Linear Geometric Analysis of Maxillary Expansion in Mixed Dentition: Rapid Palatal Expander Versus Invisalign First System

<b>Objectives</b>: This study aimed to evaluate maxillary arch width increase in juvenile patients requiring space gain, but without skeletal transverse discrepancies necessitating orthopedic expansion. The comparison focused on the effects of Rapid Maxillary Expansion (RME) using Hyrax expanders and dentoalveolar expansion via Invisalign First. <b>Methods</b>: This retrospective longitudinal study analyzed digital dental models of 38 patients (19 males and 19 females, aged 8 ± 2 years) undergoing maxillary expansion at the Department of Biomedical, Surgical, and Dental Science, Fondazione IRCCS Ca' Granda, Ospedale Maggiore Policlinico (Milan, Italy). Patients were divided into two groups: one treated with Hyrax expanders (<i>n</i> = 19) and the other with Invisalign First (<i>n</i> = 19). Intraoral scans were taken before (T0) and after treatment (T1), and measurements were performed using Mimics Materialize 21.0 software. Statistical analysis included <i>t</i>-tests, ANOVA, and regression models to assess differences in maxillary expansion between groups. <b>Results</b>: Both groups showed statistically significant transverse arch increases (<i>p</i> < 0.01). Hyrax achieved greater expansion at the deciduous canine level, while Invisalign showed more at the deciduous molar level. First permanent molar expansion was similar. ICC for reliability was excellent (>0.97). No significant differences in sex or Angle class distribution were observed. <b>Conclusions</b>: Hyrax and Invisalign First both produce measurable maxillary expansion, but they serve distinct roles. While Hyrax expanders provide rapid skeletal expansion, Invisalign First offers a less invasive alternative for dentoalveolar widening with controlled force application. They should not be used interchangeably. Appliance selection must be tailored to the severity and nature of the transverse deficiency.