Longitudinal Changes in Occlusal Outcomes and Case Complexity with Invisalign^® First in Early Mixed Dentition

Background: Interceptive orthodontic treatment during mixed dentition aims to reduce malocclusion complexity and improve occlusal development. Although Invisalign^® First was introduced for Phase I therapy in growing patients, evidence regarding longitudinal occlusal outcomes and maintenance during dental development remains limited. This study described longitudinal changes in occlusal outcomes and case complexity using standardized indices in moderate-to-severe malocclusions. Methods: Records of children in early mixed dentition treated with Invisalign^® First between 2019 and 2025 were retrospectively analyzed. The full sample comprised 55 patients with complete digital occlusal records at baseline (T0), post-treatment (T1), and after a minimum 18-month follow-up period (T2), enabling longitudinal assessment using the Peer Assessment Rating (PAR) Index. All patients underwent baseline cephalometric assessment for diagnosis and treatment planning. A nested subsample of 47 patients with complete paired T0-T1 cephalometric records was additionally evaluated using a literature-based mixed dentition complexity framework adapted from Align^® clinical recommendations. Nonparametric tests (Wilcoxon and Friedman) were applied (α = 0.05). Results: Median PAR scores decreased from 22.0 (IQR 17.0-30.0) at T0 to 6.0 (IQR 1.0-8.0) at T1 and 7.0 (IQR 2.0-10.0) at T2, with no significant difference between T1 and T2 (p = 0.546). In the cephalometric subsample, global complexity decreased by 79.49 ± 22.43%, with correction rates exceeding 85% for dentoalveolar objectives and reaching 100% for posterior crossbite and open bite. Skeletal and sagittal molar components showed more limited improvement. Conclusions: Interceptive treatment with Invisalign^® First showed longitudinal reductions in occlusal complexity and high dentoalveolar correction rates, with maintenance of occlusal outcomes during follow-up as patients transitioned from mixed to permanent dentition. These findings should be interpreted as descriptive longitudinal observations within a non-controlled design.