Predictability of mesiodistal movement of upper and lower molars with clear aligners: a systematic review

Abstract

Objectives

To analyze and summarize the current scientific evidence regarding the clinical predictability of mesiodistal movements of upper and lower molars in patients treated with clear aligners without auxiliary aid.

Materials and Methods

This review followed PRISMA guidelines and was registered in PROSPERO (CRD42022357639). Databases were searched up to September 2024. Data extraction was performed independently by two reviewers, risk of bias was assessed using the ROBINS-I tool, and certainty of evidence was evaluated qualitatively using the GRADE tool.

Results

919 articles were identified, and six prospective and retrospective studies met the inclusion criteria, predominantly using the Invisalign system. Upper molar predictability was 61.1 ± 9.1% for movements ranging from 0.45 to 3.2 mm. Lower molar distalization showed lower predictability and molar mesial movement had median predictability rates of 85.6 ± 1.1%. Moderate to serious risk of bias and very low quality of evidence was found.

Conclusions

Upper molar distalization using clear aligners appears to be predictable for distalization from 1.5 to 3.2 mm. Anchorage reinforcement or overcorrection should be considered when planning mesiodistal movements. Standardization of the measurement method is necessary to improve efficacy of these systems.

INTRODUCTION

In the last 20 years, clear aligner therapy has experienced exponential growth in orthodontics with the creation of multiple aligner systems.¹ However, there is a scarcity of robust scientific evidence regarding the effectiveness and predictability of these appliances, which primarily focus on the predictability of anterior tooth movements.^2–4 However, the lack of predictability and/or precision for specific tooth movements has been identified as one of the major limitations of treatment with aligners.⁵

In this regard, aligner predictability of different movements has been analyzed through several approaches.^6–16 It has been stated that most movements may not be predictable enough with aligners, with current evidence pointing toward a low to moderate level of certainty.⁷ More recently,⁶ rotational movement of anterior teeth and premolars was found to have the greatest levels of inaccuracy. Nevertheless, orthodontic treatment of complex clinical cases, ie, severe Class II or Class III malocclusions, demands significant sagittal movement, particularly in the posterior region. Previous scientific evidence^11,17,18 regarding predictability of distal movement of the upper and lower molars has reported a predictability around 70% for distalization of 2–3 mm in the upper molars. However, evaluating mesial movement of molars and establishing a common methodology for measuring the predictability and efficiency of this movement is essential for a comprehensive understanding. Additionally, it is crucial to assess how effective aligner systems are without the use of auxiliary elements, to determine the most effective methods for treating these malocclusions.

The primary aim of the present systematic review was to analyze and summarize the currently available scientific evidence regarding the clinical predictability of mesiodistal movement of upper and lower molars in patients treated with clear aligners.

MATERIALS AND METHODS

Eligibility Criteria (PECOS Question)

Original research articles were selected based on the following criteria:

• Participants: Patients, with no age or gender restrictions and who underwent orthodontic treatment with aligners and mesiodistal movement of upper and lower molars, were included.

• Exposure: Studies that evaluated predicted versus achieved mesiodistal movement of upper and lower molars in patients with clear aligners without auxiliary appliances.

• Comparison: Virtual planning compared to the finally achieved mesiodistal movement.

• Outcome: Predictability and/or accuracy of mesiodistal movement of upper and lower molars with clear aligners assessed in millimeters (mm) or in percentage (%).

• Study design: Included randomized and nonrandomized clinical trials, cohort studies, case-control studies, and prospective, retrospective, and cross-sectional studies.

Information Sources and Search Strategy

The following electronic databases: PubMed, Scopus, Web of science, Cochrane, and Google Scholar, were used to study search results up to September 2024, with no date or language restriction. Details regarding the search string developed and used for each database have been compiled in Supplementary Table 1. A manual search of the literature was also performed.

Data Collection Process

Data extraction was performed by two experienced reviewers independently. Inconsistencies and disagreements were resolved by discussion of the two reviewers (CGM, AOP) or the involvement of another collaborator (AIL) until a consensus was reached. Study authors were contacted by email to request missing data or clarify information relevant to the study when necessary. If no response was obtained, the available data were analyzed.

Assessment of Risk of Bias of Included Studies

Quality assessment of the included articles was carried out using the Risk Of Bias In Nonrandomized Studies—of Interventions (ROBINS‐I) tool,¹⁹ which evaluated the risk of bias across several domains, and each domain was evaluated to determine the overall risk of bias in the nonrandomized studies.

Certainty Assessment

The overall certainty of evidence of the included studies was assessed using the Grading of Recommendations, Assessment, Development, and Evaluation tool (GRADE Handbook)²⁰ rating studies as “high,” “moderate,” “low,” or “very low.” Five items were considered: risk of bias, inconsistency, indirectness, imprecision, and other considerations.

RESULTS

Study Selection

The electronic literature search yielded 919 results (Figure 1), whereas no papers were identified from manual search or references of included articles. After removal of duplicates, 637 articles were assessed for screening and, after title/abstract reading, 599 articles were excluded, leaving 38 articles to be read in full text and checked against the eligibility criteria (Supplementary Table 2). Finally, six papers (three retrospective and three prospective studies) were considered eligible and were included in this systematic review.

Figure 1.

PRISMA diagram of article retrieval.

Study Characteristics

Table 1 presents the characteristics of each study in detail. Three of the included articles were retrospective studies^21–23 and the other three were prospective studies.^24–26 All papers used the Invisalign System except one, which used Angel aligners.²² The mean age of patients ranged from 23.2 ± 6.6 years to 32.9 ± 16.3 years. Average treatment time ranged from 6.7 to 20.2 months of treatment, and the mean number of aligners used ranged from 15.2 to 43.5 aligners; however, these variables were not specified in all studies, so authors were contacted for clarification.

Table 1.

Summary of Included Studies^a–g

First Author (y)	Study Design	Sample Size	Outcome	Aligner System	Superimposition Software	Method Reliability	Mean Age (y)	Average Number of Aligners (N)/Average Treatment Time (M)	Studied Movement/Measured Teeth	Attachments	Predictability of Mesiodistal Movements				P	Authors’ Conclusions
											Differences Predicted (P) – Achieved (A) Tooth Movement (mm)		Mean Accuracy (%)
Simon et al. (2014)	Retrospective	15 molars	Treatment accuracy (%)e	Invisalign	Software Surfacer 10.0 (Imageware/Siemens PLM Software)	NR	32.9 ± 16.3	18/NR	Distalization/U6; U7	7 molars with horizontal bevelled gingival attachments	U6; U7: 1.5 – 3.2 mm (P – A)		U6 with att:88.4 ± 0.2%		.38	Bodily tooth movements such as molar distalization, incisor torque, as well as premolar, can be accomplished using the Invisalign system.
										8 molars without attachments			U6 no att: 86.9 ± 0.16%		.46
Saif et al. (2022)	Prospective	142 molars	Percentage accuracyf	Invisalign	GOM Inspect Suite Software 2020	0.96a	25.4	20/6.7	Distalization/U6; U7	56.3% of molars with attachment	U6; U7: 2.6 mm (P – A)		U6: 75.5%**		<.0001	Invisalign could be used successfully for adult patients requiring 2–3 mm of maxillary molar distalization and clinicians should be aware of the countereffects. Attachments have no enhancement effect on the outcome of maxillary molar distalization.
													U7: 72.2%**		<.0001
Cong et al. (2022)	Retrospective	160* molars	Percentage accuracyf	Invisalign	SlicerCMF version 4.11.0 Software	Mx: 0.95a	23.2 ± 6.6	43.5/16.1	Mesialization and distalization/U6; U7; L6; L7	NR	U6M: 0.5 ± 0.3 mm (P) – 0.5 ± 0.3 mm (A)d		U6M: 84.9%d		.9217	ClinCheck prediction of tooth movement was more accurate for mesial displacement than distal displacement, particularly for mandibular teeth.
											NR		U6D: 68.9%d		NR
													U7M: 84.2%d		NR
													U7D: 56.9%d		NR
						Mn: 0.99a							L6M: 84.5%d		NR
													L6D: 30.8%d		NR
													L7M: 86.6%d		NR
													L7D: 18.1%d		NR
Miao et al. (2023)	Prospective	79 molars	Percentage accuracyf	Angle Align	GOM Inspect Suite Software 2022	0.949b	25.94	15.2/NR	Distalization/U6; U7	25 U6; 20 U7 with attachments	U6: 2.17 ± 1.03 mm (P) – 1.25 ± 0.79 mm (A)**	U6 with att: 0.70 ± 0.70 mm (P – A)	57.6%c	55.9%	.0000	Approximately 2 mm maxillary molar distalization was achieved. Attachments did not substantially improve the differences in the second molar distalization between the predicted and achieved movements. However, attachment use improved first molar distal movement. Ovecorrection might be a further research interest as a solution.
												U6 no att: 1.34 ± 0.69 mm (P – A)	57.6%c		.011
						0.953b					U7: 2.66 ± 1.15 mm (P) – 1.41 ± 1.00 mm (A)**	U7 with att 1.37 ± 0.97 mm (P – A)	53%c		.0000
												U7 no att 1.14 ± 0.86mm (P – A)	53%c		.416
Castroflorio et al. (2023)	Prospective	632 molars	Lack of correction (mm) = P – A	Invisalign	Geomagic Qualify Software (3D Systems)	0.99b	30.8 ± 12.0	27/9.8	Distalization/U6; U7; L6; L7	NR	U6: 0.49 (0.74) mm (P) – 0.28 (0.78)d (A)		57.1%c		<.0001	For mild-to-moderate cases, it is demonstrated that movement of second molars is mostly unexpressed.
											U7: 0.45 (0.78) mm (P) – 0.24 (0.81) mmd (A)		53.3%c		<.0001
											L6: 0.21 (0.31) mm (P) – −0.06 (0.42) mmd (A)		−28.6%c		<.0001
											L7: 0.17 (0.27) mm (P) – −0.11 (0.36) mmd (A)		−64.7%c		<.0001
Tang et al. (2023)	Retrospective	60 molars	Differences (mm) = P – A	Invisalign	Geomagic Studio 12.0 Software (3D Systems)	0.997a	27.2 ± 6.4	NR/20.2 ± 6.5	Mesialization/L6	47 Rectangular attachment 17 Optimized attachment	L6: 1.55 ± 1.16 mm (P) – 2.45 ± 1.27 mm (A)**		41.95%c		<.0001	The average mandibular reciprocal relative loss of anchorage is 25% for L4 extraction and 40% for L5 extraction in CAT so clinicians can determine the appropriate extraction treatment plan with improved precision.

^* Estimation of the number of molars assuming that each patient has eight molars. ** Statistical significance at P value <.05.

^a Pearson correlation coefficient for superimposition method. ^b Intraclass correlation coefficient for the measurement method, intrarater reliability. ^c Value calculated with the data provided in the article. ^d Median percentage accuracy. ^e Treatment accuracy (%): between clinical achieved tooth movement and predicted tooth movement; ^f Percentage accuracy: 100% − [(Ipredicted − achievedI/IpredictedI) × 100%].

^g m indicates months; U6, upper first molar; U7, upper second molar; L6, lower first molar; L7, lower second molar; U6MC, U6 mesial cusp; U6DC, U6 distal cusp; U6M, U6 mesial movement; U6D movement, U6 distal; Not reported, NR.

Several studies assessed the predictability of mesial and distal movement of upper and lower molars in percentages,^24,25 whereas other articles used millimeters between the predicted and achieved movement.^21,22,26,27 The included studies reported the use of different superimposition software: Software Surfacer 10.0,²⁸ GOM Software Suite Software 2020,¹ and 2022 version,²² SlicerCMF version 4.11.0,²⁵ Geomagic Qualify software,²¹ and Geomagic Studio software.²⁶

Risk of Bias Assessment

The risk of bias (RoB) assessments for the included studies revealed variability in methodological quality (Table 2). Three of the included studies were classified as moderate risk of bias,^24–26 and the other three were classified as serious risk of bias.^21–23 Serious risk of bias was predominantly found in patient selection,^21,22 missing data,^18,20 and outcome measurement.¹⁹ These assessments highlighted differences in the quality of the studies that may impact the reliability of their findings.

Table 2.

Risk of Bias of Observational Studies by ROBINS-I Quality Assessment Scale

Authors	Domains
	Pre-intervention		Intervention	Post-intervention				Overall RoB Judgment
	Bias due to Confounding	Bias in Selecting Participants for Study	Bias in Classifying Interventions	Bias due to Deviations From Intended Intervention	Bias due to Missing Data	Bias in Measuring Outcomes	Bias in Selecting Reported Results
Simon et al.	Low	Moderate	Low	Moderate	Moderate	Low	Low	Moderate
Saif et al.	Serious	Moderate	Low	Low	Serious	Moderate	Moderate	Serious
Cong et al.	Moderate	Low	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate
Miao et al.	Moderate	Serious	Low	Moderate	Moderate	Serious	Moderate	Serious
Castroflorio et al.	Moderate	Serious	Moderate	Moderate	Serious	Moderate	Low	Serious
Tang et al.	Moderate	Moderate	Low	Moderate	Moderate	Low	Moderate	Moderate

Certainty of Evidence

The evaluation of the certainty of the evidence assessed by the GRADE tool is described in Table 3. A narrative synthesis was conducted due to the impossibility of performing meta-analysis since the necessary data were incomplete. The quality of evidence was graded as very low for the studied outcome. This was based on the study design, which results presented serious limitations in terms of inconsistency and imprecision.

Table 3.

Narrative GRADE Evidence Profile Table

Certainty Assessment						Certainty Assessment
Outcome	Study Design	Sample Size (Studies)	Risk of Bias	Inconsistency	Indirectness	Imprecision	Other Considerations	Impact (Summary Narrative Description)	Quality of Evidence (GRADE)
Predictability of distalization of upper first and second molars	Prospective and retrospective studies	592 molars (5 studies)	Not seriousa	Serious	Not serious	Serious	None	Mean accuracies for upper molar distalization were reported from 55.95% to 86.9%24–28	⨁◯◯◯ VERY LOW
Predictability of distalization of lower first and second molars	Prospective and retrospective studies	356 molars (2 studies)	Not seriousa	Serious	Not serious	Serious	None	Accuracies for lower molar distalization range from 30.8% L6 and 18.1% L7.28 It was found out that lower molars are the worst teeth to control when planning movements along the arch.24	⨁◯◯◯ VERY LOW
Predictability of mesialization of lower first and second molars	Retrospective studies	100 molars (2 studies)	Not seriousa	Serious	Not serious	Serious	None	Accuracy of lower molar mesialization was 84.5%–86.8%.28 In extraction cases, accuracy of lower molar mesialization in extraction cases was of 42%.29	⨁◯◯◯ VERY LOW

^a Risk of bias of observational studies by ROBINS-I quality assessment scale.

DISCUSSION

The present systematic review provided essential clinical insights regarding the predictability of mesiodistal movement of upper and lower molars, comparing predicted and achieved movement, without the use of auxiliaries such as intermaxillary elastics. Distal movement of the molars is a key movement in orthodontics due to anchorage requirements and the need for this type of movement in the correction of sagittal skeletal and dentoalveolar malocclusions. Some studies^21–26 have shed light on the predictability of upper molar distalization, revealing promising outcomes.

Several methods for measuring predictability have been described in the literature, ranging from using lateral radiographs to 3D models. As observed in clinical practice, there is a trend toward digital superimposition^1,24,25,27 using best-fit matching tools, which have shown adequate reproducibility and accuracy. However, a variety of superimposition methods are available, which may result in different superimposition and measurement techniques, leading to a relevant grade of variability in results and a lack of comparability between studies.

The variation in measurement methods could impact the reported predictability of upper molar distalization. For instance, predictability of upper molar distalization up to 1.5 mm with aligner therapy has been cited to be 87.6%, thus demonstrating better predictability of movement in molars with attachments.²⁴ Other studies have reported a relatively lower overall accuracy rate of 73.8%, highlighting also the slightly higher efficiency of maxillary first molar, compared to second molar movement, when a mean distalization movement of 2.6 mm was prescribed.²³ It may be inferred that distalization of around 2 mm is predictable at approximately 70%; however, there is no scientific evidence to support greater distalization. Conversely, other authors²⁵ found lower predictability values for distal movement of upper molars, being 68.9% for first molars and 56.9% for second molars. However, it is worth noting that predicted and achieved values for these teeth were not provided; therefore, although the reasons for these significantly lower percentages compared to other studies cannot be inferred, the magnitude of the planned movement could be a critical factor in predictability. In line with this, some authors evaluated the biomechanical behavior of 316 upper molars²⁴ and concluded that first molars lost 0.4 mm for every prescribed 1 mm. This represented 60% predictability of planned movement, and suggested that the main associated factor was the absence of attachments in 25% of the molars.²¹ Similar results of 55.9% predictability were reported with other types of aligners systems²² when studying the effect of distalization, observed in the vertical plane, on the inclination and torque of the molars (Figure 2). Despite finding similar results with Angel Aligners to those obtained with the Invisalign system, more research is needed to establish a definitive conclusion.

Figure 2.

Predictability of mesiodistal movements of upper and lower molars (P⁺). * Mean calculated based on the results obtained from the articles included in Table 1: UD;^21–25 UM;²⁵ LD;^21,25 LM.^{25 +} Median percentage accuracy. ^$ No mm movement data was reported by the authors.²⁵

Attachments may play a role in enhancing the predictability of dental movement, as they are employed to improve system efficacy,²⁸ varying in shape and size and requiring careful planning during treatment.²⁹ Despite their widespread use, several studies have found that attachments did not necessarily improve the predictability of maxillary molar distalization.^18–20 Although some researchers claimed that attachments enhanced aligner retention and orthodontic tooth movement,²⁷ others argued that their impact on efficacy was not significant in orthodontic practice and outcomes. Various authors have evaluated the impact of attachments on maxillary molar distalization, concluding that their use did not improve the predictability of this movement, with other researchers reaching the same conclusion.²² Additionally, a recent systematic review on rotational movement noted that, whereas the use of attachments has been reported as prognostic for better efficacy, this did not necessarily translate into a significant effect in practice, meaning that they may not necessarily influence treatment outcome.⁶ However, finite element analysis research found that including attachments could be an essential strategy when referring to upper second molar movement, recommending the use of rectangular vertical attachments on posterior teeth to counteract the tipping tendency caused by distalization.³⁰ Therefore, the inconsistent findings across studies suggest that the role of attachments in enhancing the predictability of distalization remains inconclusive and may likely vary by clinical scenario.

Since attachments have not been proven to significantly improve the predictability of distal movement, other auxiliaries are needed. Studies using auxiliary systems,³¹ such as intermaxillary elastics, to improve upper molar distalization have reported predictability rates from 68% to 79.9%. However, some patients would require more assistance than that provided by the auxiliary elements such as elastics and, therefore, leading to the use of temporary anchorage devices (TADs). The effectiveness of distalization with clear aligners aided by TADs was recently compared to fixed appliance therapy in a recently published randomized clinical trial,³² which concluded that clear aligners had better control of vertical dimension and molar tipping during distalization. Similarly, when comparing aligners to fixed appliances with the use of TADs as auxiliary elements during distalization, no differences were observed in the mean distalization time; however, they were found to mitigate the undesirable effects of elastics.³³ Additionally, the use of other fixed distalization methods, such as pendulum devices, have shown similar results, although they exhibit poor vertical control compared to aligner-based distalization.³⁴

A scarcity of scientific evidence was observed regarding distal movement of lower molars.³⁵ Relatively low predictability for distal movement was reported, with lower molars displaying less than 31% predictability, and 18.1% was reported for the distalization of second molars.²⁵ Also, the type of movement obtained was considered a critical issue when investigating distalization. In this respect, some authors have studied predictability of lower molar distalization with lateral cephalograms, concluding that aligners were effective in this movement primarily due to tipping.^36,37 Similar results were found when measuring predictability with cone beam computed tomography, which led to the recommendation to overcorrect distal movement, given that distalization largely occurs due to molar tipping.³⁵

For mesial movement of upper or lower molars, predictability observed for upper molars ranged from 84.2% to 84.9%, whereas lower molars were between 84.5% and 86.8%.²⁵ However, these values should be viewed with caution given that no data on the magnitude of the mesial movement planned was available in the literature. It must be highlighted that mesial movement of posterior teeth has been addressed in extraction cases and was found to exceed the predicted values. Some studies²⁵ showed that mesial movement of lower molars was greater than what was predicted, suggesting loss of anchorage when mesial space was available. Such discrepancies are of significance as they could potentially prevent achieving the intended molar position, thereby impacting treatment outcomes.

Similarly, when strictly studying the undesired loss of anchorage during sequential distalization, several authors reported that they had not considered this biomechanical effect as one of the limitations of their study.^23,24 Other authors used TADs between the upper first molar and second premolar to prevent loss of anchorage.²² However, some authors measured the effect of upper molar distalization on incisors and observed flaring of the central and lateral incisors. Therefore, to prevent flaring, they proposed the use of Class II elastics,²³ which has also been advocated by other researchers to prevent this issue.^31,38

When mesialization with clear aligners was aided by auxiliary elements such as intermaxillary elastics,^38–40 it has been consistently found that upper and lower molars migrated more mesially than predicted,^40,41 displaying an increase in mesiopalatal rotation for upper molars and mesial tipping for lower molars in cases with Class III or Class II elastics. respectively.⁴⁰ When comparing treatment outcomes between clear aligners and fixed appliances, similar results were found regarding the amount of mesial tipping observed of the first and second molars during mesialization.^39,41

When interpreting the findings of this review, several factors should be pointed out. The evidence used was identified to be of very low quality, given that appropriate randomized clinical trials in the field have not been completed to date.^20–25 The evaluation methods used varied among studies, which hindered direct comparison of results in absolute quantitative terms. This study highlights the urgent need for randomized clinical trials with rigorous methodology to develop accurate and predictable treatment protocols for mesiodistal movements.

This review conducted a thorough analysis using rigorous methodology to evaluate the predictability of mesiodistal movements of upper and lower molars with clear aligners. Including retrospective and prospective studies, as well as using digital measurement methods, the study ensured accuracy and reliability. In conclusion, the differences between predicted and achieved movements were attributed to the inability of clear aligners to fully accomplish prescribed movements. This study has addressed an important gap in the literature, laying the groundwork for future research to improve the predictability of molar movements with clear aligners.

CONCLUSIONS

Based on the data from the articles included in this review, it can be concluded that:

• Distal movement of upper molars with clear aligners has a predictability of 61.1 ± 9.1% for molar distalization of 0.45–3.2 mm.

• Distal movement of lower molars mainly occurs due to molar tipping and its predictability was the lowest among the movements analyzed.

• Clear aligners present a limitation in anchorage control for premolar extraction cases, showing 25% to 40% more mesialization than predicted.

• Overcorrection of distal movement and anchorage reinforcement should be recommended.

• No consensus has been reached regarding the role of attachments in improving the predictability of mesiodistal movements.

SUPPLEMENTAL DATA

Supplementary Tables 1 and 2 are available online.