	MD −0.72 (95% CI −1.52 to 0.08) P value = 0.08
Venancio 2014	Crossbite correction	Fixed Hyrax expander 15/15	Removable expansion plate: 15/15	NS
CI: confidence interval; MD: mean difference; NS: not significant; SD: standard deviation

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3. Removable versus removable (single study comparison).

Study	Outcome	Group A	Group B	Results
Oshagh 2012	Molar and canine expansion	Removable with conventional screw	Removable with spring‐loaded screw	Results inadequately reported

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4. Treated versus untreated (single study comparison).

Study	Outcome	Group A	Group B	Group C	Results
Thilander 1984	Crossbite correction	Grinding or grinding + expansion plate (26/33)	No treatment 6/28		RR 3.68 (95% CI 1.77 to 7.64) P value = 0.0005
Lippold 2013	Molar expansion (12 months after start of treatment)	Fixed bonded Hyrax appliance followed by U‐bow activator mean 5.1 mm	Untreated participants mean 0.8 mm		P value < 0.001 (as reported in the study ‐ no SD for mean change from baseline reported)
Lippold 2013	Canine expansion (12 months after start of treatment)	Fixed bonded Hyrax appliance followed by U‐bow activator mean 3.6 mm	Untreated participants mean 1 mm		P value < 0.001 (as reported in the study ‐ no SD for mean change from baseline reported)
Baysal 2016	Maxillary intermolar width	Maxillary expansion bonded acrylic splint expander mean 6.443 SD 1.023 mm	Untreated participants mean 0.178 SD 0.155 mm		MDs 6.265; P value = 0.000, significance 0.001
Baysal 2016	Maxillary intercanine width	Maxillary expansion bonded acrylic splint expander mean 2.465 SD 0.857 mm	Untreated participants mean 0.176 SD 0.199 mm		MDs 2.288; P value = 0.000, significance 0.001
CI: confidence interval; MD: mean difference; RR: risk ratio; SD: standard deviation

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1. Removable tooth‐borne expansion (expansion plate) versus observation

Three studies at low risk of bias compared expansion plate versus observation (Godoy 2011; Petrén 2008; Sollenius 2020). Data were available comparing outcomes at the expansion appliance removal. The meta‐analysis showed that crossbite correction was significantly more likely who had maxillary expansion with expansion plates compared with the group receiving no treatment (OR 26.24, 95% CI 13.08 to 48.77; I² = 0%; 3 studies, 148 participants). We judged the certainty of the evidence to be high (Analysis 1.1).

For molar expansion, there was a difference in final inter‐molar distance in favour of the expansion plates (MD 3.30 mm, 95% CI 2.88 mm to 3.73 mm; I² = 77%; 3 studies, 145 participants). The certainty of the evidence was moderate. Regarding canine expansion, there was a difference in final inter‐canine distance in favour of the expansion plates (MD 2.59 mm, 95% CI 2.18 mm to 3.01 mm; I² = 40%; 3 studies, 145 participants). The certainty of the evidence was moderate (Analysis 1.2).

2. Fixed tooth‐borne expansion (quad‐helix) versus observation

Three studies at low risk of bias compared quad‐helix versus observation (Godoy 2011; Petrén 2008; Sollenius 2020). The meta‐analysis showed that people in the group who had maxillary expansion with quad‐helix were more much likely to have their crossbite corrected than people in the observation group (OR 50.59, 95% CI 26.77 to 95.60; I² = 0%; 3 studies, 149 participants). The certainty of the evidence was high (Analysis 2.1).

For molar expansion, there was a difference in final inter‐molar distance in favour of the quad‐helix (MD 4.71 mm, 95% CI 4.31 mm to 5.10 mm; I² = 68%; 3 studies, 146 participants). The certainty of the evidence was moderate. Regarding canine expansion, there was a difference in final inter‐canine distance in favour of the quad‐helix (MD 3.15 mm, 95% CI 2.77 mm to 3.53 mm; I² = 94%; 3 studies, 146 participants). The certainty of the evidence was low (Analysis 2.2).

3. Fixed tooth‐borne expansion (Hyrax) versus observation

Three studies (one at low risk of bias, one at unclear risk, and one at high risk of bias) compared Hyrax versus observation (Alberta 2010; Erhamza 2018; Ottaviano 2018). The meta‐analysis showed that there was a difference in crossbite reduction in favour of Hyrax (OR 48.02, 95% CI 21.58 to 106.87; I² = 0%; 3 studies, 93 participants). It indicates that the likelihood of crossbite correction was substantially higher in the group who had maxillary expansion with Hyrax compared with no treatment. The certainty of the evidence was moderate (Analysis 3.1).

Two studies (one at low risk of bias and one at unclear risk of bias) compared Hyrax versus observation regarding inter‐molar distance (Alberta 2010; Erhamza 2018). Both of these studies were in children. The meta‐analysis showed that there was a difference in inter‐molar distance in favour of Hyrax (MD 5.80 mm, 95% CI 5.15 mm to 6.45 mm; I² = 0%; 2 studies, 71 participants). The certainty of the evidence was moderate (Analysis 3.2).

4. Fixed tooth‐borne expansion (quad‐helix) versus removable tooth‐borne expansion (expansion plate)

Three studies at low risk of bias compared quad‐helix versus expansion plate (Godoy 2011; Petrén 2008; Sollenius 2020). The meta‐analysis showed that there was a difference in crossbite correction in favour of quad‐helix (RR 1.29, 95% CI 1.13 to 1.46; I² = 76%; 3 studies, 151 participants). It indicates that quad‐helix appliances are more likely to correct crossbites than removable expansion plates. The certainty of the evidence was moderate (Analysis 4.1).

For molar expansion, there was a difference in final inter‐molar distance in favour of quad‐helix (MD 1.48 mm, 95% CI 0.91 mm to 2.04 mm; I² = 0%; 3 studies, 151 participants). The certainty of the evidence was high. Regarding canine expansion, there was evidence of a difference in final inter‐canine distance in favour of quad‐helix (MD 0.59 mm, 95% CI 0.09 mm to 1.08 mm; I² = 85%; 3 studies, 151 participants). The certainty of the evidence was low (Analysis 4.2).

There was a difference in length of treatment in favour of quad‐helix (MD −3.15 months, 95% CI −4.04 months to −2.25 months; I² = 69%; 3 studies, 148 participants). The certainty of the evidence was moderate (Analysis 4.3).

5. Fixed tooth‐tissue‐borne expansion (Haas) versus fixed tooth‐borne expansion (Hyrax)

Three studies (one at low risk of bias and two at unclear risk of bias) compared Hyrax versus Haas for posterior crossbite correction (Araujo 2020; Dindaroglu 2016; Garib 2005). The meta‐analysis showed that there was no evidence of a difference in crossbite correction comparing Hyrax with Haas (RR 1.05, 95% CI 0.94 to 1.18; I² = 0%; 3 studies, 83 participants). The certainty of the evidence was moderate (Analysis 5.1).

For molar expansion, there was no evidence of a difference in final inter‐molar distance comparing Hyrax with Haas (MD −0.15 mm, 95% CI −0.86 mm to 0.56 mm; I² = 0%; 3 studies, 46 participants). The certainty of the evidence was moderate (Analysis 5.2).

6. Fixed tooth‐borne expansion (Hyrax)versus fixed tooth‐bone‐borne expansion

Three studies (two at high risk of bias and one at unclear risk) compared Hyrax with tooth‐bone‐borne expansion (Cheung 2020; Sweden 2017; Toklu 2015). The meta‐analysis showed that there was no evidence of a difference between them for crossbite correction (RR 1.02, 95% CI 0.92 to 1.12; I² = 0%; 3 studies, 120 participants). The certainty of the evidence was low (Analysis 6.1).

Two studies (one at high risk of bias and one at unclear risk) compared inter‐molar distance. There was no evidence of a difference between the interventions in final inter‐molar distance (MD −0.66 mm, 95% CI −1.36 mm to 0.04 mm; I² = 0%; 2 studies, 65 participants). The certainty of the evidence was low (Analysis 6.2).

7. Fixed tooth‐borne expansion (Hyrax) versus bone‐borne expansion

Two studies (one at high risk of bias and one at unclear risk) compared Hyrax with bone‐borne expansion for posterior crossbite correction (Alberta 2010; Celenk‐Koca 2018). The meta‐analysis showed that there was no evidence of a difference between them (RR 1.00, 95% CI 0.94 to 1.07; I² = 0%; 2 studies, 81 participants). The certainty of the evidence was low (Analysis 7.1).

Two studies (one at high risk of bias and one at unclear risk) compared Hyrax with bone‐borne expansion for inter‐molar distance (Alberta 2010; Celenk‐Koca 2018). The meta‐analysis showed that there was no evidence of a difference between the interventions in crossbite correction (MD −0.14 mm, 95% CI −0.85 mm to 0.57 mm; I² = 0%; 2 studies, 81 participants). The certainty of the evidence was low (Analysis 7.2).

Other comparisons from studies included in the meta‐analysis

Petrén 2008 also compared quad‐helix and expansion plate with the application of composite onlays on the mandibular first molars. In the composite onlay group, a few crossbites (2 of 15) were corrected, showing no significant differences compared with the untreated group. This study was at a low risk of bias.

Alberta 2010 assessed pain perception in one study comparing tooth‐borne appliances (Haas) and tooth‐bone‐borne appliances and found no statistical difference associated with the type of appliance (P = 0.547). This study also compared bone‐borne appliances with no treatment for crossbite correction and found that bone‐borne expansion corrected all posterior crossbites (21/21) and there was no self‐correction in the untreated group. They also assessed the condylar position, comparing the changes between Hyrax and control and found no statistical difference between changes in the condyles (P < 0.05). This study was at unclear risk of bias.

Godoy 2011 also assessed cost‐benefit, success rate, number of complications, and stability. The average number of appliances used was 1.82 higher in the expansion plate group than in the quad‐helix group. Comparing the total treatment costs, the number of clinical visits, the length of treatment, and the number of appliances needed, the expansion plate group cost 10.53% more than the quad‐helix group. Additionally, quad‐helix had 100% success and expansion plate had 90.9% (P = 0.238; no statistical significance). No appliances were lost in the quad‐helix group, but 24.2% were lost in the expansion plate group, and failure occurred in one‐third of the quad‐helix participants during the treatment, but there were no failures in the expansion plate group. Breakage of appliances occurred in 18.2% of the quad‐helix participants and none of the expansion plate participants. The number of missed appointments was 12.1% higher in the quad‐helix group than in the expansion plate group. Relapses occurred in 9.1% of the two experimental groups after one year of follow‐up. This study was at a low risk of bias.

Sweden 2017 also evaluated and compared perceived pain intensity, discomfort, and jaw function impairment during the first week with tooth‐borne and tooth‐bone‐borne rapid maxillary expansion appliances. There was a significant positive correlation between age and pain and discomfort on the fourth day in treatment. No correlations were found between sex and pain and discomfort, analgesic consumption, and jaw impairment. The study also evaluated and compared the effects on nasal airflow and resistance between appliances. The results showed significantly higher post‐expansion nasal airflow values for the tooth‐bone‐borne group compared with the tooth‐borne group (mean difference 51.0 cm³/s; P = 0.018). They also evaluated and compared the skeletal and dentoalveolar effects with one‐year follow‐up between appliances. The magnitude of the expansion in the midpalatal suture was not clinically significant (MD 1 mm, 95% CI 0.5 to 1.7 mm; P = 0.001). The magnitude of the expansion at the level of the nasal cavity was higher in the tooth‐bone‐borne group (MD 1.7 mm, 95 % CI 0.7 to 2.6 mm; P = 0.001). The dental expansion, alveolar bending, tipping of the molars, and stability one year post‐expansion did not show any statistically significant differences between the groups. The stability of the treatments one year post‐expansion was the same between the groups. The direct cost of the treatment for the tooth‐bone‐borne group was approximately EUR 130 higher than the tooth‐borne group (not including extra chair time costs). This study was at high risk of bias.

Cheung 2020 also assessed the upper airway volume effects produced by Hyrax, Hybrid‐Hyrax, and Keles Keyless Expanders. The maxillary expansion resulted in considerable increases in total airway volume in the Hybrid‐Hyrax group (+5902.1 mm³) and less in the Hyrax group (+2537.9 mm³) or the Keles group (+3001.4 mm³). However, treatment‐induced changes for the primary and all secondary outcomes were of small magnitude and no significant difference was seen among the three expanders in the total airway volume in either crude or adjusted analyses (P > 0.05 in all instances). Finally, among pre‐peak participants (cervical vertebral maturation (CVM) 1–3), the Hybrid‐Hyrax expander was associated with significantly greater increases in total airway volume compared to the Hyrax expander (P = 0.02). This study was at high risk of bias.

Sollenius 2020 also evaluated societal costs, material costs, and treatment time costs. For successful treatments, the societal costs were EUR 765 for the specialist‐treated quad‐helix participants, EUR 1025 for the general dentist‐treated quad‐helix participants, EUR 844 for the specialist‐treated expansion‐plate participants, and EUR 878 for the general dentist‐treated expansion‐plate participants. The cost per participant was significantly lower for the specialist‐treated quad‐helix participants compared with the general dentist‐treated quad‐helix participants (P = 0.000). Moreover, costs for expansion plates in specialist orthodontic clinics were significantly lower compared with the general dentist‐treated quad‐helix participants (P = 0.015) (Table 2). The mean material costs were EUR 120 for the specialist‐treated quad‐helix participants and general dentistry‐treated quad‐helix participants, EUR 185 for the specialist‐treated expansion‐plate participants, and EUR 229 for the general dentistry‐treated expansion‐plate participants. The mean material cost per participant was significantly lower for quad‐helix compared to expansion plate treatments in specialist as well as in general dentistry (P = 0.000). In addition, the material costs of expansion plate treatments in specialist orthodontic clinics were lower than plate treatments in general dentistry (P = 0.013). The mean treatment time costs were EUR 601 for the specialist‐treated quad‐helix participants, EUR 788 for the general dentistry‐treated quad‐helix participants, EUR 575 for the specialist‐treated expansion‐plate participants, and EUR 569 for the general dentistry‐treated expansion‐plate participants. Thus, the general dentist‐treated quad‐helix participants' costs were significantly higher compared to both quad‐helix and expansion plate treatments in specialist orthodontic clinics (P = 0.000). Furthermore, in general dentistry, the quad‐helix treatment time costs were significantly higher than plate treatments (P = 0.000). The indirect costs were EUR 44 for the specialist‐treated quad‐helix participants, EUR 117 for the general dentistry‐treated quad‐helix participants, EUR 83 for the specialist‐treated expansion‐plate participants, and EUR 80 for the general dentistry‐treated expansion‐plate participants. Overall, the indirect costs per participant were 5 to 10 per cent of the societal costs. The indirect cost per participant was significantly lower for the specialist‐treated quad‐helix participants compared with general dentistry‐treated quad‐helix participants (P = 0.008).

See Appendix 1 for a description of appliances.

Comparisons assessed in only one study

Asanza 1997 compared Hyrax expander with two different anchorages (banded versus bonded). This study was at high risk of bias. No significant difference between the amount of dental tipping or symmetrical expansion between the two appliances was reported.

Baysal 2016 compared bonded acrylic splint expander with no treatment. This study was at high risk of bias. There were statistically significant differences in intermolar and inter‐canines distances in favour of bonded acrylic splint compared to the control group.

Cheung 2020 compared self‐ligating brackets (Damon System) with Hyrax. This study was at high risk of bias. There were considerable increases in total airway volume in the Hybrid‐Hyrax group and less in the Hyrax group or the Keles group. However, treatment changes for the primary and secondary outcomes were of small magnitude. Consequently, no significant difference was seen among the three expanders in the total airway volume. Among pre‐peak participants (CVM 1–3), the Hybrid‐Hyrax expander was associated with significantly greater increases in total airway volume compared with the Hyrax expander.

Gopalakrishnan 2017 compared Hyrax expander with fan‐type maxillary expansion. This study was at high risk of bias. Fan‐type rapid maxillary expansion appliances caused only minimal expansion of the intermolar width when compared to the Hyrax. The increase in the inter‐canine width was almost similar in both groups The ratio between the inter‐canine and intermolar width expansion was nearly 4:1 in the fan‐type rapid maxillary expansion appliances and 0.75:1 in Hyrax.

Halicioglu 2014 compared Hyrax expander using a conventional screw with Hyrax using NiTi memory screw. This study was at high risk of bias. There were no statistically significant differences for any outcome.

Kilic 2008 compared Hyrax expander with a bonded expander plate. This study was at an unclear risk of bias. Both rapid maxillary expansion appliances produced significant dentoalveolar tipping during rapid maxillary expansion, but this was greater in the Hyrax group.

Lamparski 2003 compared a four‐point expansion appliance with a two‐point expansion appliance. This study was at high risk of bias. There were no statistically significant differences for either outcome.

Lippold 2013 compared Hyrax expander + U‐bow activator with no treatment. This study was at high risk of bias. Statistically significant differences were reported for both outcomes in favour of treatment.

Martina 2012 compared two‐band palatal expander with slow expansion versus two‐band palatal expander with rapid expansion. This study was at high risk of bias. There was no statistically significant difference.

Massaro 2020 compared fixed tooth‐borne expansion using two screws with differential opening expansion versus fixed tooth‐borne expansion using fan‐type expansion. This study was at a low risk of bias. There was a statistical difference in favour of differential opening expansion for molar expansion and favour of canine expansion for fan‐type expansion.

McNally 2005 compared quad‐helix + multi‐bracket with expansion arch + multi‐bracket. This study was at high risk of bias. There were no statistically significant differences for either outcome.

Mossaz‐Joelson 1989 compared bonded Minne appliance with banded Minne appliance. This study was at high risk of bias. There were no statistically significant differences for any outcome.

Nam 2020 compared fixed tooth‐borne expansion with self‐ligating brackets (Damon System) with fixed tooth‐borne expansion (Hyrax). This study was at a low risk of bias. There was no difference for crossbite correction, but there was a greater transverse expansion of the first molar and first premolars with buccal tipping in the Hyrax group.

Oshagh 2012 compared the expansion plate with conventional expansion screw versus expansion plate with spring‐loaded expansion screw for discomfort. This study was at high risk of bias. There were no significant differences in terms of discomfort and arch size changes for both appliances.

Ramoglu 2010 compared semi‐rapid maxillary expansion with rapid maxillary expansion. This study was at unclear risk of bias. There were no statistically significant differences for either outcome.

Thilander 1984 compared grinding or grinding + expansion plate with no treatment. This study was at high risk of bias. Tooth grinding was not superior to no treatment for the outcome crossbite correction, but the combination of grinding + expansion plate was superior to no treatment.

Ugolini 2015 compared Haas expander anchored on deciduous second molars with Haas anchored on permanent first molars. This study was at low risk of bias. The deciduous group showed reduced molar angulation increases at T1 (after five months), and reduced molar angulation decreases at T2, when compared with the permanent group. At T2, the net increase of the upper inter‐canine distance in the deciduous group was still significant compared with the permanent group, indicating a more stable expansion in the anterior area.

Venancio 2014 compared Hyrax expanders with expansion plates. This study was at unclear risk of bias. There were no statistically significant differences between appliances.

Other details of these comparisons can be assessed in Table 8, Table 9, Table 10, and Table 11.

Discussion

Summary of main results

Expansion therapy with fixed and removable appliances versus no treatment

From three studies that compared expansion treatment using expansion plates or fixed appliances against no treatment, we found evidence that maxillary expansion treatment with expansion plate or quad‐helix in children (7 to 11 years old) results in better posterior crossbite correction compared to children not receiving treatment. This effect occurred if the child received treatment with removable (expansion plate, 3 RCTs) or fixed appliances (quad‐helix, 3 RCTs). The evidence certainty was high. This effect probably also occurs with the Hyrax appliance, with moderate‐certainty evidence.

The results also showed a significant increase in inter‐molar and inter‐canine distances in favour of the treated children. The inter‐molar distance was higher in the treated group than in those who did not receive the treatment (3.3 mm higher for removable expansion plate and 4.71 mm higher for quad‐helix). The certainty of the evidence was moderate. There was also a significant increase in inter‐molar distance for the Hyrax appliance (5.80 mm higher); the certainty of the evidence was moderate. The inter‐canine distance increase was higher in the treated group than in the children who did not receive the treatment (2.59 mm higher for removable expansion plate; 3.15 mm higher for quad‐helix). The certainty of the evidence was moderate for the expansion plate and low for the quad‐helix.

Expansion therapy with fixed appliances versus removable appliances

Three studies compared expansion treatment in children with quad‐helix versus removable expansion plates. Quad‐helix is more effective in crossbite reduction than using a removable expansion plate. The certainty of the evidence is moderate. Molar expansion is higher for quad‐helix appliances than expansion plates (1.48 mm higher). The certainty of the evidence is high. Although an increase was found in the inter‐canine distance in favour of quad‐helix (0.59 mm higher), the changes were so small that they may not be of clinical relevance. Additionally, the certainty of the evidence was low for this comparison.

Expansion therapy with fixed appliances versus fixed appliances

We found eight studies that compared expansion therapy using four different fixed appliances (three for Hyrax and Haas, and five for tooth‐bone‐borne expander and bone‐borne expander). Although all appliances may correct posterior crossbite, we found no evidence of a difference in favour of one appliance over another (moderate‐certainty evidence). Furthermore, there was no evidence of differences in the comparison between the different types of fixed appliances considering the inter‐molar distance. In particular, there was no evidence of a difference in crossbite correction and inter‐molar distance when comparing Hyrax with Haas appliances (3 RCTs; moderate‐certainty evidence), Hyrax with tooth‐bone‐borne expansion (2 RCTs; low‐certainty‐evidence), and Hyrax with bone‐borne expander (2 RTCs; low‐certainty evidence).

Other comparisons and outcomes

We could not include some relevant comparisons in the meta‐analysis because they were single comparisons or the outcome measures were different.

One study at high risk of bias compared grinding or grinding + expansion plate with no treatment and reported that tooth grinding was better than no treatment for the outcome crossbite correction, but the combination of grinding + expansion plate was better.

Two studies at low risk of bias compared quad‐helix with expansion plate for cost‐benefit and were not comparable because of the different outcome measures. One of them considered total treatment costs, the number of visits, length of treatment, and the number of appliances and reported that the expansion‐plate group cost 10.53% more than the quad‐helix group. The other considered societal costs, material costs, treatment‐time costs, and indirect costs and reported these costs separately. We calculated all these costs, and we found that the expansion‐plate group cost 9.3% more than quad‐helix.

One study at high risk of bias compared tooth‐borne with tooth‐bone‐borne rapid maxillary expansion appliances also for cost‐benefit. The direct cost of the treatment for the tooth‐bone‐borne group was approximately EUR 130 higher than the tooth‐borne group (not including extra chair time costs).

Two studies at high risk of bias compared tooth‐borne Hyrax with tooth‐bone‐borne Hyrax for effects on nasal airflow. We could not pool the results because of the different outcome measures. One study found significantly higher post‐expansion nasal airflow values for the tooth‐bone‐borne group compared with the tooth‐borne group (MD 51.0 cm³/s, P = 0.018). Although the other study had shown considerable increases in total airway volume in the tooth‐bone‐borne group (+5902.1 mm³) and less in the tooth‐borne group (+2537.9 mm³), the treatment‐induced changes for the primary and all secondary outcomes were of small magnitude and no significant difference was seen among the expanders in the total airway volume in either crude or adjusted analyses (P > 0.05 in all instances). Significantly, increases in total airway volume could be seen in the tooth‐bone‐borne group compared to the tooth‐borne group (P = 0.02) among pre‐peak participants (CVM 1–3).

One study at low risk of bias compared quad‐helix with expansion plate for success rate and found no statistical significance (quad‐helix had 100% of success and expansion plate had 90.9% (P = 0.238).

One study at low risk of bias compared quad‐helix with expansion plate for stability and found that relapses occurred in 9.1% of the two experimental groups after one year of follow‐up. Another study at high risk of bias compared tooth‐bone‐borne with tooth‐borne maxillary expansion for stability and also found no statistically significant differences between the groups one year post‐expansion.

One study at low risk of bias compared quad‐helix with expansion plate for the number of complications and found that quad‐helix had fewer losses (none in quad‐helix against 24.2% in expansion plate); however, quad‐helix had more failure (one‐third in quad‐helix and no failures in the expansion plate group), breakage of appliances (18.2% in quad‐helix and none in expansion plate), and more missed appointments (12.1% higher in the quad‐helix group).

Other interventions

We found other appliances and approaches for the treatment of crossbite. As they were single studies, we did not include them in the meta‐analysis.

One study at high risk of bias compared Hyrax + U‐bow activator with no treatment and reported that the first was better.

Another study at high risk of bias compared bonded acrylic splint expander with no treatment and reported more expansion (for inter‐molar and inter‐canine distance) in the bonded acrylic splint group.

Three studies compared fixed rapid appliances using different anchorages and two of them (one at high risk of bias and another unclear) reported that there were no differences between Hyrax banded or bonded and four‐point or two‐point anchorage appliance, while another study at low risk of bias reported a difference in favour of expansion anchored on deciduous molars compared to anchored on permanent molars.

Three studies compared fixed rapid appliances with fixed slow appliances for activation protocols. All three used different appliances and protocols. One study at high risk of bias considered rapid expansion with the activation protocol of one turn of the screw on the first day (four quarters/1 mm) then one half‐turn (two quarters/0.5 mm) per day until 8 mm of expansion and slow expansion the activation of two turns on the first day (eight quarters/2 mm) then one half‐turn twice a week. Another study at unclear risk of bias considered as rapid expansion the activation of two quarter turns per day throughout treatment and semi‐rapid expansion the activation of two quarter turns per day for the first week followed by one quarter turn per day every other day. The other study at high risk of bias considered as slow expansion the activation of two quarter turns twice a week and rapid expansion the initial activation of eight quarters on the first day followed by three quarters per day (0.75 mm activation per day). All three protocols were successful in correcting crossbites; however, with different lengths of treatment, where rapid expansion protocols were shorter than semi‐rapid and slow expansion protocols.

Four studies compared expansion appliances using different screws. One study at low risk of bias compared Hyrax using two screws with differential opening expansion with Hyrax using fan‐type expansion, finding a difference in favour of differential opening expansion for molar expansion and favour canine expansion for fan‐type expansion. One study at high risk of bias compared expansion plates using two different screws (conventional screw versus spring‐loaded screw) for discomfort and arch size changes finding no differences for both appliances. Another study at high risk of bias compared Hyrax using a conventional screw with Hyrax using NiTi memory screw, and there were no differences for any outcome. The other study at a high risk of bias compared Hyrax with fan‐type maxillary expansion and reported that fan‐type caused minimal expansion of the inter‐molar width compared to Hyrax.

Three studies compared different expansion appliances. One study at low risk of bias compared self‐ligating brackets (Damon system) with Hyrax and reported no difference for crossbite correction, but there was a greater transverse expansion of the first molar and first premolars with buccal tipping in the Hyrax group. One study at high risk of bias compared bonded Minne appliance with banded Minne appliance and reported that there were no statistically significant differences between them for any outcome. Another study at unclear risk of bias compared Hyrax with a bonded expander plate, and both appliances produced greater dentoalveolar tipping in the Hyrax group. One study at high risk of bias compared quad‐helix + multi‐bracket with expansion arch + multi‐bracket and reported no differences for either outcome.

Overall completeness and applicability of evidence

One relevant finding of this review was that while we identified 31 RCTs, we could pool only 11 in the meta‐analyses. The other 20 studies were not included because they were single studies that were not comparable (different outcomes, appliances, anchorages activation protocols or expansion screws) and one was not included because there were serious flaws in the study.

Another relevant finding was that only eight out of 31 studies were at low risk of bias, while around half of the included studies (15) were at high risk of bias, and eight were unclear. The most common sources of bias were attrition bias, detection bias, and selection bias. From the 11 studies included in the meta‐analysis, five were at low risk of bias, four were at unclear risk, and four were at high risk. This variation can be explained mainly by the lack of standardisation in the study designs and outcome measures. This highlights the urgent necessity to use Core Outcome Measures for orthodontics (Tsichlaki 2020) and specifically for posterior crossbite to improve the quality of studies and, consequently, the quality of systematic reviews on posterior crossbite.

The available studies were not sufficient to address some of our objectives. For example, we could not find evidence for the stability of crossbite correction, signs and symptoms of temporomandibular joint dysfunction, signs and symptoms of respiratory disease (e.g. mouth breathing, nasal airway resistance), and quality of life. We were also not able to find evidence for the orthodontic treatment of posterior crossbites for adults. This lack of evidence is probably because most of the studies on adults with posterior crossbite compared orthodontic treatments with orthodontic‐surgical interventions. We excluded surgical treatments from this review to avoid confounding bias.

Quality of the evidence

We assessed the certainty of evidence included in this review using the GRADE approach (see Table 1; Table 2; Table 3; Table 4; Table 5; Table 6; Table 7). Five studies were at low risk of bias (Erhamza 2018; Godoy 2011; Nam 2020; Petrén 2008; Sollenius 2020). We judged the certainty of evidence in this review to range from high to low. We judged comparisons of orthodontic interventions against observation as high‐ or moderate‐certainty evidence other than for one low‐certainty comparison and outcome (quad helix versus observation, inter‐canine distance). We judged the head‐to‐head comparisons as moderate‐ or low‐certainty other than one high‐certainty comparison and outcome (quad helix versus expansion plate, inter‐molar distance).

Potential biases in the review process

The authors' independent assessments of study eligibility and subsequent data extraction and risk of bias assessment minimised the potential for additional bias beyond that detailed in the risk of bias tables in the Characteristics of included studies section. The incompleteness of some of the reports and our inability to obtain clarification of certain trial details or to resolve ambiguities in the reports may have contributed to some bias in their assessment. However, where these conditions applied, this was explicitly stated in the text of our review. We attempted to minimise bias by changing our inclusion criteria to only include RCTs which, when performed to a high standard, should be less biased than other study designs. We tried to limit bias in the review process by ensuring a comprehensive search for potentially eligible studies. We received peer review feedback suggesting the search strategy did not include various appliances, for example, miniscrew‐assisted rapid palatal expansion (MARPE) and prefabricated myofunctional appliances. The Cochrane Oral Health Information Specialist ran a search incorporating these terms and confirmed that no studies were identified that had not appeared in the records retrieved by previous searches. We will, however, ensure these terms are included in the search strategy for the next update. Additionally in the next update, we will include generic adverse effects as an outcome.

Agreements and disagreements with other studies or reviews

A systematic review of maxillary expansion arches for treating crossbites included studies published from 1999 to 2011 (Zuccati 2013). The review authors assessed studies using the Consolidated Standards of Reporting Trials (CONSORT) statement (Schulz 2010). Similarly to our review, they argued that there was substantial evidence of bias, which reduced the quality of the evidence: the randomisation process was poorly described, and loss to follow‐up was unclear in many of the included studies. Their review also called for future studies to be carefully planned and reported using universal clinical practice guidelines.

Another systematic review on early orthodontic treatments of unilateral posterior crossbite included studies published from 2002 to March 2020 (Caroccia 2021), though they included both prospective and retrospective studies so it is not directly comparable with the present review. In common with our review, however, Caroccia 2021 highlighted the heterogeneity of treatments, range of different measurement strategies used, lack of a common follow‐up length, and absence of cost‐effectiveness analysis.

Authors' conclusions

Implications for practice.

For children in the early mixed dentition stage (aged 7 to 11 years old), expansion plate is more beneficial than no treatment at correcting posterior crossbites, expanding the inter‐molar, and inter‐canine distances. Quad‐helix is more beneficial than no treatment for correcting posterior crossbites and probably more successful in increasing the inter‐molar distance. Quad‐helix is probably more beneficial than expansion plate for correcting posterior crossbite and increasing the inter‐molar distance. Treatment with quad‐helix is probably shorter than with expansion plate.

For children and adolescents (aged 7 to 16 years old), there may be no difference between treatment with Haas or Hyrax for crossbite correction and inter‐molar width.

The remaining evidence we found was of low certainty and insufficient to draw any robust conclusions.

Implications for research.

More randomised controlled trials are required to address the question of what is the best treatment for posterior crossbites in children, adolescents, and adults. Such studies must be well‐designed, well‐conducted, and adequately delivered with subsequent reporting, including high‐quality descriptions of all aspects of the methodology. Reporting should conform to the Consolidated Standards of Reporting Trials (CONSORT) statement (Schulz 2010), which will enable appraisal and interpretation of results, and accurate judgements to be made about the risk of bias and the overall quality of the evidence. Although it is uncertain whether reported quality accurately reflects actual study conduct, it is noteworthy that studies with unclear methodology produce biased estimates of treatment effects (Schulz 1995).

Studies should be large enough to detect a difference, if one exists, and should assess appropriate outcomes. 'Correction of crossbite' may be the primary outcome for all studies addressing this research question. Suggested outcomes for future studies are 'treatment follow‐up', 'patient's perception', 'pain', 'stability of crossbite correction', 'cost‐benefit analysis', 'signs and symptoms of temporomandibular joint dysfunction', 'respiratory disease', 'respiratory effects', 'quality of life', 'activation protocol', 'harms' and 'sleep apnoea'. A new protocol for the treatment of posterior crossbite is required for this Cochrane systematic review. This can be based on the Core Outcome Measures published for orthodontics (Tsichlaki 2020). Additionally, it would be relevant to develop specific Core Outcome Measures for posterior crossbite. This may be done in association with the COMET (Core Outcome Measures in Effectiveness Trials) Initiative (www.comet-initiative.org), using robust methodology suggested by those working with the COMET Initiative so far (Williamson 2012).

What's new

Date	Event	Description
25 April 2021	New citation required and conclusions have changed	The certainty of the evidence to support treating crossbites with expansion plate and quad‐helix is now 'moderate'. It was previously 'low'.
25 April 2021	New search has been performed	Searches updated to 8 April 2021. We have added 16 new randomised controlled trials in this update, making 31 included studies in total. We included two new secondary outcomes: length of treatment and costs with treatment.

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History

Protocol first published: Issue 1, 1998 Review first published: Issue 4, 1998

Date	Event	Description
4 April 2014	New search has been performed	Searches updated to 21 January 2014.
4 April 2014	New citation required and conclusions have changed	Inclusion criteria changed to only include randomised controlled trials (RCTs). Therefore we have discarded six of the controlled clinical trials included in the previous version. To the remaining three RCTs we have added 12 new included RCTs in this update. All sections updated to reflect up‐to‐date Cochrane review methodology and methodological expectations of Cochrane intervention reviews (MECIR). Risk of bias assessment of included studies carried out and summary of findings tables added.
23 June 2008	Amended	Converted to new review format.
16 November 2000	New citation required but conclusions have not changed	Substantive amendment. A further CCT has been found on two‐point versus four‐point expansion (Schneiderman 1990) and follow‐up data on the treatment group from Lindner 1989 (Tsarapatsani 1999). Conclusions remain largely unchanged.

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Acknowledgements

We thank Laura MacDonald for all her support during the review process and Anne Littlewood for her help with the search of studies. We also thank Helen Worthington, Fang Hua, Trevor Johnson, Fraser McDonald and Jennifer Hilgart for their comments on earlier drafts of the review and Denise Mitchell for copy editing. Our thanks to Manuel Lagravere Vich (University of Alberta, Canada), Türkan Sezen Erhamza (Kirikkale University, Turkey) for providing additional data. We would like to thank Janet Lear (Cochrane Oral Health) for her help to find some papers and Fang Hua for his translation support from Chinese and Adrien Boillot for his support with translation from French.

Appendices

Appendix 1. Description of appliances that can be used to treat posterior crossbite

Fixed appliances (with mid‐palatal screw)

Hyrax expander

(Alberta 2010; Asanza 1997; Celenk‐Koca 2018; Dindaroglu 2016; Erhamza 2018; Garib 2005; Gopalakrishnan 2017; Halicioglu 2014; Kilic 2008; Lamparski 2003; Martina 2012; Oliveira 2004; Ottaviano 2018; Sweden 2017; Toklu 2015; Venancio 2014)

A fixed metal expander soldered to bands on the first molars (2 points) or first molars and first bicuspids (4 points) with an 0.036‐mm lingual wire connecting the bands.

Haas expander

(Dindaroglu 2016; Garib 2005; Oliveira 2004; Ugolini 2015)

A fixed maxillary expander that uses acrylic pads and heavy lingual wires to apply pressure to both the teeth and the palatal tissue during expansion. The lingual wires are soldered to bands on the first bicuspids and the first molars and extend into the palate where they are embedded in the acrylic pads.

Bonded expander (acrylic splint)

(Baysal 2016; Kilic 2008; Lippold 2013; Ramoglu 2010)

An alternative to the banded design, this fixed expander uses posterior or full acrylic coverage that is bonded directly to the teeth. The posterior bite blocks free up the occlusion by removing cuspal interferences.

Tooth‐bone‐borne expander

(Celenk‐Koca 2018; Sweden 2017; Toklu 2015)

The hybrid tooth‐bone‐borne expander design anchors the rapid maxillary expansion appliance both to the posterior teeth and also, by means of two or four mini‐implants, directly to the palatal surfaces of the maxilla.

Bone‐borne expander

(Alberta 2010)

The bone‐anchored expander is composed of two custom‐milled stainless steel onplants, two miniscrews and an expansion screw. The expansion screw placed in palatal mid‐line is anchored directly to the palatal surfaces of the maxilla (top jaw) with either bioglass‐coated aluminium oxide implants or osteosynthesis plates. Or alternatively, temporary anchorage device (miniscrew) on one side and a shortened‐implant on the other.

Quad‐helix

(Godoy 2011; McNally 2005; Petrén 2008; Sollenius 2020)

This fixed metal expander (also available as fixed/removable) is capable of applying forces in numerous directions depending upon how it is activated. The four helical loops (two in the first bicuspid region and two in the second molar region) can be activated in unison or individually to achieve the desired results. The appliance is soldered to bands on the first molars and lingual arms run from the bands forwards to the cuspids or first bicuspids as desired.

Minne expander

(Mossaz‐Joelson 1989)

This is a tooth‐borne appliance without any acrylic palatal covering. This design makes use of a spring loaded screw called a Minne expander. The first premolars and molars are banded. Metal flanges are soldered onto the bands on the buccal and lingual sides. The expander consists of a coil spring having a nut that can compress the spring. This coil spring is made to extend between the lingual metal flanges that have been soldered. The expander is activated by closing the nut so that the spring is compressed.

Memory screw expander

(Halicioglu 2014)

A fixed maxillary expander that uses memory screw with an integrated spring that ensures a consistent expansion.

Fan‐type expander

(Gopalakrishnan 2017)

A fixed maxillary expander that uses fan‐type screw. The hinge point of the fan‑type rapid maxillary expansion screw was positioned in line with the distal surface of the upper first permanent molar. The anterior arms were adapted to the lingual surface of the canines and premolars. Posterior arms were bent perpendicular to the screw body and adapted to the molars. The anterior and posterior arms along with the corresponding teeth from canines to molars were enclosed in the clear acrylic material. The incisors were left free.

Expansion arch

(McNally 2005)

The expansion arch is made from 1.135 mm round stainless steel wire bent into the shape of a dental arch and inserted into the extra‐oral traction tubes on the first molar bands.

Expander with differential opening

(Massaro 2020)

The expander with differential opening is composed of two 10‐mm screws, one posteriorly and the other anteriorly positioned on the palate. During the first six days of activation, both expander screws are activated two quarter turns in the morning and two quarter turns in the evening. For an extra four days, only the anterior screw is activated following the same activation protocol.

Keles keyless expander

(Cheung 2020)

An expander with traditional design (two/four support arms or bonded) with screw that has a built‐in activation arm, which patients can activate themselves. The activation arm is pushed backward, the two adjacent metal portions are moved laterally to initiate the same expansion mechanism as with conventional devices. The activation arm then springs back without unwinding the screw and is ready for the next activation.

Removable appliances

Upper removable appliance

(Godoy 2011; Oshagh 2012; Petrén 2008; Sollenius 2020; Thilander 1984; Venancio 2014)

The upper removable appliance is an expansion plate made of acrylic, with an expansion screw and steel clasps on the first deciduous and permanent molars.

Appendix 2. Cochrane Oral Health Trials Register search strategy

Cochrane Oral Health’s Trials Register is available via the Cochrane Register of Studies. For information on how the register is compiled, see oralhealth.cochrane.org/trials

1 ((crossbite* or cross‐bite* or "cross bite*"):ti,ab) AND (INREGISTER) 2 ((" or "posterior tooth"):ti,ab) AND (INREGISTER) 3 ((posterior and dental):ti,ab) AND (INREGISTER) 4 ((anteroposterior):ti,ab) AND (INREGISTER) 5 ("dentoalveolar inclin*":ti,ab) AND (INREGISTER) 6 (((palat* or maxilla*) AND (expand* or expansion)):ti,ab) AND (INREGISTER) 7 (#1 or #2 or #3 or #4 or #5 or #6) AND (INREGISTER) 8 ((orthodontic* or interceptive):ti,ab) AND (INREGISTER) 9 ((expan* and (appliance* or device*)):ti,ab) AND (INREGISTER) 10 (((transpalatal or trans‐palatal) and arch*):ti,ab) AND (INREGISTER) 11 (("mandibular arch*" and contract*):ti,ab) AND (INREGISTER) 12 ((ipomaxilla* and (correct* or therap*)):ti,ab) AND (INREGISTER) 13 (("criss‐cross elastic*" or "criss cross elastic*"):ti,ab) AND (INREGISTER) 14 ((occlusal and grind*):ti,ab) AND (INREGISTER) 15 (#8 or #9 or #10 or #11 or #12 or #13 or #14) AND (INREGISTER) 16 (#7 and #15) AND (INREGISTER)

Appendix 3. The Cochrane Central Register of Controlled Trials (CENTRAL) search strategy

#1 (cross‐bite* in All Text or crossbite* in All Text or "cross bite*" in All Text) #2 ((posterior in All Text near/6 teeth in All Text) or (posterior in All Text near/6 tooth in All Text) or (posterior in All Text near/6 dental in All Text)) #3 ((anteroposterior in All Text near/6 teeth in All Text) or (anteroposterior in All Text near/6 tooth in All Text) or (anteroposterior in All Text near/6 dental in All Text)) #4 "dentoalveolar inclin*" in All Text #5 ((palat* in All Text or maxilla* in All Text) and (expand* in All Text or expansion in All Text)) #6 (#1 or #2 or #3 or #4 or #5) #7 MeSH descriptor Orthodontics, corrective explode all trees #8 (orthodontic* in All Text or interceptive in All Text) #9 (expan* in All Text and (appliance* in All Text or device* in All Text)) #10 ((transpalatal in All Text or trans‐palatal in All Text) and arch* in All Text) #11 ("mandibular arch*" in All Text and contract* in All Text) #12 (ipomaxilla* in All Text and (correct* in All Text or therap* in All Text)) #13 ("criss‐cross elastic*" in All Text or "criss cross elastic*" in All Text) #14 (occlusal in All Text and grind* in All Text) #15 (#7 or #8 or #9 or #10 or #11 or #12 or #13 or #14) #16 (#6 and #15)

Appendix 4. MEDLINE Ovid search strategy

1. (cross‐bite$ or crossbite$).mp. 2. (posterior adj8 (teeth or tooth or dental)).mp. 3. (anteroposterior adj8 (teeth or tooth or dental)).mp. 4. "dentoalveolar inclin$".mp. 5. ((palat$ or maxilla$) adj3 (expand$ or expansion)).mp. 6. or/1‐5 7. exp Orthodontics, corrective/ 8. exp Orthodontics, interceptive/ 9. (expan$ and (appliance$ or device$)).mp. 10. ((transpalatal or trans‐palatal) and arch$).mp. 11. ("mandibular arch$" and contract$).mp. 12. (ipomaxilla$ and (correct$ or therap$)).mp. 13. ("criss‐cross elastic$" or "criss cross elastic$").mp. 14. (occlusal and grind$).mp. 15. or/7‐14 16. 6 and 15

The above subject search was linked with the highly sensitive search strategy designed by Cochrane for identifying randomised controlled trials and controlled clinical trials in MEDLINE (as described in Lefebvre 2021, box 3b).

1. randomized controlled trial.pt. 2. controlled clinical trial.pt. 3. randomized.ab. 4. placebo.ab. 5. drug therapy.fs. 6. randomly.ab. 7. trial.ab. 8. groups.ab. 9. or/1‐8 10. exp animals/ not humans.sh. 11. 9 not 10

Appendix 5. Embase Ovid search strategy

1. (cross‐bite$ or crossbite$).mp. 2. (posterior adj8 (teeth or tooth or dental)).mp. 3. (anteroposterior adj8 (teeth or tooth or dental)).mp. 4. "dentoalveolar inclin$".mp. 5. ((palat$ or maxilla$) adj3 (expand$ or expansion)).mp. 6. or/1‐5 7. (expan$ and (appliance$ or device$)).mp. 8. ((transpalatal or trans‐palatal) and arch$).mp. 9. ("mandibular arch$" and contract$).mp. 10. (ipomaxilla$ and (correct$ or therap$)).mp. 11. ("criss‐cross elastic$" or "criss cross elastic$").mp. 12. (occlusal and grind$).mp. 13. Orthodontics/ 14. or/7‐13 15. 6 and 14

Randomized controlled trial/
Controlled clinical study/
random$.ti,ab.
randomization/
intermethod comparison/
placebo.ti,ab.
(compare or compared or comparison).ti.
((evaluated or evaluate or evaluating or assessed or assess) and (compare or compared or comparing or comparison)).ab.
(open adj label).ti,ab.
((double or single or doubly or singly) adj (blind or blinded or blindly)).ti,ab.
double blind procedure/
parallel group$1.ti,ab.
(crossover or cross over).ti,ab.
((assign$ or match or matched or allocation) adj5 (alternate or group$1 or intervention$1 or patient$1 or subject$1 or participant$1)).ti,ab.
(assigned or allocated).ti,ab.
(controlled adj7 (study or design or trial)).ti,ab.
(volunteer or volunteers).ti,ab.
human experiment/
trial.ti.
or/1‐19
random$ adj sampl$ adj7 ("cross section$" or questionnaire$1 or survey$ or database$1)).ti,ab. not (comparative study/ or controlled study/ or randomi?ed controlled.ti,ab. or randomly assigned.ti,ab.)
Cross‐sectional study/ not (randomized controlled trial/ or controlled clinical study/ or controlled study/ or randomi?ed controlled.ti,ab. or control group$1.ti,ab.)
(((case adj control$) and random$) not randomi?ed controlled).ti,ab.
(Systematic review not (trial or study)).ti.
(nonrandom$ not random$).ti,ab.
"Random field$".ti,ab.
(random cluster adj3 sampl$).ti,ab.
(review.ab. and review.pt.) not trial.ti.
"we searched".ab. and (review.ti. or review.pt.)
"update review".ab.
(databases adj4 searched).ab.
(rat or rats or mouse or mice or swine or porcine or murine or sheep or lambs or pigs or piglets or rabbit or rabbits or cat or cats or dog or dogs or cattle or bovine or monkey or monkeys or trout or marmoset$1).ti. and animal experiment/
Animal experiment/ not (human experiment/ or human/)
or/21‐33
20 not 34

Appendix 6. US National Institutes of Health Trials Register and the World Health Organization (WHO) International Clinical Trials Registry Platform search strategy

orthodontic AND posterior AND crossbite

Data and analyses

Comparison 1. Removable tooth‐borne expansion (expansion plate) versus observation.

Outcome or subgroup title	No. of studies	No. of participants	Statistical method	Effect size
1.1 Outcomes at appliance removal: expansion plate versus observation	3		Peto Odds Ratio (Peto, Fixed, 95% CI)	Subtotals only
1.1.1 Crossbite correction in children 7‐11 years old	3	148	Peto Odds Ratio (Peto, Fixed, 95% CI)	25.26 [13.08, 48.77]
1.2 Outcomes at appliance removal: expansion plate versus observation	3		Mean Difference (IV, Fixed, 95% CI)	Subtotals only
1.2.1 Molar expansion in children 7‐11 years old (mm)	3	145	Mean Difference (IV, Fixed, 95% CI)	3.30 [2.88, 3.73]
1.2.2 Canine expansion in children 7‐11 years old (mm)	3	145	Mean Difference (IV, Fixed, 95% CI)	2.59 [2.18, 3.01]

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Comparison 2. Fixed tooth‐borne expansion (quad‐helix) versus observation.

Outcome or subgroup title	No. of studies	No. of participants	Statistical method	Effect size
2.1 Outcomes at appliance removal: quad‐helix versus observation	3		Peto Odds Ratio (Peto, Fixed, 95% CI)	Subtotals only
2.1.1 Crossbite correction in children 7‐11 years old	3	149	Peto Odds Ratio (Peto, Fixed, 95% CI)	50.59 [26.77, 95.60]
2.2 Outcomes at appliance removal: quad‐helix versus observation	3		Mean Difference (IV, Fixed, 95% CI)	Subtotals only
2.2.1 Molar expansion in children 7‐11 years old (mm)	3	146	Mean Difference (IV, Fixed, 95% CI)	4.71 [4.31, 5.10]
2.2.2 Canine expansion in children 7‐11 years old (mm)	3	146	Mean Difference (IV, Fixed, 95% CI)	3.15 [2.77, 3.53]

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Comparison 3. Fixed tooth‐borne expansion (Hyrax) versus observation.

Outcome or subgroup title	No. of studies	No. of participants	Statistical method	Effect size
3.1 Outcomes at appliance removal: Hyrax versus observation	3	93	Peto Odds Ratio (Peto, Fixed, 95% CI)	48.02 [21.58, 106.87]
3.1.1 Crossbite correction in adolescents 12‐16 years old	2	71	Peto Odds Ratio (Peto, Fixed, 95% CI)	48.84 [19.51, 122.25]
3.1.2 Crossbite correction in children 7‐11 years old	1	22	Peto Odds Ratio (Peto, Fixed, 95% CI)	45.52 [8.89, 233.04]
3.2 Outcomes at appliance removal: Hyrax versus observation	2		Mean Difference (IV, Fixed, 95% CI)	Subtotals only
3.2.1 Molar expansion in adolescents 12‐16 years old (mm)	2	71	Mean Difference (IV, Fixed, 95% CI)	5.80 [5.15, 6.45]

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Comparison 4. Fixed tooth‐borne expansion (quad‐helix) versus removable tooth‐borne expansion (expansion plate).

Outcome or subgroup title	No. of studies	No. of participants	Statistical method	Effect size
4.1 Outcomes at appliance removal: quad‐helix versus expansion plate	3		Risk Ratio (M‐H, Fixed, 95% CI)	Subtotals only
4.1.1 Crossbite correction in children 7‐11 years old	3	151	Risk Ratio (M‐H, Fixed, 95% CI)	1.29 [1.13, 1.46]
4.2 Outcomes at appliance removal: quad‐helix versus expansion plate	3		Mean Difference (IV, Fixed, 95% CI)	Subtotals only
4.2.1 Molar expansion in children 7‐11 years old (mm)	3	151	Mean Difference (IV, Fixed, 95% CI)	1.48 [0.91, 2.04]
4.2.2 Canine expansion in children 7‐11 years old (mm)	3	151	Mean Difference (IV, Fixed, 95% CI)	0.59 [0.09, 1.08]
4.3 Outcomes at appliance removal: quad‐helix versus expansion plate (months)	3	148	Mean Difference (IV, Fixed, 95% CI)	‐3.15 [‐4.04, ‐2.25]
4.3.1 Length of treatment in children 7‐10 years old (months)	3	148	Mean Difference (IV, Fixed, 95% CI)	‐3.15 [‐4.04, ‐2.25]

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Comparison 5. Fixed tooth‐borne expansion (Hyrax) versus fixed tooth‐tissue‐borne expansion (Haas).

Outcome or subgroup title	No. of studies	No. of participants	Statistical method	Effect size
5.1 Outcomes at appliance removal: Hyrax versus Haas	3	83	Risk Ratio (M‐H, Fixed, 95% CI)	1.05 [0.94, 1.18]
5.1.1 Crossbite correction in adolescents 12‐16 years old	2	41	Risk Ratio (M‐H, Fixed, 95% CI)	1.00 [0.88, 1.13]
5.1.2 Crossbite correction in children 7‐11 years old	1	42	Risk Ratio (M‐H, Fixed, 95% CI)	1.11 [0.91, 1.36]
5.2 Outcomes at appliance removal: Haas versus Hyrax	2		Mean Difference (IV, Fixed, 95% CI)	Subtotals only
5.2.1 Molar expansion in adolescents 12‐16 years old (mm)	2	46	Mean Difference (IV, Fixed, 95% CI)	‐0.15 [‐0.86, 0.56]

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Comparison 6. Fixed tooth‐borne expansion (Hyrax) versus fixed tooth‐bone‐borne expansion.

Outcome or subgroup title	No. of studies	No. of participants	Statistical method	Effect size
6.1 Outcomes at appliance removal: Hyrax versus tooth‐bone‐borne expansion	3	120	Risk Ratio (M‐H, Fixed, 95% CI)	1.02 [0.92, 1.12]
6.1.1 Crossbite correction in adolescents 12‐16 years old	2	70	Risk Ratio (M‐H, Fixed, 95% CI)	1.03 [0.88, 1.21]
6.1.2 Crossbite correction in children 7‐11 years old	1	50	Risk Ratio (M‐H, Fixed, 95% CI)	1.00 [0.93, 1.08]
6.2 Outcomes at appliance removal: Hyrax versus tooth‐bone‐borne expansion	2	65	Mean Difference (IV, Fixed, 95% CI)	‐0.66 [‐1.36, 0.04]
6.2.1 Molar expansion in adolescents 12‐16 years old	1	25	Mean Difference (IV, Fixed, 95% CI)	‐0.52 [‐2.41, 1.37]
6.2.2 Molar expansion in children 7‐11 years old	1	40	Mean Difference (IV, Fixed, 95% CI)	‐0.68 [‐1.43, 0.07]

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Comparison 7. Fixed tooth‐borne expansion (Hyrax) versus fixed bone‐borne expansion.

Outcome or subgroup title	No. of studies	No. of participants	Statistical method	Effect size
7.1 Outcomes at appliance removal: Hyrax versus bone‐borne expansion	2		Risk Ratio (M‐H, Fixed, 95% CI)	Subtotals only
7.1.1 Crossbite correction in adolescents 12‐16 years old	2	81	Risk Ratio (M‐H, Fixed, 95% CI)	1.00 [0.94, 1.07]
7.2 Outcomes at appliance removal: Hyrax versus bone‐borne expansion	2		Mean Difference (IV, Fixed, 95% CI)	Subtotals only
7.2.1 Molar expansion in adolescents 12‐16 years old	2	81	Mean Difference (IV, Fixed, 95% CI)	‐0.14 [‐0.85, 0.57]

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Characteristics of studies

Characteristics of included studies [ordered by study ID]

Alberta 2010.

*Study characteristics*
Methods	Study design: RCT (3 arms) Location: Orthodontic Clinic at the University of Alberta in Edmonton, Alberta, Canada Number of centres: 1 Recruitment period: 18‐month recruitment period Funding source: not stated Clinician experience: not stated
Participants	Inclusion criteria: skeletal transverse maxillary constriction and unilateral or bilateral posterior crossbite. Included patients were non‐syndromic and ranged from ages 11‐17 years. All had a minimum of 5 mm maxillary constriction (cusp‐to‐fossa). Exclusion criteria: none stated Number randomised: 62 (Group A: 20; Group B: 21; Group C: 21) Group A: mean age 14.05 years (SD 1.35); M/F 5/15 Group B: mean age 14.24 years (SD 1.32); M/F 8/13 Group C: mean age 12.86 years (SD 1.19); M/F 6/15 Number evaluated: not stated
Interventions	Comparison: fixed tooth‐borne expansion (Hyrax) vs fixed bone‐borne expansion vs untreated control group Group A (N = 20): TAME Hyrax with bands on the first permanent molars and first premolars. Activated 0.5 mm per day until posterior dental crossbite over correction was achieved Group B (N = 21): BAME composed of 2 custom‐milled stainless steel onplants, 2 miniscrews and an expansion screw. A healing period of 1 week was allowed before activation of the expander. Activation consisted of 0.25 mm every other day until overcorrection was achieved Group C (N = 21): treatment delayed for 12 months to serve as a control group In Groups A and B, the appliance was removed after overcorrection was achieved and left without retention for an additional 6 months.
Outcomes	Molar expansion Inclination Pain Condilar spatial changes after RME using CBCT
Notes	No sample size calculation reported
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: "Subjects were randomized into the groups by using a random numbers generated list"
Allocation concealment (selection bias)	Unclear risk	Quote: "Subjects were randomized into the groups by using a random numbers generated list" Comment: not mentioned
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	It appears that it would have been possible to blind outcome assessors as the appliances were removed before final measurements were made. However, it was not mentioned
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	No indication of whether or not there were any dropouts. Unclear how many participants were included in the outcome assessment
Selective reporting (reporting bias)	Low risk	Outcome measures described in the methods section were reported in full
Other bias	Low risk	No other sources of bias identified

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Araujo 2020.

*Study characteristics*
Methods	Study design: RCT (2 arms) Location: Dental Clinic of the University of Northern Paraná, Londrina, Brazil Recruitment period: not stated Funding source: in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) ‐ Finance Code 001 Clinician experience: participants were treated by 2 orthodontist residents and supervised by a faculty member.
Participants	Inclusion criteria: children presenting unilateral or bilateral posterior crossbite Exclusion criteria: individuals who had craniofacial anomalies, compliance problems, periodontal disease, agenesis and supernumerary teeth, anterior crossbite or open bite, permanent tooth losses, extensive cavities and previous orthodontic treatment history Number randomised: 42 people randomised into 2 groups following a 1:1 pattern Group A: Hyrax N = 21; mean age 9.67, SD 1.64; F/M 12/9 Group B: Haas N = 21; mean age 9.33, SD 1.04; F/M 11/10 Number evaluated: 38
Interventions	Comparison: fixed tooth‐borne expansion (Hyrax) vs fixed tooth‐tissue‐borne expansion (Haas) Group A: Hyrax N = 21. An expansion screw and stainless steel extensions welded on the bands of upper first molars and bonded to the upper first premolars Group B: Haas N = 21. An expansion screw and stainless steel extensions welded on the bands of upper first molars and bonded to the upper premolars with an acrylic covering the palatal surface Both appliances were activated 1 complete turn a day (1.0 mm) until the palatal cusps of the maxillary molars were in contact with the buccal cusps of the mandibular first molars.
Outcomes	Primary: correction of posterior crossbite defined as the palatal cusp of the maxillary first molar touching the buccal cusp tips of the mandibular first molar Secondary: transverse dimensions and posterior teeth inclinations obtained from measurements on tomographic images. Transverse dimensions of the maxilla were measured in 2 coronal images perpendicular to the midsagittal plane, the first one passing through the centre of the palatal root of the maxillary right permanent first molar (posterior region) and the second, displaced 15 mm anteriorly (anterior region).
Notes	"Calculation of sample size was based on the ability to detect a difference in maxillary width of 1.1 mm (SD, 1.10), measured between the external cortical to the level of the deepest region of the palate (16), with an alpha of 5% and a test power of 80%."
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote "The patients were randomly assigned to one of the two treatment groups via a block randomization procedure with a block size of four, using a computer‐generated (Microsoft Corporation ‐ Redmond, USA) (17) list of random numbers."
Allocation concealment (selection bias)	Low risk	Qupte" "The allocation sequence was concealed from orthodontists and patient’s parents. When a patient was deemed as eligible for enrollment, the patient was assigned to a treatment group using opaque and sealed envelopes containing the allocation number"
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Quote: "There was allocation concealment and blinding of outcomes assessment"
Incomplete outcome data (attrition bias) All outcomes	Low risk	Comment: dropouts were balanced between groups (3/21: 14.2% and 1/21: 4.7%) and reasons were explained. Quote: "With regards to dropouts of the sample, two individuals moved to another city, 1 patient withdrawn from treatment, and 1 patient did not attend the final CBCT scan"
Selective reporting (reporting bias)	Low risk	All results reported
Other bias	Low risk	No obvious sources of bias

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Asanza 1997.

*Study characteristics*
Methods	Study design: RCT (2 arms) Location: Orthodontics Department, Albert Einstein College of Medicine and Montefiore Medical Center, New York, USA Number of centres: 1 Recruitment period: not stated Funding source: not stated but study part of a Master's thesis Clinician experience: not stated
Participants	Inclusion criteria: children with posterior crossbites Exclusion criteria: none stated Age: 8.5‐16 years Sex: M/F 7/7 Number randomised: 14 (Group A: 7; Group B: 7) Number evaluated: 14 (Group A: 7; Group B: 7)
Interventions	Comparison: fixed tooth‐borne expansion (Hyrax) vs fixed tooth‐borne expansion (bonded Hyrax) Group A (N = 7): conventional Hyrax expander with mid‐palatal jackscrew assembly and 4 rigid steel wires that were soldered to the bands on abutment teeth usually first premolars and first molars Group B (N = 7): same Hyrax expander jackscrew assembly as Group A but wires were soldered to 0.036 wire loops bent circumferentially at the height of contour of the abutment teeth. The acrylic portion encased the occlusal, lingual and buccal surfaces of the abutment teeth Participants in both groups were instructed to turn screw twice/day (0.5 mm). After expansion, appliances remained for a 3‐month retention phase
Outcomes	Lateral and posterior‐anterior cephalometric assessment of angular and linear skeletal and dental changes Molar expansion
Notes	No sample size calculation reported
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Quote: "appliances were randomly assigned" Comment: insufficient information on the method of sequence generation
Allocation concealment (selection bias)	Unclear risk	Quote: "appliances were randomly assigned" Comment: not mentioned
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	Not mentioned
Incomplete outcome data (attrition bias) All outcomes	Low risk	No dropouts reported. Data in Table 5 implied that there were no dropouts
Selective reporting (reporting bias)	High risk	No measures of variance were reported, so we were unable to analyse the data
Other bias	Low risk	No other sources of bias identified

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Baysal 2016.

*Study characteristics*
Methods	Study design: RCT parallel‐group, with a 1:1 allocation ratio Location: Izmir Katip Celebi University, Izmir, Turkey Number of centres: 1 Recruitment period: not stated Funding source: supported by a research grants from The Scientific and Technological Research Council of Turkey (Project112R033) and Izmir Katip Celebi University, Scientific Research Projects Unit (Project 2013‐3‐TSBP‐32). Clinician experience: not stated
Participants	Inclusion criteria: maxillary transverse deficiency, assessed both clinically and radiographically, with posterior crossbite Exclusion criteria: congenitally missing or extracted permanent tooth (except 3rd molars); severe facial asymmetry determined by clinical examination; craniofacial syndrome; neuromuscular deformities; history of trauma; history of orthodontic treatment; poor oral hygiene; systemic diseases that might affect treatment results Age: treatment group 13.4, SD 1.2 years and control group 12.8, SD 1.3 years Sex: M/F 18/16 Number randomised: 34 (Group A (treatment group): 17; Group B (control group): 17) Number evaluated: 17 (Group A: 17; Group B: 17)
Interventions	Comparison: fixed tooth‐borne expansion (bonded acrylic splint) vs untreated control group Group A (N = 17): RME protocol established by a bonded acrylic splint expander. The screw was activated a quarter turn twice/day (0.5 mm) for the 1st week, then a quarter turn/day (0.25 mm), until the palatal cusps of the maxillary molar contacted the buccal cusps of the mandibular molar. After the expansion completed, the appliance was kept in the mouth passively for the first month. A Hawley retainer was delivered to all participants for the rest of the retention period. Gropu B (N = 17): untreated control group with treatment delayed for at least 6 months.
Outcomes	3‐dimensional facial surface images and PACs were taken before treatment (T0) and immediately after the retention/observation period (T1). Soft tissue changes were evaluated using 3‐D facial images, which were captured in the natural head position using the 3dMD imaging system (3dMD, Atlanta, Ga)
Notes	The optimal sample size determination prior to the statistical analyses was performed based on the effect size (Cohen’s d = 0.99) reported by Johnson et al., which indicated that group sizes of 17 (total 34) would provide at least 80% statistical power.
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	The randomization (with a 1:1 allocation ratio) was made at the start of the study with pre‐prepared random number tables.
Allocation concealment (selection bias)	Low risk	One study author evaluated the participants and the other study author did the enrolling.
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Blinding was applicable for outcome assessment only. Blinding of participant and operator was not possible but was feasible during evaluation and outcome assessment.
Incomplete outcome data (attrition bias) All outcomes	Low risk	Participant treatment and observation were completed without dropouts from either group.
Selective reporting (reporting bias)	Low risk	Outcome measures described in the methods section were reported in full.
Other bias	High risk	Participants with different malocclusions (and growth pattern) were included (Class I, N = 8, Class II, N = 20, Class III, N = 6)

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Celenk‐Koca 2018.

*Study characteristics*
Methods	Study design: RCT (2 arms) Location: Turkey; location not stated Number of centres: not stated Recruitment period: not stated Funding source: not stated Clinician experience: not stated
Participants	Inclusion criteria: individuals presenting with well‐balanced facial ratios and normal growth pattern who were planned to have an 8 mm screw activation and had their first molars and premolars completely erupted at pretreatment Exclusion criteria: individuals who had craniofacial anomalies, compliance problems, need for surgically assisted RME, and previous orthodontic treatment history Sex: Group A: M/F 8/12/; Group B: M/F 7/13 Number randomised: 40 (Group A: 20; Group B: 20) Number evaluated: 40 (Group A: 20; Group B: 20)
Interventions	Comparison: fixed tooth‐borne expansion (Hyrax) vs fixed tooth‐bone‐borne expansion Group A (N = 20): treated with a tooth‐borne expander Group B (N = 20): treated with a tooth‐bone‐borne expander with 4 miniscrews 1.8 × 9 mm (Orlus, Ortholution Co, Seoul, Korea) attaching the expander to the palate surface. Appliances in both groups were activated by 2 turns a day until the palatal cusp of the maxillary first molar touching the buccal cusp tips of the mandibular first molar
Outcomes	Primary outcome: correction of crossbite Secondary outcomes: maxillary expansion measured using molar width and promolar width; the nasal cavity width between the maxillary first premolars and maxillary first molars; the widest intercortical distance of the incisive foramen as measured perpendicular to the intermaxillary suture; the intermaxillary suture width measured between the right and left cortical border at the anteroposterior position of maxillary first premolars and first molars; the buccal width measurements obtained at the level of the maxillary first premolar bifurcation and maxillary first molar trifurcation; the buccolingual inclinations of the maxillary first premolars and first molars measured using the inclinations of the buccal and mesiobuccal roots, respectively, to the horizontal plane projected from the nasal floor.
Notes	The sample size was determined by using the mean skeletal expansion measured at the level of first molars using a preliminary study. The effect size was calculated with the G*Power 3.1 statistical program (Heinrich Heine Universitat Dusseldorf Institute fur Experimentelle Psychologie, Dusseldorf, Germany). It was estimated that to detect significant differences (P < 0.05, effect size d = 0.94, and with 80% power) between the 2 groups, 15 individuals would be required in each group. 20 individuals were recruited for each group to ensure the power in case of any dropouts.
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: "Patients were randomly assigned to one of the two treatment groups via a block randomization procedure with a block size of four, using a computer‐generated list of random numbers."
Allocation concealment (selection bias)	Low risk	Quote: “The allocation sequence was concealed from the orthodontist, researchers, and the patients. When a patient was deemed as eligible for enrollment, the patient was assigned to a treatment group using opaque and sealed envelopes containing the allocation number”
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Quote: "the researcher who traced the cone‐beam computed tomography (CBCT) images and the statistician who evaluated the data were blinded"
Incomplete outcome data (attrition bias) All outcomes	Low risk	Quote: “no drop‐outs” (answered by email)
Selective reporting (reporting bias)	High risk	The conclusions stated that, quote: “Bone‐borne expansion resulted in uprighting of the maxillary posterior teeth with significant benefit to the buccal alveolar bone support.” However, regarding the uprighting of maxillary posterior teeth, the difference between T1 and T2 was 0.6º ± 2.4 at premolars level and 1.3º ± 2.1 at molars level and 0.6º ± 2.4 at premolars level, which we consider not clinically relevant. Additionally, regarding the benefit of bone‐borne to the buccal alveolar bone, there was no clinical difference in buccal bone support between T1 and T2 at molar level (−0.1 mm ± 0.1) and at premolar level (−0.04mm ± 0.2) compared to conventional at molar level (−0.24mm ± 0.2) and premolar level (−0.29 mm ± 0.2)
Other bias	Low risk	No other sources of bias identified

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Cheung 2020.

*Study characteristics*
Methods	Study design: RCT (3 arms) Location: Sydney Dental Hospital, Sydney, Australia Number of centres: single‐centre Recruitment period: from January 2017‐July 2017 Funding source: Australian Society of Orthodontists Foundation for Research and Education Clinician experience: not stated
Participants	Inclusion criteria: unilateral or bilateral posterior crossbite; maxillary transverse deficiency of > 5 mm as measured from the cusp of the upper first permanent molars to the lower first permanent molar central groove; erupted first permanent molars and premolars; adequate oral hygiene; and no history of previous orthodontic treatment Exclusion criteria: no history of craniofacial defects, syndromes, or surgery Group A: Hyrax N = 19: F/M 10/9; age mean 13.8, SD 1.6 Group B: Hybrid‐Hyrax N = 19: F/M 8/11; age mean 14.3, SD 1.7 Group C: Keles Keyless Expander N = 13: M/F 2/11; age mean 14.6, SD 1.2 Number randomised: 66 people (median age 10‐16 years) were randomised into 3 groups following a 1:1:1 pattern. Number evaluated: 51
Interventions	Comparison: fixed tooth‐borne expansion (Hyrax) vs fixed tooth‐bone‐borne expansion vs fixed tooth‐bone‐borne expansion (Keles Keyless Expander) Group A (N = 22): treated with a traditional Hyrax soldered to bands on the first premolars and molars Group B (N = 22): treated with a Hybrid‐Hyrax soldered to bands on the first molars and fixed to 2 miniscrews of 9 mm length and 2 mm width placed bilateral to the mid‐palatal suture Group C (N = 22): treated with a traditional Hyrax with a Keles Keyless Expander soldered to bands on the first premolars and molars In all 3 groups, participants were instructed to turn the expander twice a day (0.5 mm) and to return for weekly reviews until palatal cusps of the upper first molars were in contact with the buccal cusps of the lower first molars
Outcomes	Primary: main outcome was treatment‐induced (T1–T0) changes in the overall airway dimension as measured on CBCT after expander use Secondary: included changes in the separate compartments of the airway: nasal cavity, nasopharynx, and oropharynx between devices
Notes	"Sample size calculations were based on the ability to detect a clinically relevant difference of a 20 per cent increase in airway volume, α = 0.05 and a power of 80 per cent in a two‐sided paired t‐test. A mean of 23 950 mm3 and standard deviation (SD) of 6431 mm3 data from a previous study using similar airway volume parameters were used."
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quopte: "Randomization was accomplished using the ‘RANDBETWEEN’ command in Excel 2016 software (Microsoft, Redmond, Seattle, WA, USA) to randomize patients into the three groups as blocks of 22 patients each."
Allocation concealment (selection bias)	Low risk	Quote: "Allocation concealment was performed by a staff member at the Sydney Dental Hospital not directly involved with the trial. Separate sealed and opaque envelopes with each patient’s allocation were held in a central location in the department until the day of treatment."
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Quote: "The data sets were provided with codes on the expander type (Expander 1, 2, and 3) and time points (Time point a and b) for blinded statistical analysis, and the code was broken after supplying the results of the statistical analysis."
Incomplete outcome data (attrition bias) All outcomes	High risk	Comment: dropouts unbalanced between groups: Hyrax, Hybrid‐Hyrax, and Keles (2/22: 9%, 3/22: 13.6%, and 9/22: 40.9%, respectively) Quote: "In the Keles group, eight patients were excluded from the study and were further treated with a Hyrax expander. This was deemed necessary as it was found that their Keles Keyless Expanders suffered from structural defects related to blocking of the expander not permitting further activations, which made it clinically unsatisfactory to continue their use. It was decided that it would be unethical to continue. The expanders were replaced and these cases were excluded from the analysis. Additionally, two DICOM data sets from the Hyrax group and three from the Hybrid‐Hyrax group were not of appropriate quality for performing upper airway measurements due to patient movement during image acquisition, making further analysis not feasible."
Selective reporting (reporting bias)	Low risk	Not detected
Other bias	Low risk	No obvious sources of bias

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Dindaroglu 2016.

*Study characteristics*
Methods	Study design: RCT (2 arms) Location: not stated Number of centres: 1 Recruitment period: not stated Funding source: not stated Clinician experience: not stated
Participants	Inclusion criteria: skeletal maxillary constriction with unilateral or bilateral posterior crossbite; completion of permanent dentition; no apical lesions, root canal treatment, or coronal restoration in the P1s, P2s, or M1s; people who did not exceed MP3Cap Exclusion criteria: not stated Age: mean 12.8 years Sex: M/F 16/17 Number randomised: 33 (Hyrax: 16; Haas: 17) Number evaluated: 33 (Hyrax: 16; Haas: 17)
Interventions	Comparison: fixed tooth‐borne expansion (Hyrax) vs fixed tooth‐tissue‐borne expansion (Haas) Hyrax group (N = 16): an expansion screw and stainless steel extensions welded on the bands of P1 and M1. No additional buccal or lingual bars were used to attach the P2s; were activated 0.5 mm/day until the palatal cusps of the maxillary molars were in contact with the buccal cusps of the mandibular first molars Haas group (N = 17): an expansion screw and stainless steel extensions welded on the bands of P1 and M1. No additional buccal or lingual bars were used to attach the P2s; were activated 0.5 mm/day until the palatal cusps of the maxillary molars were in contact with the buccal cusps of the mandibular first molars. All participants wore mandibular anterior bite planes to eliminate premature contacts during active expansion.
Outcomes	Root resorption and volumetric change in P1, P2, and M1
Notes	No sample size calculation reported. Voxel resolution
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Randomly divided into 2 groups
Allocation concealment (selection bias)	Unclear risk	Not stated
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Segmentation was done with Mimics software V16.0 (Materialise NV, Leuven, Belgium) randomly by the same researcher, and all CBCT images were blinded.
Incomplete outcome data (attrition bias) All outcomes	Low risk	Treatment and observation were completed without dropouts from either group.
Selective reporting (reporting bias)	Low risk	Outcome measures described in the methods section were reported in full.
Other bias	Low risk	No other sources of bias identified

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Erhamza 2018.

*Study characteristics*
Methods	Study design: RCT (2 arms) Location: Turkey, exact location not stated Number of centres: not stated Recruitment period: not stated Funding source: the Coordinatorship of Scientific Research Projects of Kirikkale University financially supported the study (grant number 2015/09). Clinician experience: not stated
Participants	Inclusion criteria: maxillary constriction, deep palatal vault, bilateral crossbite, and presence of first premolars and first molars Exclusion criteria: congenital or developmental deformities or systemic disorders, thumb sucking, tongue thrust, periodontal problems, or were using antibiotics or mouthwash, or were receiving orthodontic treatment Age: Group A: mean 12.6 ± 1.17; Group B: mean 13.06 ± 1.22 Sex: Group A: M/F 8/7; Group B: M/F 5/10 Number randomised: (Group A: 15; Group B: 15) Number evaluated: (Group A: 15; Group B: 15)
Interventions	Comparison: fixed tooth‐tissue borne expansion (Haas) vs no treatment Group A (N = 15): Hyrax Group B (N = 15): no treatment The screw was activated 0.5 mm/day until the midpalatal suture was opened. The opening of the midpalatal suture was determined with an occlusal radiograph and midline diastema of the maxilla. Then it was activated 0.25 mm/day until the palatal cusps of the maxillary molars were in contact with the buccal cusps of the mandibular first molars. RME appliances were used as retainers without debanding for 4 months.
Outcomes	Halitosis Nasal volume Plaque index Gingival index Crossbite correction Transverse maxillary expansion
Notes	Sample size calculation: the power analysis showed that for a power of 0.80, with an a error of 0.05 (effect size 1.1), 15 participants were required for each group.
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: "Thirty subjects with ages between 11 and 15 years were randomly selected. From email correspondence: “Block randomization method was used for selection of the patients. Two group, A and B, and block size as 6 was determined for the sample. Online computer program (www.random.org) was used to prepared random numbers”.
Allocation concealment (selection bias)	Low risk	Quote: "Details of the allocated groups were written on colored cards placed in sequentially numbered, opaque, sealed envelopes".
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Quote: " The assessments were made by the same examiner (T.S.E.) throughout the study." Personal communication: “the results assessment blinded to the treatment and control groups”.
Incomplete outcome data (attrition bias) All outcomes	Low risk	From email correspondence: “No participant drop out of the study”
Selective reporting (reporting bias)	Low risk	Outcome measures described in the methods section were reported in full.
Other bias	Low risk	No other sources of bias identified

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Garib 2005.

*Study characteristics*
Methods	Study design: RCT (2 arms) Location: Orthodontics Department, Bauru School of Dentistry, University of Sao Paulo, Brazil Number of centres: 1 Recruitment period: not stated Funding source: not stated Clinician experience: not stated
Participants	Inclusion criteria: girls presenting with Class I or Class II malocclusion with unilateral or bilateral crossbite Exclusion criteria: boys, aged < 11 and > 14 years, persistence of any primary tooth, absence of maxillary posterior permanent teeth, metallic restorations on the maxillary posterior teeth, previous periodontal disease, previous orthodontic treatment Age: Group A: mean 12.4 years (range 11.4‐13.6); Group B: mean 12.6 years (range 11.5‐13.9 years) Sex: 100% F Number randomised: 8 (Group A: 4; Group B: 4) Number evaluated: not stated
Interventions	Comparison: fixed tooth‐tissue‐borne expansion (Haas) vs fixed tooth‐borne expansion (Hyrax) Group A (N = 4): tooth‐tissue borne palatal acrylic Haas‐type expander Group B (N = 4): tooth‐borne Hyrax expander (no palatal acrylic) Both groups had the screw activated by a complete turn after placement, followed by a one‐quarter turn morning and evening of each day (0.5 mm/day) until full expansion on day 16 (total of 7 mm expansion). Appliance was then kept as a retainer for a further 3 months and then removed, at which point the outcomes were measured.
Outcomes	Transverse maxillary expansion Maxillary posterior teeth axial inclination
Notes	Casual and systematic errors were calculated comparing the first measurement with Dahlberg's formula and second measurement with dependent t‐test at a significance level of 5%. No sample size calculation reported, but study authors suggest that study is probably underpowered.
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Quote: "...sample of eight patients, who were randomly assigned to two groups" Comment: insufficient information on the method of sequence generation
Allocation concealment (selection bias)	Unclear risk	Quote: "...sample of eight patients, who were randomly assigned to two groups" Comment: not mentioned
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	It appears that it would have been possible to blind outcome assessors as the appliances were removed before final measurements were made; however, it was not mentioned.
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	No indication of whether or not there were any dropouts. Unclear how many participants were included in the outcome assessment
Selective reporting (reporting bias)	Low risk	Outcome measures described in the methods section were reported in full.
Other bias	Low risk	No other sources of bias identified

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Godoy 2011.

*Study characteristics*
Methods	Study design: RCT (3 arms) Location: Dental Clinic of Santo Amaro by the University of Pernambuco, Brazil Number of centres: 1 Recruitment period: not stated Funding source: University of Pernambuco and research grant from the Ministry of Education of Brazil (CAPES) Clinician experience: specialist orthodontist with > 10 years of experience
Participants	Inclusion criteria: children had both unilateral posterior dental and functional crossbites in the mixed dentition; all had skeletal posterior crossbite diagnosed by examining the casts, models and anterioposterior cephalometric radiographs Exclusion criteria: sucking habits, previous orthodontic treatment, Class III malocclusion Group A: mean age 8.00 years (SD 0.79); M/F 7/26 Group B: mean age 7.82 years (SD 0.85); M/F 15/18 Group C: mean age 8.09 years (SD 0.81); M/F 19/14 Number randomised: 99 (Group A: 33; Group B: 33; Group C: 33) Number evaluated: 99 (Group A: 33; Group B: 33; Group C: 33)
Interventions	Comparison: fixed tooth‐borne expansion (quad‐helix) vs removable tooth‐borne expansion (expansion plate) vs untreated control group Group A (N = 33): quad‐helix fixed appliance with stainless steel bands bonded to teeth (tooth borne). Appliance was activated once a month until crossbite was corrected Group B (N = 33): expansion plate removable appliance with mid‐line screw and acrylic covering (tooth‐tissue borne). Appliance worn day and night and removed for teeth brushing. Appliance had the screw opened a quarter rotation every week until the posterior crossbite was corrected Group C (N = 33): no treatment Groups A and B were evaluated every 4 weeks, no overcorrection was produced and once correction was achieved each child had a retention plate to be used 24 h/day for 3 months and then only at night for a further 3 months.
Outcomes	Correction of crossbite Maxillary and mandibular inter‐molar and inter‐canine expansion Length of treatment Cost‐benefit Success rate (stability) Number of complications (Evaluation after crossbite correction, after 6‐month retention, after retention removal)
Notes	"The sample size calculation established an error of 5% and a power of 95%. To detect any differences in length of treatment between the 2 methods, the means and standard deviations were calculated based on the data from the study of Hermanson et al (8.00 ± 3.00 for the QH [quad‐helix]; 12.00 ± 5.00 for the EP [expansion plate]). The sample should include 27 patients/group to show a statistically significant difference" Email sent to authors 22 November 2012, reply received 23 November 2012
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: "For randomization, numbers were randomly drawn from a plastic bag. Each child received a number from 1 to 99" From email correspondence: "Each child received a number according to the clinical examination order. When all 99 children had a number, a helper took the 99 pieces of papers numbering from 1 to 99 from the bag. The first number to be taken would belong to QDH [quad‐helix] group, the second one to the EP [expansion plate] group, the third one to the untreated group, the fourth to the QDH group..."
Allocation concealment (selection bias)	Low risk	The method described above would not allow anybody involved in the study to know their treatment allocation until they had been allocated.
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Quote: "Palatal expansion and crossbite correction were measured on the study casts by 1 masked investigator...The investigator was unaware of the type of appliance used by the patient and the length of treatment (pretreatment, after expansion, after 6 months of retention, or 6 months after removal of the retention plate)"
Incomplete outcome data (attrition bias) All outcomes	Low risk	"Dropouts and treatments not completed within 12 months were classified as unsuccessful" Intention‐to‐treat analysis included all randomised participants
Selective reporting (reporting bias)	Low risk	Outcome measures described in the methods section were reported in full
Other bias	Low risk	No other sources of bias identified

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Gopalakrishnan 2017.

*Study characteristics*
Methods	Study design: RCT (2 arms) Location: Department of Orthodontics, Tamil Nadu Government Dental College and Hospital, Chennai, Tamil Nadu, India Number of centres: 1 Recruitment period: not stated Funding source: not stated Clinician experience: not stated
Participants	Inclusion criteria: people in the late mixed or early permanent dentition with tapered maxillary arch and unilateral or bilateral crossbites requiring palatal expansion as part of their treatment Exclusion criteria: people with palatal clefts and premature synostosis of the palate Age: 12‐14 years Sex: not stated Number randomised: 12 (fan: 6; Hyrax: 6) Number evaluated: 12 (fan: 6; Hyrax: 6)
Interventions	Comparison: fixed tooth‐borne expansion (Hyrax) vs fixed tooth‐borne expansion (fan‐type maxillary expander) Fan‐type group (N = 6): screws were adapted parallel to the occlusal plane of the upper teeth. The hinge point of the fan‑type RME screw was positioned in line with the distal surface of the upper first permanent molar. The anterior arms were adapted to the lingual surface of the canines and premolars. Posterior arms were bent perpendicular to the screw body and adapted to the molars. The anterior and posterior arms along with the corresponding teeth from canines to molars were enclosed in the clear acrylic material. The incisors were left free. Activation was started after 24 h of cementation. The appliances were activated one quarter turn twice/day, in the morning and evening. Hyrax group (N = 6): screw was positioned parallel to the second premolars or primary molars. The anterior and posterior arms were adapted to the lingual surfaces of the canines, premolars, and molars. The anterior and posterior arms along with the corresponding teeth were included in clear acrylic material. The incisors were left free. Activation was started after 24 h of cementation. The appliances were activated one quarter turn twice/day, in the morning and evening.
Outcomes	Suture opening Upper intercanine width (mm) Upper intermolar width (mm) Skeletal and dental outcomes obtained from lateral cephalogram and frontal cephalogram
Notes	Only 12 participants; midpalatal suture patency was assessed with occlusal radiographs ‐ suture opening was confirmed with occlusal radiographs
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	The participants were divided into two groups by concealed randomisation using the opaque envelope method.
Allocation concealment (selection bias)	Unclear risk	Not stated
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	Not stated
Incomplete outcome data (attrition bias) All outcomes	Low risk	Participant treatment and observation were completed without dropouts from either group.
Selective reporting (reporting bias)	Low risk	Outcome measures described in the methods section were reported in full.
Other bias	High risk	Only 12 people included in study

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Halicioglu 2014.

*Study characteristics*
Methods	Study design: RCT (2 arms) Location: Department of Orthodontics, Faculty of Dentistry, Atatürk University, Erzurum, Turkey. Number of centres: 1 Recruitment period: not stated Funding source: not stated Clinician experience: not stated
Participants	Inclusion criteria: bilateral maxillary crossbites and caused by basal apical narrowness. Exclusion criteria: not stated Age: 11‐14.5 years (memory‐screw group: 13.00 ± 1.29 years, Hyrax‐screw group: 12.58 ± 1.50 years) Sex: 17 F/15 M (memory‐screw group: 9 F/8 M), Hyrax‐screw group: 8 F/7 M) Number randomised: 32 (memory‐screw group: 17; Hyrax‐screw group: 15) Number evaluated: 32 (memory‐screw group: 17; Hyrax‐screw group: 15)
Interventions	Comparison: fixed tooth‐borne expansion (RME memory‐screw) vs fixed tooth‐borne expansion (Hyrax) Hyrax + memory‐screw (NiTi) group (N = 17): participants were instructed to activate the jack screw (Product number: 167M1529 — memory expander type ‘N’; Forestadent; Forestadent USA) 6 times (0.2 × 6 = 1.2 mm)/day: 2 in the morning, 2 after lunch, and 2 in the evening. Hyrax + conventional screw (N = 15): participants were instructed to activate the jack screw (Product number: 167‐1633 — Palatal split screw type ‘N’; Forestadent, Pforzheim, Germany; Forestadent USA, St Louis, Missouri, USA) twice (0.225 × 2 = 0.45 mm) a day until the suture was opened and then 1 turn/day. Both screws were activated until the occlusal aspect of the maxillary lingual cusp of the upper first molars contacted the occlusal aspect of the facial cusp of the mandibular first molars, thus producing the desiring expansion. Furthermore, the mean retention period was 6.42 ± 0.59 months in the memory‐screw group and 6.17 ± 0.32 months in the Hyrax‐screw group. The splitting at the suture palatine media was observed on different days of the expansion treatment. In all participants, sutural opening without any problem and suture palatine media that was filled with bone after the retention periods were determined using occlusal radiographs.
Outcomes	Skeletal and dental outcomes obtained from plaster models and postero‐anterior cephalograms Skeletal, dental and soft tissue outcomes obtained from lateral cephalograms
Notes	Same study as Halicioglu 2016 (see Halicioglu 2014 for reference)
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Participants randomly assigned to 2 groups
Allocation concealment (selection bias)	High risk	Not stated
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	Not stated
Incomplete outcome data (attrition bias) All outcomes	Low risk	Participant treatment and observation were completed without dropouts from either group.
Selective reporting (reporting bias)	Low risk	Outcome measures described in the methods section were reported in full.
Other bias	High risk	No sample size calculation reported

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Kilic 2008.

*Study characteristics*
Methods	Study design: RCT (2 arms) Location: Department of Orthodontics, Faculty of Dentistry, Atatürk University, Erzurum, Turkey Number of centres: 1 Recruitment period: not stated Funding source: not stated Clinician experience: not stated
Participants	Inclusion criteria: participants with severe maxillary arch width deficiency, bilateral crossbite and deep palatal vault Exclusion criteria: people with mucosal swelling during maxillary expansion Age: Group A: 13.75 years (SD 17 months); Group B: 13.5 years (SD 12 months) Gender: M/F 10/29 Number randomised: 39 (Group A: 21; Group B: 18) Number evaluated: 39 (Group A: 21; Group B: 18)
Interventions	Comparison: fixed tooth‐borne expansion (Hyrax) vs fixed tooth‐borne expansion (acrylic bonded Hyrax) Group A (N = 21): conventional Hyrax tooth‐borne appliance for RME Group B (N = 18): acrylic bonded appliance for RME Appliances were activated twice a day, one quarter turn each time (0.5 mm/day)
Outcomes	Maxillary molar expansion Molar crown tipping Alveolar inclination
Notes	No sample size calculation reported Email sent 22 November 2012
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Quote: "Randomly assigned to two groups" Comment: insufficient information on the method of sequence generation
Allocation concealment (selection bias)	Unclear risk	Quote: "Randomly assigned to two groups" Comment: not mentioned
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	It may have been possible to blind outcome assessors if the appliances were removed before final measurements were made; however, this was not mentioned.
Incomplete outcome data (attrition bias) All outcomes	Low risk	No dropouts reported. Numbers of participants/group reported in the results tables matches the numbers that were randomised.
Selective reporting (reporting bias)	Low risk	Outcome measures described in the methods section were reported in full.
Other bias	Low risk	No other sources of bias identified

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Lamparski 2003.

*Study characteristics*
Methods	Study design: RCT (2 arms) Location: private practice, Natrona Heights, PA, USA Number of centres: 1 Recruitment period: not stated Funding source: not stated Clinician experience: Board‐certified orthodontist with 27 years of experience
Participants	Inclusion criteria: crossbite; mixed or permanent dentition Exclusion criteria: systemic disease, dental pathology; maxillary tori Group A: mean age 11.33 years (range 7.75‐13.92); M/F 9/6 Group B: mean age 10.8 years (range 6.58‐14.58); M/F 6/9 Number randomised: 30 (Group A: 15; Group B: 15) Number evaluated: not stated
Interventions	Comparison: fixed tooth‐borne expansion (Hyrax with 4‐point anchorage) vs tooth‐borne expansion (Hyrax with 2‐point anchorage) Group A (N = 15): the 4‐point expansion appliance consisted of a Hyrax expansion screw with bands cemented to the maxillary first permanent molars and either the maxillary first premolars or the maxillary deciduous first molars. Activated 0.5 mm/day until posterior dental crossbite overcorrection was achieved Group B (N = 15): the 2‐point expander was banded only to the maxillary first permanent molars and had a jackscrew (a typical Hyrax appliance with both anterior wires removed). Activated 0.5 mm/day until posterior dental crossbite overcorrection was achieved
Outcomes	Molar, canine and sutural expansion
Notes	No sample size calculation reported
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Quote: "The subjects were randomly assigned" Comment: insufficient information on the method of sequence generation
Allocation concealment (selection bias)	Unclear risk	Quote: "The subjects were randomly assigned" Comment: not mentioned
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	It appears that it would have been possible to blind outcome assessors as the appliances were removed before final measurements were made; however, it was not mentioned
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	No indication of whether or not there were any dropouts. Unclear how many participants were included in the outcome assessment
Selective reporting (reporting bias)	Unclear risk	Outcomes planned in the methods section were reported in the results; however, means and SDs in each group were not reported.
Other bias	Low risk	No other sources of bias identified

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Lippold 2013.

*Study characteristics*
Methods	Study design: RCT (2 arms) Location: Orthodontics Department, Münster University's Medical Sciences Division and University Hospital Münster, Germany Number of centres: 1 Recruitment period: not stated Funding source: not stated Clinician experience: "Two specialists in orthodontics"
Participants	Inclusion criteria: functional unilateral posterior crossbite in the late deciduous or early mixed dentition Exclusion criteria: mid‐line deviation during orthodontic treatment; persisting habits; general diseases with permanent medication (e.g. diabetes mellitus); syndromes; cleft lip and palate; general impairments; structural orthopaedic diseases Age: Group A: mean 7.3 years (SD 2.2); Group B: mean 7.2 years (SD 2) Sex: "The gender ratio was nearly equal at the beginning of the study" Number randomised: 82 (Group A: 40; Group B: 42), but 5 participants dropped out after randomisation but before treatment (Group A: 37; Group B: 40) Number evaluated: 66 (Group A: 31; Group B: 35)
Interventions	Comparison: fixed tooth‐borne expansion (bonded Hyrax) followed by U‐bow activator therapy vs no treatment Group A (N = 37): bonded Hyrax appliance worn 24 h/day. Once/day activation (0.2 mm) was mandatory and the maxillary expansion was achieved in a mean of 3.2 weeks (SD 1.2). This was followed by a retention period of a mean of 12.6 weeks (SD 1.8). The U‐bow activator was then applied for a mean of 36.8 weeks (SD 5.4). This was a double‐plate activator combined with eponymous U‐shaped wire bows on each side (protrusive and labial bows on the upper and lower jaws). The maxillary plate had an additional transversal expansion screw for retention of the achieved expansion Group B (N = 40): no treatment (after the final follow‐up point, these children received the same treatment as those in the therapy group)
Outcomes	Sagittal, vertical and transversal dimensions of the maxilla and mandible Mid‐line deviation between the anterior teeth of the maxilla and mandible Sagittal overjet and vertical overbite Outcomes were measured 1 year after the start of treatment
Notes	No sample size calculation reported
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: "Block randomisation with a block length of 20 and an allocation ratio of 1:1"
Allocation concealment (selection bias)	Unclear risk	Quote: "Block randomisation with a block length of 20 and an allocation ratio of 1:1" Comment: allocation concealment not mentioned
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	Comment: not mentioned. It would have been possible to blind outcome assessors as the appliances could have been removed before final measurements were made, but who was in the control group may have been obvious due to it being an untreated control group whose crossbites would not be properly corrected.
Incomplete outcome data (attrition bias) All outcomes	Low risk	Numbers of dropouts and reasons were clearly described in a study workflow (Figure 1), and were sufficiently similar in each group.
Selective reporting (reporting bias)	High risk	No SD for mean change from baseline reported
Other bias	Low risk	No other sources of bias identified

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Martina 2012.

*Study characteristics*
Methods	Study design: RCT (2 arms) Location: Department of Oral Science, University of Naples Federico II, Italy Number of centres: 1 Recruitment period: May 2006‐October 2007 Funding source: grant from the Italian Ministry of University and Research Clinician experience: not stated
Participants	Inclusion criteria: children up to 13 years old (M) and 12 years old (F), with erupted permanent first molars and unilateral or bilateral full cusp posterior crossbite Exclusion criteria: severe periodontal disease (probing depth > 4 mm), congenital syndromes, defects, previous orthodontic treatment Group A: mean age 10.3 years (SD 2.5); M/F 7/5 Group B: mean age 9.7 years (SD 1.5); M/F 6/8 Number randomised: 50 (Group A: 23; Group B: 27). However, 9 children dropped out after randomisation but before treatment (Group A: 19; Group B: 22) Number evaluated: 26 (Group A: 12; Group B: 14)
Interventions	Comparison: fixed tooth‐borne expansion (Hyrax with 2‐point anchorage with slow expansion) vs fixed tooth‐borne expansion (Hyrax with 2‐point anchorage with rapid expansion) A 2‐band palatal expander was used for both groups Group A (N = 12): slow expansion, screw turned twice a week (0.50 mm activation/week) Group B (N = 14): rapid expansion, screw initially turned 8 times (2.0‐mm screw activation) at chair side 2 h after curing. Thereafter, the children's parents were trained to turn the screw 3 times/day (0.75 mm activation/day) In both groups, a 2‐band palatal expander was used and the jackscrew was activated until a 2 mm molar transverse overcorrection was achieved. After the required expansion was achieved, the screw was locked with light‐cure flow composite. The appliances were removed 7 months after treatment started, at which point the outcomes were measured
Outcomes	Molar expansion at molar cusp Antero and posterior maxillary expansion Pterygoid expansion Molar tipping
Notes	The power calculation was based upon previous estimates of RME transverse skeletal effects. This indicated that 12 children were needed for each treatment group (to detect 2.5 mm difference, SD = 2.0 mm, significance level = 0.05, power 80%).
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: "Balanced block randomisation using gender as a stratifying factor"
Allocation concealment (selection bias)	Low risk	Quote: "A single operator allocated the patients by means of a custom‐made Java script and was responsible for the allocation concealment, that is, the allocation was disclosed only when a new patient was enrolled in the trial"
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Measurements were made by a single operator blinded to participant allocation.
Incomplete outcome data (attrition bias) All outcomes	High risk	Numbers of dropouts and reasons were clearly described in a study workflow (Figure 4), and were similar. However, dropout rate was very high (Group A: 48%; Group B: 48%). If the missing participants had higher mean scores in 1 group than the other, as the attrition rate increased, so would over/understatement of the mean difference.
Selective reporting (reporting bias)	Low risk	Outcome measures described in the methods section were reported in full.
Other bias	Low risk	No other sources of bias identified

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Massaro 2020.

*Study characteristics*
Methods	Study design: RCT (2 arms) Location: Orthodontic Clinic of Bauru Dental School, University of São Paulo, Brazil Number of centres: 1 Recruitment period: January 2017‐ June 2018 Funding source: study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior ‐ Brasil (CAPES) ‐ Finance Code 001, and by the São Paulo Research Foundation (FAPESP) ‐ grant numbers 2017/12911‐9 and 2017/24115‐2 Clinician experience: not stated
Participants	Inclusion criteria: Class I and Class II patients from 7‐11 years of age with maxillary constriction and posterior crossbites Exclusion criteria: individuals with a Class III malocclusion, craniofacial syndromes, clinical absence of maxillary deciduous canines, and history of previous orthodontic treatment Group A: mean age 7.62 years (SD 0.92); M/F 11/13 Group B: mean age 7.83 years (SD 0.96); M/F 10/14 Number randomised: 48 (Group A: 24; Group B: 24). No dropouts Number evaluated: 48 (Group A: 24; Group B: 24)
Interventions	Comparison: fixed tooth‐borne expansion (differential opening expansion) vs fixed tooth‐borne expansion (fan‐type expansion) Group A (N = 24): the EDO was composed of two 10‐mm screws, one posteriorly and the other anteriorly positioned on the palate. During the first 6 days of activation, both expander screws were activated 2 quarter turns in the morning and 2 quarter turns in the evening. For extra 4 days, only the anterior screw was activated following the same activation protocol. The total expansion was 4.8 mm in the posterior screw and 8 mm in the anterior screw. Group B (N = 24): the fan expander was composed of one 11‐mm screw anteriorly positioned on the palate. For 10 consecutive days, the expander screw was activated 2 quarter turns in the morning and 2 quarter turns in the evening, resulting in an expansion of 8 mm in the screw.
Outcomes	Frequency of crossbite correction and the maxillary arch width changes Interincisal diastema, arch perimeter, arch length, arch size, arch shape, and mandibular dental arch changes The amount of differential expansion between the anterior and posterior region of the maxillary dental arch Maxillary dentoskeletal lateral displacements Changes in molars and canines buccolingual inclination Anteroposterior and vertical displacements and maxillary rotation
Notes	In order to detect a difference of 2 mm in intercanine width change, with a standard deviation of 2.18 mm, power of 80 per cent, and alpha of 5 per cent, a sample size of 20 participants per group was required. Considering possible losses, 24 participants were selected for each group.
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: "After recruitment, patients were randomly allocated into two study groups. A block randomization was performed before trial commencement using the Web site Randomization.com (http://www. randomization.com) to ensure that the trial arms have equal numbers of participants"
Allocation concealment (selection bias)	Low risk	Quote: "Opaque, sealed, and sequentially numbered envelopes containing the treatment allocation cards were prepared before trial commencement. The envelopes were sequentially opened for each participant during recruitment. The initials of the name of the participant were written on the envelope before opening it. The generation of randomization list, allocation concealment, and implementation were performed independently by different researches.“
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Quote: "Blinding was accomplished during outcome assessment since all data were unidentified before analysis and all the digital dental models were taken without the expander in the oral cavity."
Incomplete outcome data (attrition bias) All outcomes	Low risk	Quote: "No patients were lost during the follow‐up period."
Selective reporting (reporting bias)	Low risk	Not detected
Other bias	Low risk	No obvious sources of bias

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McNally 2005.

*Study characteristics*
Methods	Study design: RCT (2 arms) Location: Queen's Hospital, Burton on Trent and The University of Birmingham, School of Dentistry, Birmingham Number of centres: 2 Recruitment period: not stated Funding source: 3M Unitek supplied the quad‐helix arches used in this study. Clinician experience: the study authors had used expansion arches of the type tested for a number of years.
Participants	Inclusion criteria: either a unilateral or bilateral crossbite that required correction according to the consultant orthodontist Exclusion criteria: none stated Age: 11‐16 years (not reported by group) Sex: M/F 30/30 (not reported by group) Number randomised: 60 (Group A: 30; Group B: 30) Number evaluated: 55 (Group A: 28; Group B: 27) for molar expansion; 52 (Group A: 26; Group B: 26) for canine expansion (the lower numbers were due to unerupted canines)
Interventions	Comparison: fixed tooth‐borne expansion (quad‐helix + multi‐bracket) vs fixed tooth‐borne expansion (expansion arch + multi‐bracket) Group A (N = 30): quad‐helix plus multi‐bracket Group B (N = 30): expansion arch plus multi‐bracket In both groups, the appliance was activated in order to deliver 1.8‐N expansion force
Outcomes	Molar and canine expansion Comfort Outcomes were measured 4, 8 and 12 weeks after the start of treatment.
Notes	Altman nomogram for an SD of 1.5 mm at 80% power and 1% significance level suggests a total sample size of 46 children, with 23 in each group
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	High risk	Quote: "Using random number tables the first 30 participants were allocated for treatment with either appliance according to an odd or even number. The next 30 were then allocated in order to receive the alternative treatment to those in the initial allocation" Comment: this does not represent a truly random sequence generation.
Allocation concealment (selection bias)	High risk	Quote: "Using random number tables the first 30 participants were allocated for treatment with either appliance according to an odd or even number. The next 30 were then allocated in order to receive the alternative treatment to those in the initial allocation" Comment: potential to foresee group allocation
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	Quote: "The group of the participant was obvious from the model since the imprint of each type of appliance could be seen. However, since all measurements were made without reference to previous values, there could be no bias" Comment: we cannot completely rule out the possibility of bias in this situation.
Incomplete outcome data (attrition bias) All outcomes	Low risk	Numbers of dropouts and reasons are clearly described in a study workflow (Figure 4), and are sufficiently similar in each group.
Selective reporting (reporting bias)	Low risk	Outcome measures described in the methods section were reported in full.
Other bias	Low risk	No other sources of bias identified

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Mossaz‐Joelson 1989.

*Study characteristics*
Methods	Study design: reclassified as RCT following correspondence (2 arms) Location: Department of Orthodontics, Dental School, University of Geneva, Switzerland Number of centres: 1 Recruitment period: not stated Funding source: not stated Clinician experience: not stated
Participants	Inclusion criteria: children with bilateral or functional unilateral crossbite. No further definition provided Exclusion criteria: none stated Age: Group A: 8.6‐10.8 years; Group B: 8.7‐12 years Sex: M/F 6/4 (not reported by group) Number randomised: 10 (Group A: 5; Group B: 5) Number evaluated: not stated
Interventions	Comparison: fixed tooth‐borne expansion (bonded Minne) vs fixed tooth‐borne expansion (banded Minne) Group A (N = 5): conventional Minne expander embedded in acrylic and bonded to upper deciduous first and second molars (or first and second premolars) Group B (N = 5): conventional Minne expander soldered to bands to the upper first primary molars (or first permanent premolars) and upper first molars A continuous 2‐pound (0.9‐kg) force coil spring was used to obtain expansion. The spring was re‐activated every other week. The slow expansion period lasted 7‐15 weeks. After expansion was complete, the fixed appliances were removed, apart from the bonded vertical tubes in Group A, and the maxillary first molar bands in Group B. Conventional maxillary retainer worn full time for 12 weeks (retention period). There were then 12 more weeks of observation without the retainers to evaluate relapse
Outcomes	Molar and canine expansion Angular changes Cephalometric variables Stability (relapse of molar and canine expansion)
Notes	No sample size calculation reported
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	From correspondence with study authors: "...randomly divided into two groups..." Comment: insufficient information on the method of sequence generation
Allocation concealment (selection bias)	Unclear risk	From correspondence with study authors: "...randomly divided into two groups..." Comment: not mentioned
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	It appears that it would have been possible to blind outcome assessors as the appliances were removed before final measurements were made; however, it was not mentioned.
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	No indication of whether or not there were any dropouts. Unclear how many children were included in the outcome assessment.
Selective reporting (reporting bias)	Low risk	Outcome measures described in the methods section were reported in full.
Other bias	High risk	No sample size calculation reported.

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Nam 2020.

*Study characteristics*
Methods	Study design: RCT (2 arms) Location: Orthodontic Clinic in the University of Alberta (Alberta, Canada) Recruitment period: May 2010‐May 2016 Funding source: no external funding Clinician experience: not stated
Participants	Inclusion criteria: adolescents from 11‐16 years of age with diagnosis of maxillary transverse deficiency with unilateral or bilateral crossbite requiring maxillary expansion. Participants were in permanent dentition. All participants had a minimum of 5 mm maxillary constriction determined by calculating the differences between intermolar widths of maxilla and mandible (palatal cusp tips of upper molars to the central fossae of lower molars). Exclusion criteria: participants with syndromic characteristics, systemic diseases, or history of previous maxillary expansion/orthodontic treatment Group A: Damon system N = 41; F/M 28/13; age mean 13.8 SD 1.6 Group B: Hyrax N = 41; F/M 22/19; age mean 13.3 SD 1.5 Number randomised: 90 participants were randomised into 2 groups following a 1:1 pattern Number evaluated: 82
Interventions	Comparison: fixed tooth‐borne expansion (Hyrax) vs fixed tooth‐borne expansion (Damon system) Group A: Damon system N = 41 consisted of fixed appliance (0.022 inch (5.64 mm) dimension, self‐ligating brackets). Initial alignment was done sequentially with Insignia prefabricated 0.014 NiTi, 0.016 NiTi, and 0.014 x 0.025 NiTi archwires in Damon Arch Form. In addition, participants wore crossbite elastics full‐time until maxillary expansion was overcorrected by 20%. Group B: Hyrax N = 41 expansion appliance attached to the upper first premolars and first permanent molars activated with 1 turn of the screw/twice a day (0.25 mm/turn, 0.5 mm daily) until 20% over‐correction was achieved. On the same day of the Hyrax insertion, non‐self‐ligating brackets were bonded from maxillary right canine to left canine and mandibular right first molar to left first molar −
Outcomes	Skeletal and dental changes
Notes	"a minimum sample size of 44 patients/group was calculated to be needed when the effect size index was 0.70, α = 0.05, and power = 0.90"
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: "A person, external to the research group, generated random number blocks for all patients using an Excel worksheet to randomly allocate patients to each group, once they accepted participation in the study"
Allocation concealment (selection bias)	Low risk	Quote: "A person, external to the research group, generated random number blocks for all patients using an Excel worksheet to randomly allocate patients to each group, once they accepted participation in the study"
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Quote: "All diagnostic records were coded. The principal investigator was blinded with respect to treatment group and timing of each record when analyzing the diagnostic records"
Incomplete outcome data (attrition bias) All outcomes	Low risk	Quote: "A total of 41 patients in the Damon group and 41 patients in the Hyrax group started and completed treatment"
Selective reporting (reporting bias)	Low risk	Not detected
Other bias	Low risk	No other sources of bias identified

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Oliveira 2004.

*Study characteristics*
Methods	Study design: RCT (2 arms) Location: Department of Orthodontics and the Craniofacial Center, University of Illinois at Chicago, USA Number of centres: 3 university orthodontic clinics and 1 private practice Recruitment period: not stated Funding source: partially funded by a grant from the American Association of Orthodontists Foundation Clinician experience: not stated
Participants	Inclusion criteria: children who had maxillary permanent first premolars or non‐mobile maxillary deciduous first molar and unilateral or bilateral crossbite Exclusion criteria: any craniofacial anomaly Group A: mean age 11.9 years (range 10.7‐13.4); M/F 5/4 Group B: mean age 11.1 years (range 7.3‐14.6); M/F 1/9 Number randomised: 19 (Group A: 9; Group B: 10) Number evaluated: 19 (Group A: 9; Group B: 10)
Interventions	Comparison: fixed tooth‐tissue‐borne expansion (Haas) vs fixed tooth‐borne expansion (Hyrax) Group A (N = 9): tooth‐tissue‐borne palatal acrylic Haas‐type expander Group B (N = 10): tooth‐borne Hyrax expander (no palatal acrylic) Appliances were activated twice a day, 1 quarter turn each time (0.5 mm/day) until overcorrection was obtained. Appliance was then kept as a retainer for a further 3 months and then removed, at which point the outcomes were measured.
Outcomes	Molar expansion Anteroposterior cephalometry
Notes	"...projected sample size of 50 subjects" ‐ sample size not calculated
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: "Randomization was carried out by using a table of random numbers"
Allocation concealment (selection bias)	Low risk	Quote: "A staff member in the clinic was informed about every new patient suitable for the study and assigned the type of expander following the list order" Comment: it appears that allocation was done by someone not involved in the study. Probably done
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	It appears that it would have been possible to blind outcome assessors as the appliances were removed before final measurements were made; however, it was not mentioned.
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	Quote: "Randomization was carried out by using a table of random numbers based on a projected sample size of 50 subjects" Comments: no dropouts reported, but it is not clear if the 19 participants were gathered from the 50 previously selected
Selective reporting (reporting bias)	Low risk	Outcome measures described in the methods section were reported in full.
Other bias	High risk	No sample size calculation

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Oshagh 2012.

*Study characteristics*
Methods	Study design: RCT (2 arms) Location: Orthodontic Department, School of Dentistry, Shiraz University of Medical Sciences, Iran Number of centres: 1 Recruitment period: not stated Funding source: not stated Clinician experience: not stated
Participants	Inclusion criteria: healthy children with posterior bilateral crossbites and with growth potential Exclusion criteria: systemic diseases, previous orthodontic treatment, requiring headgear or an auxiliary component, cleft palate, anterior crossbite, children with extractions Group A: mean age 9.85 years (SD 1.87); M/F 6/19 Group B: mean age 11.2 years (SD 2.46); M/F 5/5 Number randomised: 35 (Group A: 25; Group B: 10) Number evaluated: not stated
Interventions	Comparison: removable tooth‐borne expansion (expansion plate with conventional expansion screw) vs removable tooth‐borne expansion (expansion plate with spring‐loaded expansion screw) Group A (N = 25): conventional screw activated by parent with a 1‐quarter turn twice/week (total 0.5 mm) Group B (N = 10): spring‐loaded screw activated by the orthodontist each month Both groups were treated by the same orthodontist following the same protocol. Active expansion continued until overcorrection (mean duration of treatment 5.3 months)
Outcomes	Molar and canine expansion Arch size changes Participant discomfort
Notes	No sample size calculation reported
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: "Randomization was accomplished by a randomization table"
Allocation concealment (selection bias)	Unclear risk	Quote: "Randomization was accomplished by a randomization table" Comment: allocation concealment not mentioned
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	It appears that it would have been possible to blind outcome assessors as the appliances were removed before casts were made; however, it was not mentioned.
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	No indication of whether or not there were any dropouts. Unclear how many children were included in the outcome assessment
Selective reporting (reporting bias)	High risk	Poor reporting of results. Only the overall mean monthly changes were reported. The study authors did not report the mean and SD for changes between the baseline and final measurement.
Other bias	High risk	Imbalance in the numbers/group (Group A: 25; Group B: 10) and sex (Group A: M/F 6/19; Group B: M/F 5/5) suggests a problem with the randomisation

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Ottaviano 2018.

*Study characteristics*
Methods	Study design: RCT treated vs untreated Location: ENT Clinic, Department of Neurosciences of Padua University Number of centres: 1 Recruitment period: from June 2015‐ January 2016 Funding source: this research did not receive any specific grant from funding agencies in the public, commercial, or not‐for‐profit sectors. Clinician experience: not stated
Participants	Inclusion criteria: with unilateral or bilateral posterior crossbite, transversal discrepancy of 4 mm between upper jaw and mandible (grade 3c or 4c of Index of Treatment Needs). mixed or early permanent dentition phase, with the first upper molars fully erupted and in prepubertal phase (stage CS1‐CS2) according to Cervical Vertebral Maturation method Exclusion criteria: genetic disease or congenital syndromes, systemic diseases, periodontal disease, ENT diseases, bad habits and mouth breathing; previous orthodontic treatment, not undergone any previous ENT surgery. Group A: RME 11 participants; mean age 8.27 ± 1.62 years Group B: untreated controls 11 participants; mean age 8.27 ± 1.25 years Number randomised: 22 (11 boys and 11 girls) Number evaluated: 22
Interventions	Comparison: fixed tooth‐borne expansion (Hyrax) vs untreated controls Group A RME (N = 11): for each child in the study group an Hyrax‐type RME was installed, designed with bands on permanent molars, an 8‐mm expansion screw (Leone S.p.A., Sesto Fiorentino, Firenze) and palatal arms resting on deciduous canines. Parents were instructed to activate the screw, with a quarter turn each day. The average expansion was of 6.04 ± 1.07 mm (range 5‐8 mm) with an average of 30.18 ± 5.33 total activations of the screw (range 25‐40 mm). Group B untreated controls (N = 11): untreated
Outcomes	Olfactory function Nasal resistances Nasal flows
Notes	Both groups underwent all nasal procedures (PNIF, AAR, and olfactory measurements) and the evaluation of nasal symptoms by means of SNOT 22. In particular, at the enrolment into the study before RME application (T0), at the end of the active phase of the expansion (T1, 25 to 40 days after RME application; mean 30.18 ± 5.33) and 6 months after the second examination (T2) for the study group and at the enrolment into the study (T0), after 1 month (T1) and 6 months after the second examination (T2) for the control group.
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Quote:"The subjects were then randomized into two groups" Comment: insufficient information on the method of sequence generation
Allocation concealment (selection bias)	Unclear risk	Not stated
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	Not stated
Incomplete outcome data (attrition bias) All outcomes	Low risk	Quote: "Of the 22 children enrolled none was lost during the 6‐months follow‐up and they all met scrupulously the three appointments"
Selective reporting (reporting bias)	Unclear risk	Poor reporting of results. Only graphs and figures without tables
Other bias	High risk	No sample size calculation reported

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Petrén 2008.

*Study characteristics*
Methods	Study design: RCT (4 arms) Location: Public Dental Health Service, Skane County Council, Sweden, and the Department of Orthodontics, Faculty of Odontology, Malmö University, Malmö, Sweden Number of centres: 3 Recruitment period: 2001‐2005 Funding source: supported by the Swedish Dental Society, Skane County Council, and the Faculty of Odontology, Malmö University, Sweden Clinician experience: 5 experienced general practitioners under the supervision of specialist orthodontists
Participants	Inclusion criteria: mixed dentition (all incisors and first molars erupted); unilateral posterior crossbite Exclusion criteria: sucking habits or sucking habit discontinued at < 1 year before the trial; previous orthodontic treatment Group A: mean age 9.1 years (SD 1.03); M/F 6/9 Group B: mean age 8.7 years (SD 0.82); M/F 6/9 Group C: mean age 8.3 years (SD 0.7); M/F 7/8 Group D: mean age 8.8 years (SD 0.7); M/F 7/8 Number randomised: 60 (15/group) Number evaluated: 60 (15/group)
Interventions	Comparison: fixed tooth‐borne expansion (quad‐helix) vs removable tooth‐borne expansion (expansion plate) vs composite onlays vs untreated control group Group A (N = 15): the quad‐helix consisted of a standard stainless steel arch with stainless steel bands attached with glass ionomer cement on the maxillary first molars. The quad‐helix was activated 10 mm before placement and then re‐activated every 6 weeks until a crossbite correction. The treatment result was retained for 6 months. Group B (N = 15): the expansion plate consisted of acrylic with an expansion screw and stainless steel clasps on the deciduous and permanent first molars. The plate was activated 0.2 mm once a week by the child until a normal transverse relationship was achieved. The treatment result was retained for 6 months. Group C (N = 15): the composite onlay technique was based on the theory that bite raising will inhibit the forced lateral movement and consequently allow the maxilla to grow and develop in the transverse dimension without locking the mandible in occlusion. The bite raising was achieved by bonding composite on the occlusal surfaces on both mandibular first molars. The composite onlay was checked every 6 weeks and removed after 1 year. Group D (N = 15): no treatment
Outcomes	Crossbite correction Molar and canine expansion Time to correction
Notes	The sample size for each group had previously been calculated and was based on a significance level 5% and a power of 90%, to detect a mean inter‐group difference in expansion of 2 mm (SD 1.5 mm). The sample size calculation showed that each group needed a minimum of 12 children. To increase the power even further and to compensate for conceivable dropouts during the study, it was decided to select 15 for each group. Furthermore, when planning the follow‐up study, it was decided to add 10 more participants (5 in each group) to increase the power even more and compensate for conceivable dropouts.
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: "The subjects were randomized as follows: 4 opaque envelopes were prepared with 20 sealed notes in each (5 notes for each group). Thus, for every new patient in the study, a note was extracted from the first envelope. When the envelope was empty, the second envelope was opened, and the 20 new notes were extracted as patients were recruited to the study. This procedure was then repeated 2 more times"
Allocation concealment (selection bias)	Low risk	Quote: "The envelope was in the care of 1 investigator...who was contacted and randomly extracted a note and informed the dentist which treatment strategy to use"
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Quote: "Measurements were blinded; the examiner was unaware of which treatment the patients had received or which models were taken at T0 and T1"
Incomplete outcome data (attrition bias) All outcomes	Low risk	Quote: "Data on all patients were analyzed on an intention‐to‐treat (ITT) basis... all patients, successful or not, were included in the final analysis" and "all patients finished the trial (Fig 5)"
Selective reporting (reporting bias)	Low risk	Outcome measures described in the methods section were reported in full.
Other bias	Low risk	No other sources of bias identified

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Ramoglu 2010.

*Study characteristics*
Methods	Study design: RCT Location: Department of Orthodontics of Selçuk University, Turkey Number of centres: 1 Recruitment period: not stated Funding source: Selçuk University Research Projects (2003‐2004) Clinician experience: not stated
Participants	Inclusion criteria: functional unilateral or bilateral posterior crossbite with transverse deficiency; eruption of the first permanent molars Exclusion criteria: sagittal skeletal problem; > 1 missing maxillary tooth in the right and left sides of the dentition Group A: mean age 8.63 years (SD 1.09); M/F 7/11 Group B: mean age 8.78 years (SD 1.21); M/F 6/11 Number randomised: 35 (Group A: 18; Group B: 17) Number evaluated: 35 (Group A: 18; Group B: 17)
Interventions	Comparison: fixed tooth‐tissue‐borne expansion (bonded acrylic appliance with semi‐rapid expansion) vs fixed tooth‐tissue‐borne expansion (bonded acrylic appliance with rapid expansion) A splint type tooth‐ and tissue‐borne modified bonded appliance was used for both groups. Group A (N = 18): semi‐RME activation was 2 quarter turns/day for the first week followed by 1 quarter turn/day every other day. The mean treatment time was 57.16 days (SD 21.52). Group B (N = 17): RME activation was 2 quarter turns/day throughout treatment, and the mean treatment time was 21.23 days (SD 8.36). Screw activation was ended when approximately 2 mm of overcorrection was achieved, and the screw was fixed by a ligature wire.
Outcomes	Molar and canine expansion Cephalometric treatment effects
Notes	No sample size calculation reported
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Quote: "The subjects were randomly divided into two groups" Comment: insufficient information on the method of sequence generation
Allocation concealment (selection bias)	Unclear risk	Quote: "The subjects were randomly divided into two groups" Comment: allocation concealment not mentioned
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	It may have been possible to blind outcome assessors if the appliances were removed before final measurements were made; however, this was not mentioned.
Incomplete outcome data (attrition bias) All outcomes	Low risk	No dropouts reported. Numbers of participants/group reported in the results tables matches the numbers that were randomised.
Selective reporting (reporting bias)	Low risk	Outcome measures described in the methods section were reported in full.
Other bias	Low risk	No other sources of bias identified

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Sollenius 2020.

*Study characteristics*
Methods	Study design: RCT Location: Public Dental Health Service, Halland County Council Number of centres: 10 Recruitment period: December 2013‐November 2018 Funding source: European Orthodontic Society Research grant (2015), Region Halland, Sweden. Clinician experience: all children were treated at 3 orthodontic specialist clinics and 10 general dentistry clinics by 2 orthodontic specialists and 17 general dentists. The 2 orthodontists who provided the information to the patients had 15 years of experience and treated all the patients in the specialist orthodontic clinics and did not treat any patients at the general dental clinics. The 17 general dentists had between 1 and 30 years of experience of interceptive orthodontic treatments.
Participants	Inclusion criteria: consecutive children, boys and girls; mixed dentition (between 8‐10 years of age); unilateral posterior crossbite including the first permanent molar and with a functional shift of > 1 mm; Class I occlusion Exclusion criteria: children with sucking habits or ceased sucking habits during the year before the study was started, who had previously undergone orthodontic treatment, who had severe crowding of teeth (extraction of teeth necessary) or with craniofacial syndromes Group A: quad‐helix treated by specialists (N = 28) (M/F 11/17; mean age 9.3 (7.4‐11.5) Group B: quad‐helix treated by general dentistry (N = 27) M/F 14/13; mean age 9.5 (8.1‐12.4) Group C: expansion plate treated by specialists (N = 27) M/F 15/12; mean age 8.7 (7.6‐10.7) Group D: expansion plate treated by general dentistry (N = 28) M/F 11/17; mean age 9.2 (6.5‐10.6) Group E: postponed treatment (N = 25) M/F 16/9; mean age 8.5 (7.7‐11.1) Group F: untreated control (N = 25) M/F 17/8; mean age 9.3 (6.5‐12) Number randomised: 110 (Groups 5 and 6 were not randomised)
Interventions	Comparison: fixed tooth‐borne expansion (quad‐helix) vs removable tooth‐borne expansion (expansion plate) vs no treatment Group A: quad‐helix treated by specialists ‐ consisted of a standard stainless steel arch with stainless steel bands attached with glass ionomer cement on the maxillary first molars Group B: quad‐helix treated by general dentistry ‐ consisted of a standard stainless steel arch with stainless steel bands attached with glass ionomer cement on the maxillary first molars Group C: expansion plate treated by specialist ‐ consisted of acrylic, with an expansion screw and stainless steel clasps on the primary and permanent first molars Group D: expansion plate treated by general dentistry ‐ consisted of acrylic, with an expansion screw and stainless steel clasps on the primary and permanent first molars Group E: postponed treatment Group F: untreated control
Outcomes	Palatal surface area changes Palatal projection area changes Palatal shell volume Linear maxillary and mandibular intercanine and intermolar distances as well as the success rate of crossbite correction Clinical effectiveness Cost analysis
Notes	Sample size calculation provided. Qutote: "With the assumption of means in variances for different groups taken from Primožič et al, and using multiple testing adjustment suggested by Dunnett and Tamhane, with standard parameters of 80% power and α 0.05, a number of 23 children/group was needed, and as we expected some dropouts to occur, the required sample size was estimated to be 25 in each of the groups."
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: "The randomization was prepared and carried out by an independent person not involved in the study and the randomization used blocks of 25 (5 + 5 + 5 + 5 + 5)."
Allocation concealment (selection bias)	Low risk	Quote: "Six opaque envelopes were prepared with 25 sealed notes in each (5 notes for each group), and for every new patient in the study, a note was extracted from the first envelope."
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Quote: "Blinding was performed of the outcome evaluator and the person who analysed the data."
Incomplete outcome data (attrition bias) All outcomes	Low risk	Quote: "Data on all children were evaluated on an intention‐to‐treat (ITT) basis; consequently, if the crossbite was not corrected during the trial period of 1 year, the outcome was judged as unsuccessful, and the possible change in palatal area and volume as well as linear expansion effect was quoted. Accordingly, all children, successful or not, were incorporated in the final analysis. Moreover, any withdrawals in the course of the trial were regarded as unsuccessful and with no expansion or changes. If data were lacking at the follow‐up or after treatment, the last observation carried forward principle was used."
Selective reporting (reporting bias)	Low risk	Outcome measures described in the methods section were reported in full.
Other bias	Low risk	No other sources of bias identified

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Sweden 2017.

*Study characteristics*
Methods	Study design: RCT (2 arms) Location: Postgraduate Dental Education Centre, Department of Orthodontics, Region Örebro County, Sweden Number of centres: 1 Recruitment period: from September 2010‐December 2015 Funding source: study was supported by the Uppsala‐Örebro Regional Research Council, Sweden, (grant number RFR‐72021). The funding sources had no role in design and conduct of the study; collection, management, analysis, and interpretation of the data; and preparation of the manuscript. The authors’ work was independent of the funders Clinician experience: all participants were treated by the same orthodontist (F.B.).
Participants	Inclusion criteria: uni‐ or bilateral crossbite with constricted maxilla and; age at diagnosis of 8‐13 years, with dental stage in the early or late mixed dentition Exclusion criteria: previous or ongoing orthodontic treatment, craniofacial syndromes, or cleft lip or palate Age: Group A (tooth‐borne), mean age 9.7 years (SD 1.5); Group B (tooth‐bone‐borne), mean age 10.2 years (SD 1.4) Sex: M/F 21/19 Number randomised: 54 (tooth‐borne: 27; tooth‐bone‐borne: 27) Number evaluated: 52 (tooth‐borne: 26; tooth‐bone‐borne: 26)
Interventions	Comparison: fixed tooth‐borne expansion (Hyrax) vs fixed tooth‐bone‐borne Group A (tooth‐borne) N = 26: treated with a tooth‐borne expander Group B (tooth‐bone‐borne) N = 26: treated with a tooth‐bone‐borne expander with two 1.7 × 8 mm mini‐screw implants (Orthoeasy®; Forestadent, Pforzheim, Germany) attaching the expander to the palate surface Both expanders were activated 2 quarter turns/day (0.5 mm) until the palatal cusps of the maxillary first molars contacted the buccal cusps of the mandibular first molars. Hence, both groups were overexpanded the same amount and had the same endpoint.
Outcomes	Pain and discomfort during the first week of RME, analgesic consumption, and daily activities Nasal airflow and nasal airway resistance were assessed for each nasal cavity separately, while a reference pressure (150 Pa) was measured for the contralateral nostril. Intermolar distances pre‐ and post‐expansion evaluated on study casts at the shortest intermolar linear distance at the gingival margins and the mesiobuccal cusp tips of the teeth, using a digital caliper
Notes	Dropouts 25%
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Participants were randomly allocated in blocks of different sizes, using the concealed allocation principle in a 1:1 ratio, to two groups, a tooth‐borne group and a tooth‐bone‐borne group.
Allocation concealment (selection bias)	Low risk	A computer‐generated randomisation list was created using SPSS software (version 17.0; SPSS, Chicago, IL, USA) and stored with a research secretary at the Postgraduate Dental Education Centre. Each time a participant gave his/her consent, the secretary was contacted by email to provide the information about which type of expander the participant should receive.
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Bazargani 2018: intermolar measurements were blinded; the examiner was unaware of which treatment the participants had received or which models were taken at pre‐ and post‐expansion. The care providers at the ENT unit who conducted all the rhinomanometry examinations were blinded to which group the participants were allocated to. Feldmann 2017: questionnaires were analysed by one of the co‐authors, who was blinded to the study and performed no orthodontic treatment on the participants.
Incomplete outcome data (attrition bias) All outcomes	High risk	Bazargani 2020: 52 of the enrolled 54 participants were randomised. Bazargani 2018: all performed rhinomanometric registration at T0, but only 30 showed up at T1 for the post‐expansion registration, of whom 16 had been randomised to the tooth‐borne and 14 to the tooth‐bone‐borne group. Dropouts: 25%: 3 in the tooth‐borne group and 7 in the tooth‐bone‐borne group did not show up for the follow‐up rhinomanometry appointments. Feldmann 2017: 50 of the enrolled 54 participants completed both questionnaires. One boy and one girl each in group A and group B did not submit their questionnaires despite several reminders. The overall response rate was 91%.
Selective reporting (reporting bias)	Low risk	Outcome measures described in the methods section were reported in full.
Other bias	High risk	A very large age range was used for inclusion in the sample: age at diagnosis of 8‐13 years, with dental stage in the early or late mixed dentition.

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Thilander 1984.

*Study characteristics*
Methods	Study design: RCT Location: Public Dental Health Service, Enköping, Sweden Number of centres: 1 Recruitment period: 1965‐1967 Funding source: Grant of Swedish Medical Research Council Clinician experience: orthodontist
Participants	Inclusion criteria: children with posterior crossbites. No further definition provided Exclusion criteria: none stated Group A: all children 5 years of age; M/F 13/20 Group B: all children 5 years of age; M/F 11/17 Number randomised: 68 (Group A: 34; Group B: 34) Number evaluated: 61 (Group A: 33; Group B: 28)
Interventions	Comparison: removable tooth‐borne expansion (grinding or grinding + expansion plate) vs no intervention Group A (N = 34): treatment procedure comprised interceptive measures in 2 steps: grinding and, in the event of unsatisfactory results, expansion plates Group B (N = 34): no treatment Group A were treated with grinding at age 5 years. Group B remained untreated until the study finished 8 years later at age 13 years. Follow‐up measurements to assess crossbite correction were made at age 6, 7, 10 and 13 years
Outcomes	Correction of crossbite
Notes	No sample size calculation reported
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Quote: "68 children...were randomly divided into two groups" Comment: insufficient information on the method of sequence generation
Allocation concealment (selection bias)	Unclear risk	Quote: "68 children...were randomly divided into two groups" Comment: allocation concealment not mentioned
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	Blinding not mentioned
Incomplete outcome data (attrition bias) All outcomes	High risk	The dropout rate was very different between the treated group (3%) and the untreated group (18%). Reasons reported were lack of interest in the untreated group and relocation from the town.
Selective reporting (reporting bias)	Low risk	Outcome measures described in the methods section were reported in full.
Other bias	Low risk	No other sources of bias identified

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Toklu 2015.

*Study characteristics*
Methods	Study design: RCT Location: Orthodontic clinic of Yeditepe University, Istanbul, Turkey Number of centres: 1 Recruitment period: 2‐year period Funding source: not stated Clinician experience: not stated
Participants	Inclusion criteria: patients who required maxillary expansion with unilateral or bilateral posterior crossbite and maxillary constriction in permanent dentition Exclusion criteria: previous orthodontic treatment; presence of congenitally missing; extracted maxillary canines, premolars, and first molars; presence of systemic or genetic disease Group A (Hyrax): mean age 14.3 (SD 2.3); M/F 5/8 Group B (Hybrid Hyrax): mean age 13.8 (SD 2.2); M/F 6/6 Number randomised: 26 (Group A: 13; Group B: 13) Number evaluated: 25 (Group A: 13; Group B: 12)
Interventions	Comparison: fixed tooth‐borne expansion (Hyrax) vs tooth‐bone‐borne expansion Group A (N = 13): treated with a traditional tooth‐borne Hyrax appliance attached to the maxillary first premolars and molars Group B (N = 12): treated with a tooth‐bone‐borne Hyrax appliance attached to the maxillary first molars and two miniscrews (1.8 mm diameter and 9 mm length)
Outcomes	Periodontal, dentoalveolar, and skeletal effects measured using CBCT
Notes	The sample size was calculated and indicated a total of 26 patients (13 participants in each group) for a study with a power of 0.90 and an alpha of 0.05.
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: "According to the order of referral with a randomization ratio of 1:1, they were randomly allocated to 2 groups by an orthodontist (D.G.‐C.) who did not know in advance which treatment the next patient would get."
Allocation concealment (selection bias)	Unclear risk	Not stated
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Quote: “Blinding was used at the analysis level”
Incomplete outcome data (attrition bias) All outcomes	Low risk	Only one lost Quote: "One patient who lost the palatal miniscrews 2 days after insertion of the expander because of consuming hard foods was excluded from the study"
Selective reporting (reporting bias)	Low risk	Outcome measures described in the methods section were reported in full.
Other bias	Low risk	No other sources of bias identified

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Ugolini 2015.

*Study characteristics*
Methods	Study design: RCT Location: Universities of Genova, Siena, and Insubria (Varese), Italy Number of centres: 3 Recruitment period: not stated Funding source: not stated Clinician experience: not stated
Participants	Inclusion criteria: in mixed dentition; unilateral posterior crossbite at least of the first permanent molar; upper deciduous second molars available as RME anchoring teeth; before the pubertal peak (CVM 1–3) Exclusion criteria Primary: previous orthodontic treatment; hypodontia in any quadrant excluding third molars; inadequate oral hygiene; temporomandibular joint disorders; craniofacial abnormalities Secondary: lack of records; loss of lower ‘‘E’’ and need for lingual arch, lip bumper, or space maintainer; lack of consensus; need for other orthodontic treatment during RME Group A (GrE): mean age 8.4 years (SD 1.1) Group B (Gr6): mean age 8.6 years (SD 1.3) Number randomised: 70 (31 boys and 39 girls), (Group A: 35; Group B: 35) Number evaluated: 70 (Group A: 35; Group B: 35)
Interventions	Comparison: fixed tooth‐bone‐borne expansion (Haas anchored on second deciduous molars) vs fixed tooth‐bone‐borne (Haas anchored on first permanent molars) Group A (N = 35): RME on second deciduous molars Group B (N = 35): RME on first permanent molars In GrE or Gr6, when RME was in situ, participants started the screw activation of 1 quarter turn a day (0.22 mm) until overcorrection was achieved and the RME remained in place for 10 months.
Outcomes	Crossbite correction Arch width measured on 3D dental casts
Notes	Sample size was calculated a priori based on a primary outcome (molar expansion, as continuous outcome) to obtain a statistical power of the study > 0.85, using the mean values and standard deviations of maxillary molar expansion after RME therapy found by Cozzani et al., at alpha of 0.05 and beta of 0.2). Based on these parameters, to have an 85% chance of detecting as significant (at the 2‐sided 5% level) a 5‐point difference between the 2 groups, with an assumed standard deviation of 20 and a loss to follow‐up of 20%, the sample size required was 30 participants in each group.
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: "Subjects were randomly assigned to group GrE (RME on second deciduous molars; Figure 1a) or Gr6 (RME on first permanent molars; Figure 1b) by using a stratified blocked randomization with random block sizes performed by an electronic computer program."
Allocation concealment (selection bias)	Low risk	Quote: "The treating clinician was blinded from the randomization procedure" After mail with the authors: "Randomization was carried out using an excel worksheet with random number generator"
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Quote: "The cast examiner was also blinded from the treatment protocol."
Incomplete outcome data (attrition bias) All outcomes	Low risk	No dropouts
Selective reporting (reporting bias)	Low risk	Outcome measures described in the methods section were reported in full.
Other bias	Low risk	No other sources of bias identified

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Venancio 2014.

*Study characteristics*
Methods	Study design: RCT Location: Pediatric Clinic at the School of Dentistry of the University Complutense of Madrid, Spain Number of centres: 1 Recruitment period: not stated Funding source: Fundación Investigación Médica Mutua Madrileña Clinician experience: not stated
Participants	Inclusion criteria: unilateral posterior cross‐bite and functional mandibular shift (≥1.5 mm) to the crossbite side Exclusion criteria: presence of skeletal asymmetries (measured on frontal and Hirtz radiographs), craniofacial anomalies, TMJ dysfunction, a history of neuromuscular disease or disease affecting neuromuscular performance, dental caries, extensive restorations, dental pain, previous or current orthodontic treatment and deciduous tooth mobility during functional evaluation Group A (EP) + Group B (HE): aged 7.1‐11.8 years Number randomised: 30 (15 boys and 15 girls), (Group A: 15; Group B: 15) Number evaluated: 30 (Group A: 15; Group B: 15)
Interventions	Comparison: removable tooth‐bone‐borne expansion (expansion plate) vs fixed tooth‐bone‐borne expansion (Hyrax) Group A (N = 15): expansion plate Group B (N = 15): Hyrax The EP had a midline 10‐mm screw, 4 stainless steel clasps on the deciduous and permanent first molars, an acrylic covering, and acrylic flat coverage of the occlusal surfaces of the left and right posterior teeth. The participants were instructed to use the EP day and night except for meals and tooth brushing. The Hyrax expander was an acrylic coverage bonded cap splint Hyrax*. In both groups, children or parents were instructed to turn the screw a quarter rotation (0.2 mm) twice a week (slow maxillary expansion), until overcorrection of the transverse relationship of 3 mm was observed. Participants were monitored weekly.
Outcomes	Changes in the maximum vertical opening Mandibular lateral shift and lateral excursion
Notes	No sample size calculation reported
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: "Patients were randomly assigned to two groups through a computer‐generated list: EP (N = 15) and HE [Hyrax expander] (N = 15)".
Allocation concealment (selection bias)	Unclear risk	Not stated
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Quote: "One calibrated examiner performed all kinesiographic measurements in a blinded manner, unaware of the presence of a posterior cross‐bite or the type of treatment"
Incomplete outcome data (attrition bias) All outcomes	Low risk	Quote: "No attrition bias occurred in this study because there were no dropouts"
Selective reporting (reporting bias)	Low risk	Outcome measures described in the methods section were reported in full.
Other bias	Low risk	No other sources of bias identified

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AAR: active anterior rhinomanometric; BAME: bone‐anchored maxillary expander; CBCT: cone‐bean computed tomography; CVM: cervical vertebral maturation; EDO: expander with differential opening: ENT: Ear, Nose and Throat; EP: expansion plate; F: female; M: male; N: number; PAC: posteroanterior cephalogram; PNIF: peak nasal inspiratory flow; RCT: randomised controlled trial; RME: rapid maxillary expansion; SD: standard deviation; SNOT 22: 22‐item SinoNasal Outcomes Test; TAME: tooth‐anchored maxillary expander; TMJ: temperomandibular joint

Characteristics of excluded studies [ordered by study ID]

Study	Reason for exclusion
Akim 2021	Not RCT
Alghamdi 2017	Not RCT
Altieri 2020	Crossbite not considered as an inclusion criteria
Altindis 2016	Crossbite not considered as an inclusion criteria
Alves 2020	Crossbite not considered as an inclusion criteria
Annarumma 2021	Not RCT
Baldini 2018	Crossbite not considered as an inclusion criteria
Baratieri 2014	Crossbite not considered as an inclusion criteria
Barone 2020	Not RCT
Biondi 2017	Not RCT
Calil 2020	Not RCT
Canan 2017	Crossbite not considered as an inclusion criteria
Caprioglio 2020	Not RCT
Cossellu 2019	Primary outcome was medicine effects on expanders
Cunha 2019	Crossbite not considered as an inclusion criteria
Davami 2020	Crossbite not considered as an inclusion criteria
Digregorio 2019	Not RCT
Erdur 2020	Not RCT
Farret 2015	Not RCT
Fastuca 2017	Not RCT
Ferreira 2016	Randomisation was performed after crossbite correction
Garcia 2016	Same appliance used in both groups
Garib 2021	Included Class I and Class III patients
Gianoni‐Capenakas 2021	Patients with no crossbites were included in both groups
Jacob 2019	It was not reported if there was crossbite correction as they stopped expansion at 8 mm
Jia 2021	Crossbite not considered as an inclusion criteria
Kabalan 2015	Crossbite was not considered as an inclusion criteria
Kayalar 2019	Patients were submitted to surgery
Kilic 2016	Not RCT
Lee 2021	Not RCT
Lione 2018	Crossbite not considered as an inclusion criteria
Malkoc 2021	Crossbite not considered as an inclusion criteria
Matos 2020	Crossbite not considered as an inclusion criteria
Melgaco 2014	Crossbite not considered as an inclusion criteria
Michelotti 2019	Not RCT
Mohan 2016	Not RCT
Nagrik 2020	Crossbite not considered as an inclusion criteria
Nieri 2020	Crossbite not considered as an inclusion criteria
Petrén 2011	Subset of participants from one of the included studies (Petrén 2008), plus other participants and matched controls
Ribeiro 2020	It was not reported if there was crossbite correction as they stopped expansion at 8 mm. Same sample used for Jacob 2019
Rinaldi 2018	Crossbite not considered as an inclusion criteria
Ring 2020	Crossbite not considered as an inclusion criteria
Taner 2018	Not RCT
Ugolini 2020	Crossbite not considered as an inclusion criteria
Weissheimer 2011	Crossbite not considered as an inclusion criteria
Yagci 2016	Crossbite not considered as an inclusion criteria

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RCT: randomised controlled trial

Differences between protocol and review

The original protocol and previous versions of this review included controlled clinical trials (CCTs) and quasi‐RCTs. To attempt to limit bias, we only included RCTs in this update.
We clarified that 'correction of crossbite' is the primary outcome of this review by listing it as such in the 'Methods' section.

Contributions of authors

Jayne Harrison (JH) and Deborah Ashby (DA) wrote the protocol and previous versions of the review.

For this update:

screening the search results and retrieving the papers: Alessandro Ugolini (AU), Klaus Batista (KB), Armando Silvestrini‐Biavati (ASB)
data extraction and risk of bias assessment: AU, KB
analysing the data and interpreting the results: AU, KB, JH
creating the summary of findings (GRADE) tables: KB
writing the results, discussion, and conclusions: AU, KB, Paola Agostino, ASB, JH

Sources of support

Internal sources

Department of Surgical and Diagnostic Sciences, University of Genoa, Italy

Provision of IT support and open access to the university library.
Royal Liverpool and Broadgreen University Hospitals Trust (RLBUHT)/NHS, UK
School of Dentistry, The University of Manchester, UK
Manchester Academic Health Sciences Centre (MAHSC) and the NIHR Manchester Biomedical Research Centre, UK

External sources

National Health Service (NHS), UK

Jayne Harrison received research and development grant funding support for previous versions of this review.
Cochrane Oral Health Group Global Alliance, UK

The production of Cochrane Oral Health reviews has been supported financially by our Global Alliance since 2011 (oralhealth.cochrane.org/partnerships-alliances). Contributors in the last 2 years have been the American Association of Public Health Dentistry, USA; AS‐Akademie, Germany; the British Association for the Study of Community Dentistry, UK; the British Society of Paediatric Dentistry, UK; the Canadian Dental Hygienists Association, Canada; the Centre for Dental Education and Research at All India Institute of Medical Sciences, India; the National Center for Dental Hygiene Research & Practice, USA; New York University College of Dentistry, USA; and Swiss Society of Endodontology, Switzerland.
National Institute for Health Research (NIHR), UK

This project was supported by the NIHR, via Cochrane Infrastructure funding to Cochrane Oral Health. The views and opinions expressed herein are those of the review authors and do not necessarily reflect those of the Evidence Synthesis Programme, the NIHR, the NHS, or the Department of Health and Social Care.

Declarations of interest

Alessandro Ugolini: involved in running and reporting of Ugolini 2015; however, he was not involved in the quality assessment of this study. Armando Silvestrini‐Biavati: involved in running and reporting of Ugolini 2015; however, he was not involved in the quality assessment of this study. Klaus Batista: no interest to declare Paola Agostino: no interest to declare Jayne Harrison: no interest to declare

New search for studies and content updated (conclusions changed)

References

References to studies included in this review

Alberta 2010 {published and unpublished data}

Lagravere MO, Carey J, Heo G, Toogood RW, Major PW. Transverse, vertical, and anteroposterior changes from bone-anchored maxillary expansion vs traditional rapid maxillary expansion: a randomized clinical trial. American Journal of Orthodontics and Dentofacial Orthopedics 2010;137(3):304. [DOI] [PubMed] [Google Scholar]
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Halıcıoğlu K, Yavuz I. Comparison of the effects of rapid maxillary expansion caused by treatment with either a memory screw or a Hyrax screw on the dentofacial structures--transversal effects. European Journal of Orthodontics 2014;36(2):140-9. [PMID: ] [DOI] [PubMed] [Google Scholar]
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Kilic 2008 {published data only}

Kilic N, Kiki A, Oktay H. A comparison of dentoalveolar inclination treated by two palatal expanders. European Journal of Orthodontics 2008;30(1):67-72. [DOI] [PubMed] [Google Scholar]

Lamparski 2003 {published data only}

Lamparski DG Jr, Rinchuse DJ, Close JM, Sciote JJ. Comparison of skeletal and dental changes between 2-point and 4-point rapid palatal expanders. American Journal of Orthodontics and Dentofacial Orthopedics 2003;123(3):321-8. [DOI] [PubMed] [Google Scholar]

Lippold 2013 {published data only}

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Massaro 2020 {published data only}

Massaro C, Garib D, Cevidanes L, Janson G, Yatabe M, Lauris JRP, et al. Maxillary dentoskeletal outcomes of the expander with differential opening and the fan-type expander: a randomized controlled trial. Clinical Oral Investigations 2021;25(9):5247-56. [DOI: 10.1007/s00784-021-03832-9] [PMID: ] [DOI] [PMC free article] [PubMed] [Google Scholar]
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McNally 2005 {published and unpublished data}

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Mossaz‐Joelson 1989 {published and unpublished data}

Mossaz-Joelson K, Mossaz C. Slow maxillary expansion: a comparison between bonded and banded appliances. European Journal of Orthodontics 1989;11:67-76. [DOI] [PubMed] [Google Scholar]

Nam 2020 {published and unpublished data}

Nam HJ, Gianoni-Capenakas S, Major PW, Heo G, Lagravere MO. Comparison of skeletal and dental changes obtained from a tooth-borne maxillary expansion appliance compared to the Damon system assessed through a digital volumetric imaging: a randomized clinical trial. Journal of Clinical Medicine 2020;9(10):1-14. [DOI: 10.3390/jcm9103167] [DOI] [PMC free article] [PubMed] [Google Scholar]

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Oshagh M, Momeni Danaei Sh, Hematiyan MR, Hajian Kh, Shokoohi Z. Comparison of dental arch changes and patients' discomforts between newly designed maxillary expansion screw and slow expansion procedures. Journal of Dentistry, Shiraz University of Medical Sciences 2012;13(3):110-9. [Google Scholar]

Ottaviano 2018 {published and unpublished data}

Ottaviano G, Maculan P, Borghetto G, Favero V, Galletti B, Savietto E, et al. Nasal function before and after rapid maxillary expansion in children: a randomized, prospective, controlled study. International Journal of Pediatric Otorhinolaryngology 2018;115:133-8. [DOI: 10.1016/j.ijporl.2018.09.029] [DOI] [PubMed] [Google Scholar]

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Petrén S, Bjerklin K, Bondemark L. Stability of unilateral posterior crossbite correction in the mixed dentition: a randomized clinical trial with a 3-year follow-up. American Journal of Orthodontics and Dentofacial Orthopedics 2011;139:e73-81. [10.1016/j.ajodo.2010.06.018] [DOI] [PubMed] [Google Scholar]
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Sweden 2017 {published and unpublished data}

Bazargani F, Lund H, Magnuson A, Ludwig B. Skeletal and dentoalveolar effects using tooth-borne and tooth-bone-borne RME appliances: a randomized controlled trial with 1-year follow-up. European Journal of Orthodontics 2021;43(3):245-53. [DOI: 10.1093/ejo/cjaa040] [DOI] [PubMed] [Google Scholar]
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Toklu MG, Germec-Cakan D, Tozlu M. Periodontal, dentoalveolar, and skeletal effects of tooth-borne and tooth-bone-borne expansion appliances. American Journal of Orthodontics and Dentofacial Orthopedics 2015;148:97-109. [DOI: ] [DOI] [PubMed] [Google Scholar]

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References to studies excluded from this review

Akim 2021 {published data only}

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Digregorio 2019 {published data only}

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Erdur 2020 {published data only}

Erdur EA, Yıldırım M, Karatas RMC, Akin M. Effects of symmetric and asymmetric rapid maxillary expansion treatments on pharyngeal airway and sinus volume: a cone-beam computed tomography study. Angle Orthodontist 2020;90(3):425-31. [DOI] [PMC free article] [PubMed] [Google Scholar]

Farret 2015 {published data only}

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Fastuca 2017 {published data only}

Fastuca R, Lorusso P, Lagravere MO, Michelotti A, Portelli M, Zecca PA, et al. Digital evaluation of nasal changes induced by rapid maxillary expansion with different anchorage and appliance design. BMC Oral Health 2017;17(1):113. [DOI] [PMC free article] [PubMed] [Google Scholar]

Ferreira 2016 {published data only}

Ferreira FN, Gondim JO, Neto JJ, Dos Santos PC, Freitas Pontes KM, Kurita LM, et al. Effects of low-level laser therapy on bone regeneration of the midpalatal suture after rapid maxillary expansion. Lasers in Medical Science 2016;31(5):907-13. [DOI] [PubMed] [Google Scholar]

Garcia 2016 {published data only}

Garcia VJ, Arnabat J, Comesana R, Kasem K, Ustrell JM, Pasetto S, et al. Effect of low-level laser therapy after rapid maxillary expansion: a clinical investigation. Lasers in Medical Science 2016;31(6):1185-94. [DOI] [PubMed] [Google Scholar]

Garib 2021 {published data only}

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Gianoni‐Capenakas 2021 {published data only}

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Jacob 2019 {published data only}

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Michelotti 2019 {published data only}

Michelotti A, Rongo R, Valentino R, D'Antò V, Bucci R, Danzi G, et al. Evaluation of masticatory muscle activity in patients with unilateral posterior crossbite before and after rapid maxillary expansion. European Journal of Orthodontics 2019;41(1):46-53. [DOI] [PubMed] [Google Scholar]

Mohan 2016 {published data only}

Mohan CN, Araujo EA, Oliver DR, Kim KB. Long-term stability of rapid palatal expansion in the mixed dentition vs the permanent dentition. American Journal of Orthodontics and Dentofacial Orthopedics 2016;149(6):856-62. [DOI] [PubMed] [Google Scholar]

Nagrik 2020 {published data only}

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Nieri 2020 {published data only}

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Ribeiro 2020 {published data only}

Ribeiro GL, Jacob HB, Brunetto M, Pereira JS, Tanaka OM, Buschang PH. A preliminary 3-D comparison of rapid and slow maxillary expansion in children: a randomized clinical trial. International Journal of Paediatric Dentistry 2020;30(3):349-59. [DOI] [PubMed] [Google Scholar]

Rinaldi 2018 {published data only}

Rinaldi MR, Azeredo F, Lima EM, Rizzatto SM, Sameshima G, Menezes LM. Cone-beam computed tomography evaluation of bone plate and root length after maxillary expansion using tooth-borne and tooth-tissue-borne banded expanders. American Journal of Orthodontics and Dentofacial Orthopedics 2018;154(4):504-16. [DOI] [PubMed] [Google Scholar]

Ring 2020 {published data only}

Ring IJ, Nevéus T, Markström A, Magnuson A, Bazargani F. Rapid maxillary expansion in children with nocturnal enuresis: a randomized placebo-controlled trial. Angle Orthodontist 2020;90(1):31-8. [DOI] [PMC free article] [PubMed] [Google Scholar]

Taner 2018 {published data only}

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Weissheimer 2011 {published data only}

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PERMALINK

Orthodontic treatment for posterior crossbites

Alessandro Ugolini

Paola Agostino

Armando Silvestrini-Biavati

Jayne E Harrison

Klaus BSL Batista

Abstract

Background

Objectives

Search methods

Selection criteria

Data collection and analysis

Main results

Authors' conclusions

Plain language summary

Summary of findings

Summary of findings 1. Removable tooth‐borne expansion (expansion plate) versus observation for posterior crossbite.

Summary of findings 2. Fixed tooth‐borne expansion (quad‐helix) compared with observation for posterior crossbite.

Summary of findings 3. Fixed tooth‐borne expansion (Hyrax) compared with observation for posterior crossbite.

Summary of findings 4. Fixed tooth‐borne expansion (quad‐helix) compared with removable tooth‐borne expansion (expansion plate) for posterior crossbite.

Summary of findings 5. Fixed tooth‐tissue‐borne expansion (Haas) versus fixed tooth‐borne expansion (Hyrax) for posterior crossbite.

Summary of findings 6. Fixed tooth‐borne expansion (Hyrax) versus tooth‐bone‐borne expansion for posterior crossbite.

Summary of findings 7. Fixed tooth‐borne expansion (Hyrax) versus bone‐borne expansion for posterior crossbite.

Background

Description of the condition

Description of the intervention

How the intervention might work

Why it is important to do this review

Objectives

Methods

Criteria for considering studies for this review

Types of studies

Types of participants

Types of interventions

Types of outcome measures

Primary outcomes

Secondary outcomes

Search methods for identification of studies

Electronic searches

Searching other resources

Data collection and analysis

Selection of studies

Data extraction and management

Assessment of risk of bias in included studies

Measures of treatment effect

Unit of analysis issues

Dealing with missing data

Assessment of heterogeneity

Assessment of reporting biases

Data synthesis

Subgroup analysis and investigation of heterogeneity

Sensitivity analysis

Summary of findings and assessment of the certainty of the evidence

Results

Description of studies

Results of the search

1.

Included studies

Characteristics of trial setting

Location

Details of studies that reported location

Number of centres

Clinician experience

Details of studies that reported clinical experience

Funding

Details of studies that reported funding

Recruitment dates

Characteristics of the participants

Age

Dentition

Malocclusion

Characteristics of the interventions and comparisons

Single study comparisons

Characteristics of the outcomes

Excluded studies

Risk of bias in included studies

Overall risk of bias

2.

Allocation