Qualitative Comparison of Stress Distribution of Different Fixation Techniques of Sagittal Split Ramus Osteotomy (SSRO) in Mandibular Setback Surgery in Asymmetric Cases Using Three-Dimensional Finite Element Analysis (FEA)

Background: Bilateral sagittal split osteotomy (BSSO) is the most common procedure in orthognathic Surgey of mandible also in asymmetries.However, the methods of fixation are different.Few studies worked on asymmetric cases especially with the use of finite element analysis (FEA).We aimed to evaluate stress distribution of two different Fixation techniques in mandibular setback surgery in asymmetric cases using FEA. Methods:A 3-dimensional model of asymmetric mandible was obtained.SSRO with modified osteotomy was simulated unilaterally and another side osteotomized as common.Then differential set back was done and rigid fixation of that modified side with miniplate and monocortical screws was simulated then rigid fixation of other side by different fixation technics include 2 or 3 bicortical screws.With the force of 132N and 300N on the occlusal surface of first molars, the Von Mises Stress (VMS) distribution was calculated.Results: Stress distribution in threads of screws in use of three bicortical screws was higher than two bicortical screws (161%).VMS distribution in spongy bone of left ramus in use of three bicortical screws was higher than the use of two bicortical screws (78% difference).VMS distribution in cortical bone of mandible body in use of three bicortical screws was significantly higher than the use of two bicortical screws (1.3% difference) (P<0.5). Conclusion:The use of modified osteotomy and fixation with rigid fixation of two bicortical screws can create a more predictable and uniform stress distribution in mandibular setback surgery in asymmetric cases.