Abstract
Objective
To compare the efficacy and stability of maxillary arch expansion using clear aligners (CAs) and slow maxillary expanders (SMEs) in growing patients with maxillary transverse deficiency.
Methods
This prospective multicenter cohort study included 48 growing patients (mean age, 9.3 ± 1.7 years; CA group n = 24; SME group n = 24) requiring maxillary expansion. The maxillary arch dimensions at the crown and gingival levels, arch perimeter, and molar buccolingual inclination were assessed at pretreatment (T0), post-expansion (T1), and post-retention (T2). The primary outcome was intermolar transverse expansion. A predefined non-inferiority analysis was performed to determine whether the expansion achieved with CAs was non-inferior to that achieved with SMEs within a clinically acceptable margin. Secondary analyses assessed the magnitude, pattern, and stability of the expansion through intra- and intergroup comparisons across time points.
Results
The average duration of the expansion phase (T0–T1) was 8.9 ± 3.4 months in the CA group and 6.3 ± 6.1 months in the SME group, followed by a retention phase (T1–T2) of 8.8 ± 5.0 and 8.3 ± 5.7 months, respectively. Both groups showed significant increases in the arch dimensions at T0–T1 and T0–T2. The SME group showed a greater bodily expansion pattern in the molars at T0–T2, although this difference was not statistically significant (P = 0.26). Significantly greater expansion at the canine cusp tip was observed in the CA group. Age was a negative predictor of expansion at the canine region.
Conclusions
Based on a predefined non-inferiority margin, the maxillary arch expansion achieved with CAs was not inferior to that obtained with SMEs in growing patients.
Keywords: Orthodontic appliances, Palatal expansion technique, Treatment outcome
INTRODUCTION
Maxillary transverse deficiency (MTD) is a common condition that contributes to malocclusion in growing patients. It often presents as a narrow maxilla with posterior crossbite or crowding, with a reported prevalence of 8–21% in children and approximately 10% in adults.1 The etiology of MTD is multifactorial, involving genetic, functional, and environmental factors such as mouth-breathing and adenoidal hypertrophy, which can disrupt normal orofacial development. Untreated MTD may lead to functional mandibular shifts, asymmetric growth, and dental compensation. Because transverse maxillary growth is completed earlier than sagittal or vertical development, early intervention is critical.2
Conventional appliances for the treatment of MTD include removable plates with jackscrews, such as the Schwarz appliance, fixed expanders such as Haas and Hyrax expanders, and spring appliances, such as the quad-helix expander.3 Both rapid maxillary expansion (RME) and slow maxillary expansion can achieve dento-skeletal expansion of the maxilla with similar skeletal expansion outcomes.4-6
Clear aligners (CAs) offer esthetic and hygienic advantages and have been increasingly used for arch development in mixed dentition. Despite early concerns regarding the predictability of expansion with CAs, studies have reported accuracy rates ranging from approximately 63% to 87%, with higher predictability observed in the premolar region and progressively lower accuracy toward the molars.7 In a recent mixed-dentition study, the mean differences between programmed and achieved movements at the cusp tip were –0.77 mm for deciduous canines, –1.24 mm for deciduous molars, and –1.27 mm for first molars.8
Comparative studies of CAs and conventional expanders (CEs) have reported that both types of appliances yield significant transverse expansion. However, they show discrepancies in the pattern of expansion; CEs tend to produce greater skeletal changes, whereas CAs often rely more on dentoalveolar tipping.7,9,10 In contrast, some studies have reported bodily molar movement with aligners, indicating the potential for skeletal adaptation when proper staging and specialized features, such as optimized attachments, are used.11,12
Despite the growing interest, most aligner studies have reported only short-term outcomes immediately after expansion. Since relapse can occur after expansion, retention is essential for evaluating treatment stability. Although the stability of conventional maxillary expansion has been extensively studied,13 long-term data on aligner-based expansion are scarce.
Therefore, this prospective cohort study aimed to compare the magnitude and pattern of maxillary arch expansion after active expansion and the retention phase in growing patients with MTD who underwent treatment using CAs and slow maxillary expanders (SMEs). The null hypothesis was that the expansion achieved by CAs was clinically inferior to that achieved by SMEs by more than the pre-specified non-inferiority margin (Δ).
MATERIALS AND METHODS
The research protocol for this multicenter prospective cohort study was approved by the institutional review boards of Asan Medical Center (2022-1182) and National Institute for Bioethics Policy (P01-202305-01-045). This study was conducted and reported in accordance with the STROBE guidelines for cohort studies.14 Patients diagnosed with MTD who were indicated for maxillary expansion were enrolled consecutively in the Department of Orthodontics, Asan Medical Center, and a private orthodontic clinic in Seoul, Korea. Patients were eligible if they met the following inclusion criteria: (1) MTD indicated for expansion; (2) mixed or early permanent dentition stage; and (3) Cervical Vertebral Maturation Stage IV or lower. Patients were excluded if they (1) had previously undergone any form of orthodontic expansion, (2) had craniofacial deformities or syndromes (e.g., cleft lip/palate), or (3) had poor oral hygiene or dental health that would contraindicate orthodontic treatment.
The primary outcome was intermolar transverse expansion. A predefined non-inferiority analysis was performed to determine whether the expansion achieved with CAs was non-inferior to that achieved with SMEs within a clinically acceptable margin. Secondary analyses included intra- and intergroup comparisons across time points to evaluate the magnitude, pattern, and stability of expansion.
The data for this study were collected between September 2022 and September 2024. After written informed consent was obtained from the guardians of all participants, patients were allocated to the CA or SME group based on their preference, after receiving detailed information about the differences between CAs and SMEs, rather than through randomization. Each clinic contributed cases to both groups based on the patient’s needs and preferences. At each clinic, the treatments were performed by an orthodontist with 15 years of hospital experience and 35 years in private practice.
Sample-size calculation
The sample size was calculated to achieve a non-inferiority comparison of the mean intermolar expansion between the two groups.15,16 The non-inferiority margin was set at 1.6 mm, based on the mean expansion values reported in previous studies and representing the largest clinically acceptable difference that would not influence clinical decision-making in maxillary expansion, as agreed by consensus among the authors.4,6,17-20 A standard deviation of 1.9 mm for maxillary first molar transverse measurements, which corresponded to the upper margin of reported variability,19,20 was selected as a conservative estimate to avoid underestimation of variance. With a one-sided alpha level of 0.025 and 80% power, and assuming independent groups with equal variances and a true mean difference of zero under the non-inferiority framework, the required sample size was calculated to be 23 participants per group. Although this was a multicenter study, both study groups were represented at each clinic and treated according to standardized protocols; therefore, potential center-level clustering effects were minimized, and no design-effect inflation factor was applied.
Treatment protocols
The CA group underwent maxillary arch expansion using the Invisalign® system (Align Technology, San Jose, CA, USA). Each case was digitally planned with ClinCheck® software (Align Technology). Deciduous canines, first and second deciduous molars (or premolars), and first molars were expanded simultaneously in the transverse direction by approximately 0.2 mm per stage (0.1 mm per side), using the contralateral teeth as reciprocal anchorage. Optimized expansion-support attachments were applied as needed to enhance aligner retention and generate a force system for root control of deciduous molars (premolars) and molars. Additional attachments, such as conventional rectangular attachments and optimized rotation attachments, were placed according to the individual tooth-movement objectives. Patients were instructed to wear the aligners for 20–22 hours daily. The number of aligners varied in relation to treatment needs, with patients changing aligners every 7–14 days depending on compliance. Expansion was considered complete when the planned transverse width was achieved.
The SME group was treated using conventional maxillary expanders. At the university hospital, a Hyrax expander with a bonded acrylic plate covering the canines to the first molars was used, whereas in private practice, a Haas-type expander with bands on the first molars was used, representing the standard protocol for conventional expansion at each clinic. First molars were selected as the anchoring teeth for the Haas-type expander, since patients exhibited varying degrees of root resorption in the deciduous molars at the start of treatment.21 The type of conventional maxillary expander used differed between the two clinics; however, previous studies comparing banded and bonded maxillary expanders have reported no significant differences in dental or skeletal expansion outcomes or in long-term stability.22,23 In addition, no significant differences have been observed between these appliance types in the magnitude of dental tipping or the symmetry of transverse expansion.24 Parents activated the jackscrew (¼ turn) twice per week, producing approximately 0.5 mm of expansion per week, until the palatal cusp tips of the maxillary molars came into contact with the lingual inclines of the mandibular buccal cusps. After active expansion, the appliances were retained passively for 2–6 months to allow consolidation and stabilization.
Data collection and measurements
Maxillary digital models were obtained using an iTero® intraoral scanner (Align Technology) at three time points: pretreatment (T0), post-expansion (T1, after expansion), and post-retention (T2, at least 3 months after completion of expansion). T2 measurements were obtained to evaluate the long-term retention and stability of maxillary expansion.
Transverse inter-arch widths were measured at the crown and gingival levels for the deciduous canines, first and second deciduous molars (or premolars), and first molars. Crown-level measurements (CL3–CL6) were obtained between the cusp tips, and gingival-level measurements (GL3–GL6) were obtained at the palatal gingival margins (Figure 1A and 1B). The ratio of gingival to crown expansion (G-C ratio 3–6) was calculated to assess the nature of tooth movement (bodily vs. tipping).
Figure 1.
Measurement scheme for maxillary arch analysis. A, Gingival-level transverse widths (GL3–GL6) measured at the palatal gingival margins of the deciduous canines (GL3), first and second deciduous molars or premolars (GL4, GL5), and permanent first molars (GL6). B, Crown-level transverse widths (CL3–CL6) measured between the cusp tips of the canines (CL3), buccal or mesiobuccal cusp tips of the first and second deciduous molars or premolars (CL4, CL5), and the mesiobuccal cusp tips of the permanent first molars (CL6). C, Arch perimeter, which was defined as the sum of linear segments (a + b + c + d) connecting the contact points from the mesial surface of one first molar to the contralateral side. D, Molar inclination (MI) defined as buccolingual inclination of the maxillary first molars, which was measured as the angle between buccal and palatal cusp tips of the right and left maxillary first molars, in accordance with the method described by Handelman et al.25
Arch perimeter was defined as the sum of the linear distances along the arch from the mesial side of the first molar to the contralateral side through the contact points (Figure 1C). Molar buccolingual inclination was measured as the angle between the buccal and palatal cusp tips of the right and left maxillary first molars, in accordance with the method described by Handelman et al. (Figure 1D).25
Measurements and reliability
All measurements were performed by a single examiner (JYK) using Meshlab software (Visual Computing Lab, ISTI-CNR, Pisa, Italy).26 Each measurement was repeated after a 2-week interval to assess intra-examiner reliability, which was high (intraclass correlation coefficient [ICC] > 0.95) for all variables. To assess inter-examiner reliability, a second examiner (YJK) independently measured models from 20 randomly selected patients, and ICCs exceeded 0.93 for all variables.
Statistical analysis
Data were analyzed using SPSS 22.0 (IBM Corp., Armonk, NY, USA), with statistical significance set at P < 0.05. Primary analyses were conducted using a per-protocol approach, excluding participants who discontinued follow-up. Descriptive statistics were calculated for all variables at T0, T1, and T2. Normality was assessed using the Shapiro–Wilk test. Paired t tests were used for within-group comparisons, and independent t tests were used for between-group comparisons.
To evaluate the effect of treatment modality, multiple linear regression analyses were conducted using the amount of change (Δ = T1–T0 or T2–T0) as the dependent variable. The primary independent variable was the treatment group (CA vs. SME); age at T0 and sex were included as covariates to adjust for potential confounding. The regression coefficients and 95% confidence intervals were estimated using ordinary least-squares analysis.
Additionally, a subgroup analysis comparing patients with ≤ 6 months and > 6 months of retention follow-up was performed using the Mann–Whitney U test to assess the potential influence of the follow-up duration on retention outcomes.
RESULTS
Patient characteristics and treatment duration
To account for potential attrition, 25 participants were initially enrolled in each group (total: N = 50). One participant in each group was lost to follow-up due to failure to attend scheduled clinic visits, resulting in a final sample of 48 participants included in the analysis (CA group: n = 24, 13 females, mean age = 9.8 ± 1.4 years; SME group: n = 24, 11 females, mean age = 8.8 ± 1.8 years; Figure 2). All variables showed a normal distribution, except for the arch perimeter at T2. At T0, significant differences were observed in the intercanine width at the gingival level and arch perimeter (Table 1). The expansion phase (T0–T1) lasted 8.9 ± 3.4 months in the CA group and 6.3 ± 6.1 months in the SME group. The retention phase (T1–T2) averaged 8.8 ± 5.0 months in the CA group and 8.3 ± 5.7 months in the SME group. The total observation period (T0–T2) was 17.3 ± 5.7 months in the CA group and 13.8 ± 7.8 months in the SME group. The two groups showed no significant differences in the treatment duration.
Figure 2.
Study flow diagram. Flowchart illustrating participant enrollment, allocation, follow-up, and analysis. A total of 50 patients were assessed for eligibility and enrolled. Among these, 25 patients each were allocated to the clear aligner (CA) and slow maxillary expander (SME) groups. During follow-up, one participant in each group was lost to follow-up due to failure to attend scheduled clinic visits. Consequently, 24 participants in each group were included in the final analysis.
Table 1.
Comparison of pretreatment maxillary transverse dimensions between the CA and SME groups
| Variable | CA group (n = 24) | SME group (n = 24) | P value |
|---|---|---|---|
| GL3_T0 (mm) | 27.8 ± 2.0 | 25.9 ± 2.7 | 0.032* |
| GL4_T0 (mm) | 27.7 ± 1.8 | 28.3 ± 2.5 | 0.349 |
| GL5_T0 (mm) | 31.9 ± 1.7 | 31.5 ± 3.1 | 0.606 |
| GL6_T0 (mm) | 33.8 ± 1.9 | 33.8 ± 2.8 | 0.914 |
| CL3_T0 (mm) | 35.4 ± 2.3 | 33.9 ± 3.5 | 0.142 |
| CL4_T0 (mm) | 41.2 ± 2.4 | 41.4 ± 3.2 | 0.795 |
| CL5_T0 (mm) | 47.7 ± 2.2 | 46.8 ± 3.4 | 0.285 |
| CL6_T0 (mm) | 53.1 ± 2.3 | 53.0 ± 3.3 | 0.885 |
| APE_T0 (mm) | 80.2 ± 4.8 | 76.1 ± 4.1 | 0.003** |
| MI_T0 (°) | 160.1 ± 9.5 | 158.1 ± 10.0 | 0.479 |
Values are presented as mean ± standard deviation. P values were obtained using independent t tests.
T0, pretreatment; CA group, patients treated with clear aligners; SME group, patients treated with conventional slow maxillary expanders; GL3–GL6, gingival-level transverse width at the palatal gingival margins of deciduous canines (GL3), first and second deciduous molars or premolars (GL4, GL5), and permanent first molars (GL6); CL3–CL6, crown-level transverse width at the cusp tips of deciduous canines (CL3), first and second deciduous molars or premolars (CL4, CL5), and permanent first molars (CL6); APE, arch perimeter; MI, molar inclination measured as the angle between the buccal and palatal cusp tips of the right and left maxillary first molars.
*P < 0.05, **P < 0.01.
Within-group changes after expansion (T0–T1) and retention (T0–T2)
Immediately after expansion (T0–T1), both groups showed significant increments in transverse arch dimensions at the crown and gingival levels across all tooth regions as well as in the arch perimeter (Table 2). The molar inclination (MI) angle decreased slightly, indicating buccal tipping, in both groups; however, these changes were not statistically significant (Table 2).
Table 2.
Mean and standard deviations of the variables at T0, T1, and T2, and changes between T0 and T1 and between T0 and T2 in the clear aligner and slow maxillary expander groups
| Variable | CA group (n = 24) | SME group (n = 24) | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| T0 | T1 | T2 | T0–T1 | P value | T0–T2 | P value | T0 | T1 | T2 | T0–T1 | P value | T0–T2 | P value | ||
| GL3 (mm) | 27.8 ± 2.0 | 29.6 ± 1.5 | 28.4 ± 1.4 | 1.9 ± 1.5 | < 0.001*** | 0.3 ± 2.9 | 0.797 | 25.9 ± 2.7 | 27.8 ± 2.5 | 26.9 ± 3.2 | 2.0 ± 1.1 | < 0.001*** | 1.0 ± 1.4 | 0.063 | |
| GL4 (mm) | 27.7 ± 1.8 | 29.9 ± 1.7 | 30.6 ± 0.8 | 2.2 ± 1.7 | < 0.001*** | 2.5 ± 1.3 | 0.001** | 28.3 ± 2.5 | 30.6 ± 2.5 | 30.5 ± 2.4 | 2.2 ± 0.9 | < 0.001*** | 2.1 ± 1.4 | 0.001** | |
| GL5 (mm) | 31.9 ± 1.7 | 34.3 ± 1.7 | 33.8 ± 1.3 | 2.6 ± 1.7 | < 0.001*** | 2.5 ± 1.9 | 0.004** | 31.5 ± 3.1 | 33.8 ± 3.0 | 32.8 ± 3.4 | 2.5 ± 1.1 | < 0.001*** | 2.5 ± 1.1 | < 0.001*** | |
| GL6 (mm) | 33.8 ± 1.9 | 35.8 ± 1.7 | 36.0 ± 1.5 | 1.9 ± 1.7 | < 0.001*** | 2.2 ± 1.3 | < 0.001*** | 33.8 ± 2.8 | 36.5 ± 2.9 | 36.2 ± 2.5 | 2.5 ± 1.1 | < 0.001*** | 2.6 ± 1.5 | < 0.001*** | |
| CL3 (mm) | 35.4 ± 2.3 | 38.1 ± 1.7 | 37.9 ± 2.3 | 2.5 ± 1.2 | < 0.001*** | 2.9 ± 1.5 | 0.004** | 33.9 ± 3.5 | 35.8 ± 3.2 | 35.5 ± 3.9 | 2.1 ± 0.9 | < 0.001*** | 1.9 ± 1.3 | 0.002** | |
| CL4 (mm) | 41.2 ± 2.4 | 44.5 ± 2.4 | 44.7 ± 1.9 | 3.0 ± 1.8 | < 0.001*** | 3.4 ± 2.0 | 0.002** | 41.4 ± 3.2 | 43.8 ± 3.2 | 43.3 ± 3.3 | 2.4 ± 1.0 | < 0.001*** | 2.1 ± 1.9 | 0.004** | |
| CL5 (mm) | 47.7 ± 2.2 | 50.6 ± 2.4 | 50.5 ± 1.8 | 3.2 ± 2.4 | < 0.001*** | 3.1 ± 2.5 | 0.003** | 46.8 ± 3.4 | 49.7 ± 3.3 | 48.4 ± 3.2 | 3.0 ± 1.3 | < 0.001*** | 2.2 ± 1.6 | 0.001** | |
| CL6 (mm) | 53.1 ± 2.3 | 55.6 ± 2.5 | 55.9 ± 2.2 | 2.5 ± 2.1 | < 0.001*** | 3.1 ± 1.8 | < 0.001*** | 53.0 ± 3.3 | 56.0 ± 3.3 | 55.0 ± 2.8 | 2.8 ± 1.2 | < 0.001*** | 2.3 ± 1.8 | 0.001** | |
| APE (mm) | 80.2 ± 4.8 | 81.9 ± 4.2 | 81.1 ± 4.0 | 1.8 ± 2.6 | 0.003** | 1.5 ± 3.1 | 0.065 | 76.1 ± 4.1 | 77.8 ± 4.3 | 74.9 ± 7.3 | 1.5 ± 1.6 | < 0.001*** | –1.2 ± 4.6 | 0.324 | |
| MI (º) | 160.1 ± 9.5 | 156.6 ± 9.2 | 154.7 ± 7.8 | –3.5 ± 8.9 | 0.069 | –7.1 ± 9.8 | 0.030* | 158.1 ± 10.0 | 155.1 ± 10.5 | 157.7 ± 9.4 | –2.3 ± 6.2 | 0.092 | –1.0 ± 10.9 | 0.759 | |
Values are presented as mean ± standard deviation. P values were obtained using paired t tests.
T0, pretreatment; T1, post-expansion; T2, post-retention (> 3 months after expander removal and consolidation); T0–T1, interval from T0 to T1; T0–T2, interval from T0 to T2; CA group, patients treated with clear aligners; SME group, patients treated with conventional slow maxillary expanders; GL3–GL6, gingival-level transverse width at the palatal gingival margins of deciduous canines (GL3), first and second deciduous molars or premolars (GL4, GL5), and permanent first molars (GL6); CL3–CL6, crown-level transverse width at the cusp tips of deciduous canines (CL3), first and second deciduous molars or premolars (CL4, CL5), and permanent first molars (CL6); APE, arch perimeter; MI, molar inclination measured as the angle between the buccal and palatal cusp tips of the right and left maxillary first molars.
*P < 0.05, **P < 0.01, ***P < 0.001.
After retention (T0–T2), the transverse gains were largely maintained with minimal relapse, except at the gingival level of the canines in both groups (Table 2). The changes from T0 to T2 at the canine gingival level (GL3) were limited (CA: 0.3 ± 2.9 mm; SME: 1.0 ± 1.4 mm; Table 2). The arch perimeter also regressed toward near-pretreatment levels. Significant buccal molar tipping was observed in the CA group (∆MI: –7.1° ± 9.8°, P = 0.030), whereas the SME group showed nonsignificant changes (∆MI: –1.0° ± 10.9°, P = 0.759).
Intergroup comparison of transverse dimension changes
Intergroup comparisons using the independent t test showed no statistically significant differences in terms of the amount of expansion or the ratio of gingival to crown expansion across all tooth regions during both the post-expansion (T0–T1) and post-retention (T0–T2) phases (Table 3). Notably, the ratio of gingival to crown expansion at the first molar decreased to 0.6 in the CA group over T0–T2, but the intergroup difference was not statistically significant (P = 0.259; Table 3).
Table 3.
Intergroup comparison of changes between T0–T1 and T0–T2 for the clear aligner and slow maxillary expander groups
| Variable | T0–T1 | T0–T2 | |||||
|---|---|---|---|---|---|---|---|
| CA group (n = 24) |
SME group (n = 24) |
P value | CA group (n = 24) |
SME group (n = 24) |
P value | ||
| ΔGL3 (mm) | 1.9 ± 1.5 | 2.0 ± 1.1 | 0.818 | 0.3 ± 2.9 | 1.0 ± 1.4 | 0.604 | |
| ΔGL4 (mm) | 2.2 ± 1.7 | 2.2 ± 0.9 | 0.865 | 2.5 ± 1.3 | 2.1 ± 1.4 | 0.511 | |
| ΔGL5 (mm) | 2.6 ± 1.7 | 2.5 ± 1.1 | 0.929 | 2.5 ± 1.9 | 2.5 ± 1.1 | 0.956 | |
| ΔGL6 (mm) | 1.9 ± 1.7 | 2.5 ± 1.1 | 0.196 | 2.2 ± 1.3 | 2.6 ± 1.5 | 0.487 | |
| ΔCL3 (mm) | 2.5 ± 1.2 | 2.1 ± 0.9 | 0.408 | 2.9 ± 1.5 | 1.9 ± 1.3 | 0.155 | |
| ΔCL4 (mm) | 3.0 ± 1.8 | 2.4 ± 1.0 | 0.154 | 3.4 ± 2.0 | 2.1 ± 1.9 | 0.176 | |
| ΔCL5 (mm) | 3.2 ± 2.4 | 3.0 ± 1.3 | 0.768 | 3.1 ± 2.5 | 2.2 ± 1.6 | 0.326 | |
| ΔCL6 (mm) | 2.5 ± 2.1 | 2.8 ± 1.2 | 0.566 | 3.1 ± 1.8 | 2.3 ± 1.8 | 0.220 | |
| ΔAPE (mm) | 1.8 ± 2.6 | 1.5 ± 1.6 | 0.656 | 1.5 ± 3.1 | –1.2 ± 4.6 | 0.058 | |
| ΔMI (º) | –3.5 ± 8.9 | –2.3 ± 6.2 | 0.581 | –7.1 ± 9.8 | –1.0 ± 10.9 | 0.177 | |
| G-C ratio 3 | 0.6 ± 1.0 | 1.0 ± 0.7 | 0.229 | 0.1 ± 1.1 | –0.3 ± 1.9 | 0.610 | |
| G-C ratio 4 | 0.9 ± 1.1 | 0.9 ± 0.6 | 0.842 | 2.3 ± 4.4 | 3.8 ± 5.0 | 0.508 | |
| G-C ratio 5 | 0.9 ± 0.8 | 0.8 ± 0.3 | 0.463 | 1.6 ± 1.7 | 1.7 ± 1.1 | 0.908 | |
| G-C ratio 6 | 1.0 ± 1.1 | 1.0 ± 0.3 | 0.958 | 0.6 ± 0.3 | 1.0 ± 1.2 | 0.259 | |
Values are presented as mean ± standard deviation. P values were obtained using independent t tests.
T0, pretreatment; T1, post-expansion; T2, post-retention (> 3 months after expander removal and consolidation); T0–T1, interval from T0 to T1; T0–T2, interval from T0 to T2; CA group, patients treated with clear aligners; SME group, patients treated with conventional slow maxillary expanders; GL3–GL6, gingival-level transverse width at the palatal gingival margins of deciduous canines (GL3), first and second deciduous molars or premolars (GL4, GL5), and permanent first molars (GL6); CL3–CL6, crown-level transverse width at the cusp tips of deciduous canines (CL3), first and second deciduous molars or premolars (CL4, CL5), and permanent first molars (CL6); G-C ratio 3–6, ratio of gingival-level to crown-level transverse expansion for teeth 3 to 6, used to assess the degree of bodily movement versus tipping; APE, arch perimeter; MI, molar inclination measured as the angle between the buccal and palatal cusp tips of the right and left maxillary first molars.
Regression analyses revealed that the CA group exhibited significantly greater expansion at the canine cusp tip than the SME group at both T0–T1 and T0–T2 (Table 4). No significant group differences were found in the expansion at the other sites. In the subgroup analysis comparing the short- and long-term retention periods, no significant differences were found between the groups for any transverse dimension or MI (Table 5).
Table 4.
Multiple linear regression analyses for changes in transverse dimensions and molar inclination angles at T0–T1 and T0–T2
| Independent variable | Dependent variable |
T0–T1 | T0–T2 | |||
|---|---|---|---|---|---|---|
| β | P value | β | P value | |||
| ΔGL3 (mm) | Group (CA) | 0.436 | 0.334 | –0.209 | 0.841 | |
| Age_T0 | –0.382 | 0.006** | –0.523 | 0.088 | ||
| Sex (female) | –0.302 | 0.458 | –1.327 | 0.310 | ||
| ΔGL4 (mm) | Group (CA) | –0.189 | 0.658 | 0.483 | 0.482 | |
| Age_T0 | 0.115 | 0.365 | –0.005 | 0.980 | ||
| Sex (female) | 0.066 | 0.872 | –0.421 | 0.569 | ||
| ΔGL5 (mm) | Group (CA) | –0.133 | 0.785 | 0.219 | 0.763 | |
| Age_T0 | 0.152 | 0.319 | –0.020 | 0.931 | ||
| Sex (female) | –0.166 | 0.714 | –1.138 | 0.142 | ||
| ΔGL6 (mm) | Group (CA) | –0.696 | 0.155 | –0.344 | 0.533 | |
| Age_T0 | 0.214 | 0.201 | –0.082 | 0.642 | ||
| Sex (female) | 0.432 | 0.357 | –0.571 | 0.305 | ||
| ΔCL3 (mm) | Group (CA) | 0.898 | 0.017* | 1.483 | 0.015* | |
| Age_T0 | –0.383 | 0.001** | –0.549 | 0.003** | ||
| Sex (female) | 0.062 | 0.847 | 0.628 | 0.340 | ||
| ΔCL4 (mm) | Group (CA) | 0.503 | 0.286 | 1.302 | 0.195 | |
| Age_T0 | 0.154 | 0.274 | 0.075 | 0.775 | ||
| Sex (female) | 0.207 | 0.645 | –0.528 | 0.610 | ||
| ΔCL5 (mm) | Group (CA) | –0.187 | 0.765 | 1.101 | 0.274 | |
| Age_T0 | 0.311 | 0.115 | 0.003 | 0.994 | ||
| Sex (female) | –0.277 | 0.635 | –1.229 | 0.246 | ||
| ΔCL6 (mm) | Group (CA) | –0.395 | 0.492 | 0.857 | 0.212 | |
| Age_T0 | 0.281 | 0.151 | –0.086 | 0.689 | ||
| Sex (female) | 0.063 | 0.911 | –0.529 | 0.441 | ||
| ΔMI (º) | Group (CA) | –0.959 | 0.675 | –7.859 | 0.085 | |
| Age_T0 | –0.086 | 0.901 | 1.971 | 0.152 | ||
| Sex (female) | –5.142 | 0.023* | –3.521 | 0.428 | ||
β, standardized regression coefficient; T0, pretreatment; T1, post-expansion; T2, post-retention (> 3 months after expander removal and consolidation); T0–T1, interval from T0 to T1; T0–T2, interval from T0 to T2; CA, patients treated with clear aligners; GL3–GL6, gingival-level transverse width at the palatal gingival margins of deciduous canines (GL3), first and second deciduous molars or premolars (GL4, GL5), and permanent first molars (GL6); CL3–CL6, crown-level transverse width at the cusp tips of deciduous canines (CL3), first and second deciduous molars or premolars (CL4, CL5), and permanent first molars (CL6); MI, molar inclination measured as the angle between the buccal and palatal cusp tips of the right and left maxillary first molars.
*P < 0.05, **P < 0.01.
Table 5.
Subgroup analysis of post-retention changes (T0–T2) across short-term (≤ 6 months) and long-term (> 6 months) follow-up periods
| Short-term (≤ 6 mo) group | Long-term (> 6 mo) group | ||||||
|---|---|---|---|---|---|---|---|
| CA group (n = 7) | SME group (n = 9) | P value | CA group (n = 13) | SME group (n = 8) | P value | ||
| ΔGL3 (mm) | 1.7 ± 0.7 | 2.7 ± 0.9 | 0.400 | –2.1 ± 1.9 | 0.7 ± 1.6 | 0.095 | |
| ΔGL4 (mm) | 2.3 ± 1.1 | 1.6 ± 0.9 | 0.486 | 3.2 ± 1.3 | 1.9 ± 1.7 | 0.202 | |
| ΔGL5 (mm) | 2.5 ± 1.1 | 2.4 ± 2.3 | 0.690 | 2.7 ± 1.4 | 2.5 ± 1.3 | 0.999 | |
| ΔGL6 (mm) | 2.8 ± 1.6 | 1.5 ± 1.6 | 0.310 | 2.6 ± 1.0 | 2.4 ± 1.6 | 0.733 | |
| ΔCL3 (mm) | 2.1 ± 0.7 | 3.4 ± 2.1 | 0.400 | 2.5 ± 0.7 | 1.8 ± 1.6 | 0.714 | |
| ΔCL4 (mm) | 2.1 ± 1.5 | 2.0 ± 1.8 | 0.886 | 4.8 ± 1.0 | 2.1 ± 2.2 | 0.073 | |
| ΔCL5 (mm) | 2.0 ± 1.2 | 2.5 ± 3.2 | 0.999 | 3.7 ± 1.6 | 2.4 ± 1.9 | 0.329 | |
| ΔCL6 (mm) | 2.0 ± 1.5 | 2.4 ± 2.5 | 0.999 | 3.5 ± 1.1 | 2.7 ± 2.1 | 0.404 | |
| ΔMI (°) | –3.3 ± 7.4 | –10.2 ± 5.1 | 0.190 | –5.5 ± 11.5 | 0.8 ± 13.6 | 0.228 | |
Values are presented as mean ± standard deviation. P values were obtained using Mann–Whitney U tests.
T0, pretreatment; T2, post-retention (> 3 months after expander removal and consolidation); T0–T2, interval from T0 to T2; CA group, patients treated with clear aligners; SME group, patients treated with conventional slow maxillary expanders; GL3–GL6, gingival-level transverse width at the palatal gingival margins of deciduous canines (GL3), first and second deciduous molars or premolars (GL4, GL5), and permanent first molars (GL6); CL3–CL6, crown-level transverse width at the cusp tips of deciduous canines (CL3), first and second deciduous molars or premolars (CL4, CL5), and permanent first molars (CL6); MI, molar inclination measured as the angle between the buccal and palatal cusp tips of the right and left maxillary first molars.
DISCUSSION
Both CAs and SMEs produced comparable magnitudes of maxillary arch expansion in patients with mixed and early permanent dentition. Significant increases in transverse width were observed at both the crown and gingival levels, and the expansion was well maintained during the retention phase in both groups. These findings suggest that the maxillary arch expansion achieved with CAs is not inferior to that achieved with SMEs in terms of efficacy and stability. In a randomized controlled trial by Bruni et al.27 comparing Invisalign® First and RME, no significant differences were found in intercanine width, palatal surface area, or palatal volume. Pamukçu et al.11 reported no significant differences in intercanine or intermolar widths, although removable acrylic expanders produced greater increases in palatal surface area and volume. However, the amount of expansion achieved with aligners tends to be under-expressed relative to ClinCheck® predictions, with higher predictability in canines and premolars than in molars, and at the crown level than at the gingival level.8,28-31 These findings suggest that some degree of overcorrection is necessary when planning arch expansion with aligners.
Regarding the tendency for buccal tipping of the molars, the mean increase in intermolar width at the gingival level was lower in the CA group both immediately after expansion and after retention, indicating greater buccal tipping; however, these differences were not statistically significant. For the long-term observation period (T0–T2), the CA group also showed a reduction in the MI angles (i.e., buccal tipping), indicating relapse, although this did not reach statistical significance in the intergroup comparison (P = 0.177). Similarly, in the regression analysis of ΔMI (T0–T2), after adjusting for age and sex, the CA group showed a greater tendency toward buccal tipping of the molars, with borderline nonsignificance (P = 0.085). Bruni et al.27 reported that intermolar width gain at the gingival level was significantly lower in the CA group, indicating a greater degree of buccal tipping of the first molars than in the RME group. In contrast, Lu et al.32 and Wang et al.12 found that Invisalign® First produced less buccal tipping than RME and SME, respectively. Similarly, Pamukçu et al.11 observed less buccal tipping with Invisalign® First than with removable acrylic expanders and suggested that the bodily movement achieved with aligners may be attributable to optimized attachments that deliver buccal root torque. Zhou and Guo10 reported that the efficiency of bodily buccal expansion for maxillary first molars averaged 36.35% and recommended incorporating sufficient buccal root torque when needed.
Another finding from long-term observations was the limited stability of intercanine expansion at the gingival level. Although both groups showed significant increases in intercanine width at the crown and gingival levels immediately after expansion, the gingival intercanine width relapsed toward pretreatment values during retention. This pattern may reflect the well-documented tendency of intercanine width to relapse over time,33-35 whereas the appliance may have maintained the crown level during the consolidation period and subsequent treatment.
In our study, the retention phase (T1–T2) varied widely from 3 to 25 months. To assess whether this variability influenced the evaluation of expansion stability, we conducted a subgroup analysis comparing the treatment groups across short- and long-term follow-up periods. No statistically significant differences in transverse changes were observed across subgroups, suggesting that retention duration within this range did not substantially affect post-expansion stability. However, given the relatively small sample size of each subgroup, these results should be interpreted with caution.
This study had several limitations that require acknowledgment. First, the study was non-randomized, as patients were allocated to groups based on their treatment preferences, which may have introduced selection bias. Due to the multicenter design, potential clustering effects related to the treatment center could not be completely excluded. Future studies with larger samples or mixed-effects modeling that incorporates the center as a random effect would help to further control for this source of variability. In addition, the SME group included two appliance designs with different biomechanical characteristics, which may have introduced heterogeneity in the dentoalveolar responses, particularly regarding molar tipping. Although existing evidence suggests comparable transverse expansion outcomes between bonded and banded expanders, appliance-related effects cannot be fully excluded and should be considered when interpreting the results. Our comparison involved CA-based expansion versus the expansion induced by SMEs; assessments of RME were not included. Although previous studies have reported no significant differences in skeletal expansion outcomes between SME and RME,6 a direct comparison of the long-term stability of CAs with RME could provide additional insights. In addition, the relatively large standard deviations and borderline significance of variables such as MI warrant further study with a larger sample size to confirm these findings with greater statistical power.
CONCLUSIONS
Within the predefined non-inferiority margin, CAs and SMEs achieved comparable transverse maxillary expansion in growing patients with mild-to-moderate MTD. Both treatment modalities demonstrated clinically significant transverse gains, generally maintained during the post-expansion follow-up period. However, given the variability in follow-up durations and the limited sample size in the subgroup analyses, our conclusions regarding long-term stability should be interpreted with caution. Further studies with standardized and longer follow-up intervals are warranted to more definitively assess the long-term stability of CA-based maxillary expansion.
Footnotes
AUTHOR CONTRIBUTIONS
Conceptualization: YJK, UBB. Data curation: JYK, DJ. Formal analysis: YJK. Funding acquisition: YJK. Investigation: JYK, DJ. Methodology: YJK, UBB. Project administration: YJK, UBB. Resources: JYK, DJ. Software: JYK, DJ. Supervision: YJK, UBB. Validation: YJK, UBB. Visualization: JYK. Writing–original draft: JYK, YJK. Writing–review & editing: YJK, UBB.
CONFLICTS OF INTEREST
The authors declare that this study was supported by a research grant from Invisalign® Korea. The funding source played no role in the study design, data collection, data analysis, data interpretation, or writing of the report.
FUNDING
This research received a research grant from Invisalign® Korea (2022OM0473-1).
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