Stability of transverse changes with clear aligners and hyrax expanders from the mixed to the permanent dentition

This study evaluated the longitudinal stability of transverse changes in patients treated with the Invisalign First system (IFS; Align Technology Inc, Tempe, Ariz) and hyrax expanders during the mixed dentition, followed through the permanent dentition. The objective was to determine whether expansion achieved during the mixed dentition was maintained through the transition to the permanent dentition.

This retrospective longitudinal study included 119 patients: 44 patients treated with IFS, 36 patients treated with hyrax, and 39 untreated controls. Arch width changes at the canines, premolars, and first molars were measured on digital study models at 3 time points: pre-phase 1 treatment (T1), post-phase 1 treatment (T2), and pre-phase 2 evaluation (T3). Vertical skeletal changes were evaluated in a cephalogram subgroup. Statistical analyses included analysis of variance and linear mixed-effects models.

Both IFS and hyrax groups showed greater maxillary arch width increases from T1 to T3 (IFS: 1.8-2.8 mm; hyrax: 2.2-3.7 mm) than controls (0.4-1.2 mm; P <0.0001). At T2, hyrax showed significantly greater interpremolar and intermolar expansion (4.1-4.8 mm) than IFS (1.9-2.8 mm), but more relapse from T2 to T3. The IFS group also showed increased mandibular arch width from T1 to T3 (1.9-2.9 mm) compared with controls (P <0.05). No significant vertical skeletal changes from T1 to T3 were observed.

Both IFS and hyrax produced maxillary arch expansion that was maintained throughout the transition from mixed to permanent dentition. The IFS also showed stable mandibular arch expansion, supporting its use for mild crowding or transverse deficiencies without long-term vertical effects.