Biomechanical rationale for a hybrid mIVSRO-SSRO surgical strategy in mandibular asymmetry: a retrospective cohort study.

PURPOSE: The surgical correction of mandibular asymmetry presents a biomechanical challenge regarding iatrogenic condylar torque on the temporomandibular joint (TMJ). This study aimed to characterize the unique biomechanical effects and clinical outcomes of a hybrid modified intraoral vertico-sagittal ramus osteotomy (mIVSRO) and sagittal split ramus osteotomy (SSRO) strategy, using bilateral SSRO (BSSRO) and unilateral SSRO (USSRO) as established reference techniques.

METHODS

This retrospective study included 120 patients with mandibular asymmetry treated between January 2016 and April 2024. Inclusion criteria were skeletal class III or open bite requiring mandibular osteotomy. Patients with syndromes, prior TMJ surgery, or incomplete data were excluded. The cohort was stratified into three groups: BSSRO (n = 98), USSRO (n = 8), and Hybrid (n = 14). Primary outcomes were the 6-month postoperative change in the condylar long axis angle and the change in TMJ symptoms (improved, unchanged, or worsened). Analysis of covariance (ANCOVA) and multivariate ordinal logistic regression were used for statistical analysis.

RESULTS

The Hybrid group demonstrated significant and unique external condylar rotation on the deviated side (mean adjusted angle: 11.81 ± 0.90°), whereas both the BSSRO (16.46 ± 0.34°, p < .001) and USSRO (16.07 ± 1.18°, p = 0.013) groups exhibited internal rotation. High rates of TMJ symptom improvement were observed in the Hybrid (83.3%) and BSSRO (65.1%) groups, but the intergroup difference was not statistically significant (p = 0.144). Multivariate analysis identified preoperative TMJ symptoms as the sole significant predictor of postoperative improvement (p < .001), irrespective of surgical technique.

CONCLUSION

The hybrid mIVSRO-SSRO technique produces a distinct biomechanical outcome by inducing external condylar rotation on the deviated side, contrasting with the internal rotation observed in BSSRO and USSRO. While this rotational difference was significant, high rates of symptom improvement were observed across groups without a statistically significant difference (p = 0.144), suggesting that patient-specific factors may play a larger role in clinical outcomes than the osteotomy design alone.