Transverse Changes in Combined Orthodontic and Surgical Treatment with Segmental LeFort I Osteotomy: Comparing Aligners with Fixed Appliances

Abstract

Background and Objectives:Transverse maxillary expansion by segmental LeFort I osteotomy is historically considered the least stable of all orthognathic movements, with reported molar relapse rates exceeding 30 to 50% in prior literature. Despite this common perception among practitioners, a comprehensive characterization of dental and skeletal transverse dimensional change across all three phases of surgical-orthodontic treatment has not been established. These phases include pre-surgical orthodontics, surgery, and post-surgical orthodontics. Furthermore, no studies have evaluated transverse stability specifically in patients treated with clear aligners and segmental maxillary expansion-despite growing clinical adoption of aligners in orthognathic surgery protocols. The primary objectives of this study were: (1) to characterize transverse dimensional changes at the skeletal and dental levels across each treatment phase, (2) to compare outcomes between patients treated with clear aligners versus fixed appliances, and (3) to identify factors associated with post-surgical transverse constriction. Materials and Methods: This was a retrospective cohort study of 34 patients (15 females, 19 males; mean age 36.1 years) who underwent segmental LeFort I osteotomy for transverse maxillary expansion combined with orthodontic treatment. Patient records were collected from two private oral surgery offices and four private practice orthodontists in the greater Seattle area. Fourteen patients were treated with clear aligners and 20 with fixed appliances. Cone-beam computed tomography was obtained at up to four time points: pre-treatment (T0), before surgery (T1), immediately post-surgery (T2), and completion of treatment (T3). Four transverse measurements were evaluated at each time point: posterior skeletal width, anterior skeletal width, intermolar width (IMW; posterior dental), and inter-canine width (ICW; anterior dental), as well as bilateral buccal-lingual first molar inclination. The fixed appliance group presented with larger pre-treatment transverse discrepancies, a different distribution of treatment time across phases, and was planned for greater surgical expansion compared with the clear aligner group. Statistical analyses included Welch two-sample t-tests, Wilcoxon rank-sum tests, McNemar's test, Pearson correlation, and bivariate linear regression. Results: Pre-surgical orthodontic phase (T1–T0): Pre-surgical orthodontics produced small expansionary changes at both dental levels (IMW: 1.18 ± 1.78 mm; ICW: 0.56 ± 1.78 mm), with no significant between-group differences (IMW: p = 0.572; ICW: p = 0.952). Pre-surgical orthodontics contributed approximately one-third of total net IMW expansion and half of total net ICW expansion. Surgical phase (T2–T1): Mean posterior skeletal expansion was 4.31 ± 2.32 mm overall, with the fixed appliance group achieving significantly greater expansion than the clear aligner group (5.14 ± 2.16 mm vs. 3.04 ± 2.00 mm; p = 0.008). No significant differences between-group were found for anterior skeletal expansion (3.49 ± 2.66 mm overall; p = 0.495), or ICW expansion (0.68 ± 0.88 mm overall; p = 0.854). For IMW expansion, there was a trend toward more expansion in the fixed appliance group (2.61 ± 1.92 mm overall; fixed appliances: 3.04 mm vs. clear aligners: 1.99 mm; p = 0.107). Surgical expansion was greater posteriorly than anteriorly at both skeletal and dental levels. Planned IMW expansion was strongly correlated with achieved expansion across all patients (r = 0.904, p < 0.001), with both groups achieving approximately 96% of planned expansion. Post-surgical phase (T3–T2): Posterior and anterior skeletal changes were minimal and non- significant between groups (posterior: 0.06 ± 0.66 mm overall, p = 0.635; anterior: −0.05 ± 1.93 mm overall, p = 0.200). Significant IMW constriction occurred overall (−2.04 ± 1.35 mm), with greater constriction in the fixed appliance group (−2.62 mm vs. −1.21 mm; p < 0.001). ICW constriction was −0.92 ± 1.12 mm overall (fixed appliances: −1.01 mm; clear aligners: −0.80 mm; p = 0.557). Significant lingual tipping of the right (−3.36 ± 3.56° overall; fixed appliances: −4.40°; clear aligners: −1.88°; p = 0.037) and left first molars (−2.77 ± 3.02° overall; fixed appliances: −3.88°; clear aligners: −1.20°; p = 0.005) occurred, with greater tipping in the fixed appliance group. Regression analysis: Total net IMW expansion (β = −0.28, p = 0.009, R² = 0.22), surgical expansion (β = −0.31, p = 0.010, R² = 0.19), and appliance type (β = −1.41, p = 0.002, R² = 0.27) were significant predictors of post-surgical IMW constriction. Total net ICW expansion (T2–T0) was a significant predictor of post-surgical ICW constriction (β = −0.30, p = 0.004, R² = 0.26). Treatment duration, gender, pre-treatment transverse discrepancy magnitude, and anteroposterior classification were not significantly associated with post- surgical constriction in any measurement. Occlusal outcomes: No patients presented with a posterior crossbite at treatment completion, with no significant difference between appliance groups. Four of 30 patients (13%) who presented with a pre-treatment crossbite ended with edge-to-edge posterior overjet, while none of the four patients without a pre-treatment crossbite developed edge-to-edge overjet (p < 0.001). The rate of edge-to-edge posterior overjet did not differ significantly between appliance groups (clear aligners: 7.1%; fixed appliances: 15.0%; p = 0.627). Conclusions: Skeletal expansion following segmental LeFort I osteotomy was highly stable through treatment completion. Post-surgical changes were predominantly dentoalveolar in nature, with IMW demonstrating the greatest constriction. Expansion magnitude was the most consistent predictor of post-surgical IMW constriction. The greater constriction observed in the fixed appliance group could be attributed to greater expansion magnitude, case complexity, and larger pre-treatment transverse discrepancies in that group. Planned and achieved IMW expansion were strongly correlated, confirming high surgical predictability regardless of appliance type. Within the scope of this study, clear aligners demonstrated adequate ability for achieving surgical expansion and maintaining the transverse dimension during the post-surgical phase through the removal of appliances. These findings should be interpreted within the context of the study's retrospective design, modest sample size, short-term follow-up, and bivariate linear regression approach. Prospective studies with larger sample sizes, multivariable modeling, and long-term follow-up are needed to confirm these findings.