Two-stage mandibular symmetry improvement after BSSRO: A CBCT analysis of 60 consecutive patients with asymmetric prognathism

The remodeling process of asymmetrically prognathic mandibles following bilateral sagittal split ramus osteotomy (BSSRO) remains incompletely understood. This study investigates the sequential changes in mandibular symmetry after BSSRO. A retrospective analysis was conducted on 60 consecutive patients with asymmetric prognathism who underwent BSSRO between January 2018 to December 2023. CBCT scans were obtained at three different time points: preoperatively (T0, within 2 weeks before surgery), immediately postoperatively (T1, within 2 days) and long-term postoperatively (T2, at least 6 months later). Mandibular symmetry was quantified using root mean square error (RMSE), derived from spatial discrepancies between the right-deviated mandibles and its mirrored counterpart. Additionally, positional discrepancies of 12 landmark points and volumetric differences of 6 mandibular segments were assessed using paired t-test. At T0, the mean RMSE was 2.729, which significantly decreased to 2.163 immediately after BSSRO (p < 0.0001), and further to 1.916 at T2 (p < 0.001). During T0-T1, significant corrections were observed in X-directional discrepancies in CoG-C, MF and CoG-B. However, BSSRO increased Y-directional discrepancies in Go and Cd, as well as Z-directional discrepancies in CoG-R, while volumetric asymmetry expanded in three mandibular segments (C: p = 0.002; B: p = 0.003; R: p < 0.001). During T1-T2, significant reductions occurred in X- and Z-directional discrepancies of Go and Z-directional discrepancies of CoG-R, with resolution of volumetric asymmetry in the mandibular body (p = 0.001) and ramus (p < 0.001). Mandibular symmetry improvement after BSSRO occurs in two distinct stages. The first stage is primarily due to surgical repositioning of the deviated distal segment, while the second stage involves orthodontic treatment and bony remodeling of the morphologically asymmetric proximal segments.