Beyond Sagittal and Vertical: Transverse Constriction as a Feature of the Collapsed Arch in Angle Class II Post-Extraction Retreatment Patients: A Retrospective CBCT Pilot Study

Background: Orthodontic retreatment patients with a history of premolar extraction for Angle Class II malocclusion exhibit distinct dental arch morphologies; however, studies characterizing these features remain limited. This pilot study aimed to objectively assess baseline dental arch morphology and preliminary retreatment-related changes in this specific patient population. Methods: In this retrospective pilot investigation, a total of 21 subjects were consecutively enrolled and divided into two groups according to orthodontic history: the Retreatment Group (n = 9, patients with Class II malocclusion and a history of prior orthodontics with four-premolar extractions) and the Control Group (n = 12, first-time orthodontic patients with Class II malocclusion undergoing identical four-premolar extraction therapy during the current treatment). All participants were treated with a standardized 0.022-inch MBT self-ligating system. Pre-treatment (T0) and post-treatment (T1) CBCT data were analyzed to evaluate the following variables: intercanine width, interpremolar width (IPMW), intermolar width (IMW), anterior arch depth, total arch depth, arch perimeter, intercanine angle, mesiodistal angulation (MA), buccolingual inclination (BI), depth of the curve of Spee, overbite, and overjet. Arch width was designated as the primary outcome. Student’s t-tests were used for statistical analysis (α = 0.05). Results: At T0, the Retreatment Group had narrower posterior arches than the controls (p < 0.01; Hedges’ g = 1.55–1.94), with IPMW smaller by 4.9 ± 1.11 mm (maxillary) and 5.37 ± 1.25 mm (mandibular), and IMW smaller by 4.85 ± 1.06 mm (maxillary) and 4.17 ± 1.13 mm (mandibular). They also had greater overbite and overjet, greater maxillary anterior depth, smaller intercanine angle, and lower incisor BI and canine MA (p < 0.05). From T0 to T1, the main treatment changes included arch width expansion and adjustments in MA and BI. At T1, most parameters were similar between groups, but mandibular IPMW remained narrower in the Retreatment Group (p < 0.01). Conclusions: The Retreatment Group showed a distinct “arch collapse” pattern characterized by posterior constriction, increased overbite and overjet, an elongated and tapered arch, and abnormal incisor torque and canine angulation. Orthodontic retreatment corrected most of these features, but mandibular IPMW correction remained limited.