Effect of Inferior Border and Lingual Vertical Osteotomy Line Length on Splitting Mechanics in Bilateral Sagittal Split Ramus Osteotomy: A Biomechanical Study

Background Bilateral sagittal split ramus osteotomy (BSSRO) is a widely used technique for mandibular correction; however, controlling fracture propagation and preventing unfavorable splits remain major challenges. Adjunctive osteotomy lines, including inferior border cuts and lingual vertical cuts, have been proposed to improve predictability, yet their optimal design and dimensions remain controversial. This study aimed to evaluate how variations in the length of the inferior border cut (fourth osteotomy line) and lingual vertical osteotomy line (fifth osteotomy line) influence splitting mechanics and fracture outcomes during BSSRO. Methods Cone beam computed tomography (CBCT) data from patients with three mandibular angle types (ectropion, upright, and involution) were imported into a 3D segmentation software. Virtual mandibular models were reconstructed and fabricated via additive manufacturing, yielding 33 models (11 per type). 15 freshly isolated porcine mandibles were used for validation. A custom stainless steel test rig with fixed and movable fixtures was designed to standardize the splitting process. A digital torque gauge attached to a chisel continuously recorded real-time torque during osteotomy. Additional osteotomy lines were created: inferior border cuts (fourth line, 5–20 mm) and lingual vertical cuts (fifth line, 5–10 mm). Both sides of each mandible served as self-controls. Torque values, fracture patterns, and “bad split” incidence were analyzed using two-way analysis of variance, t-tests, and Cochran–Mantel–Haenszel tests, with significance set at p  < 0.05. Results Application and incremental lengthening of the additional osteotomy lines significantly reduced the torque required for mandibular splitting (from 1.608 to 0.461 Nm, p  < 0.01) and decreased the incidence of “bad splits.” No significant association was observed between mandibular angle type and torque requirements. Conclusions Extending the inferior border cut up to 20 mm improves the predictability and safety of BSSRO by reducing torque demands and minimizing unfavorable fractures. Trial registration: not applicable.