Skeletal and Dental-Alveolar Changes With Invisalign First Expansion System in the Mixed Dentition: A Retrospective Study.

backgroundThis retrospective study aimed to evaluate the skeletal and dentoalveolar effects of maxillary expansion using the Invisalign First system in the mixed dentition.

methodsThe study was conducted in the orthodontic department of the Fourth Affiliated Hospital of Zhejiang University School of Medicine between 2021 and 2024. Inclusion criteria were mixed dentition patients with maxillary transverse deficiency, fully erupted first molars, arch width discrepancy ≤ 5 mm, mild to moderate crowding and pre-peak growth status (CS2). Exclusion criteria were Angle's Class III malocclusion, previous orthodontic treatment, congenitally missing teeth, TMJ disorders, cleft lip/palate, or use of additional appliances. All patients were treated with a standardised digital protocol using the Invisalign First system with optimised expansion support attachments. No additional buccal root torque was programmed, and Class II elastics were not used. Pre- and post-treatment intraoral scan digital models and cone-beam computed tomography data were obtained. Measured parameters included arch width, nasal width, apical base width, alveolar width, palatal depth and first molar inclination. Statistical analyses were performed using paired t-tests for normally distributed data and Wilcoxon signed-rank tests for non-normally distributed data, with a significance level set at p < 0.05.

resultsThe study included 45 patients (mean age 8.84 ± 1.01 years; mean treatment duration 18.26 ± 0.95 months). Significant transverse increases were observed in all maxillary arch widths, particularly in the canine and deciduous molar regions (canine dental width: 4.17 ± 1.91 mm, p < 0.001; first deciduous molar width: 3.86 ± 1.93 mm, p < 0.001; second deciduous molar width: 4.38 ± 1.78 mm, p < 0.001). Corresponding significant expansion was noted at the alveolar bone level (anterior alveolar process width: 4.04 ± 3.37 mm, p < 0.001; posterior alveolar process width: 2.51 ± 1.71 mm, p < 0.001). The maxillary first molars showed controlled buccal inclination of 1.6° ± 4.09° (p < 0.01). The upper arch perimeter increased by 3.68 ± 2.95 mm (p < 0.001) with crowding reduction of 3.83 ± 3.00 mm (p < 0.001). No significant palatal depth changes occurred (0.42 ± 3.36 mm, p = 0.41).

conclusionIn the mixed dentition, the Invisalign First system can effectively expand the maxillary dental-alveolar width, maintain molar inclination and improve the transverse deficiencies and arch form in the canine-premolar region.