Comparative Study of Mandibular Stability After Sagittal Split Osteotomies: Biocortical Versus Monocortical Osteosynthesis

Objective: A comparative study of clinical mandibular stability following bilateral sagittal split osteotomies by means of monocortical versus bicortical osteosynthesis was undertaken.

Design: This retrospective study utilized cephalometric radiographs, which were taken at 1 week and 6 months postoperatively.

Setting: The research was carried out at the Department of Plastic and Reconstructive Surgery of the Nagasaki University School of Medicine.

Patients: A total of 32 patients who underwent only sagittal split osteotomies and mandibular set back in our unit was included in this study. Of these patients, 17 patients were osteosynthesized monocortically, and 15 patients were osteosynthesized bicortically.

Main Outcome Measures: Four measurements--gonial angle (GA), mandibular plane (MP), SNB, and percentage of upper face to total face height (percent upper to total face)--were completed to evaluate postoperative mandibular movement.

Result: Statistical analyses of cephalometric measurements (GA, MP, SNB, and percent upper face to total face height) showed that monocortical fixed mandibles were more changeable postoperatively on the GA and percent upper face to total face height, but MP and SNB showed no significant differences among the groups.

Conclusion: These findings suggested that the postoperative excessive shear force stress, produced by the compressive action of the masseter muscle, transformed the mandibular shape as the distal segment rotated clockwise and proximal segment rotated counterclockwise. Consequently, the mandible was bent at the miniplate. It was concluded that bicortical osteosynthesis was more rigid against this shearing stress than monocortical osteosynthesis.