Andrew Tsai

Date of Award

9-26-2025

Department

Department of Orthodontics

First Advisor

Heeyeon Suh

First Committee Member

Heesoo Oh

Second Committee Member

Sandra Khong Tai

Abstract

Introduction: This study evaluated the stability of transverse changes in patients treated with the Invisalign First System (IFS) compared to those treated with a Hyrax expander and fixed appliances (Hyrax) and a control group. The objective was to determine whether transverse changes achieved during the mixed dentition were maintained into the permanent dentition at the start of Phase II orthodontic treatment. Material and Methods: This retrospective longitudinal observational study included 119 subjects: 44 IFS patients, 36 Hyrax patients, and 39 untreated controls. Arch width changes at the canines, premolars, and molars were measured on digital study models at three timepoints: pre-Phase I treatment (T1), post-Phase I treatment (T2), and pre-Phase II evaluation (T3). Vertical skeletal changes were evaluated in a cephalogram subgroup. Results: Maxillary arch width increased significantly from T1 to T3 in both IFS and Hyrax groups (IFS: 1.8-2.8 mm; Hyrax: 2.2-3.7 mm) compared with controls (0.4-1.2 mm; P < .0001). The Hyrax group demonstrated significantly greater interpremolar and intermolar expansion at T2 (Hyrax: 4.1-4.8 mm, IFS:1.9-2.8 mm) but experienced more relapse by T3 from T2 to T3: Hyrax –1.9 to – 2.4 mm vs. IFS – 0.1 to – 0.5mm). Additionally, the IFS group showed a statistically significant increase in mandibular arch width from T1 to T3 (1.9 – 2.9 mm) compared with controls (P < .05). No vertical skeletal measurements differed significantly between T1 and T3. Conclusions: Both IFS and Hyrax produced significant maxillary arch width increases that were maintained into the permanent dentition compared with the controls. The IFS group also showed an increase in the mandibular arch width into the permanent dentition relative to the control group. IFS may be considered a suitable option for mild arch length discrepancies and crowding in the mixed dentition. No significant differences in long-term vertical skeletal changes were observed across groups.

Download

Share

COinS