Differential Stress Generation during Saggittal Split Osteotomy

Objective: Concerns exist when performing a saggittal split ramus osteotomy in the mandible that an untoward fracture may occur. The purpose of this investigation was to determine the stresses produced on the proximal segment while torque is being applied with the osteotome at the time of the saggittal split procedure. Methods: A photoelastic model of a mandible including individual simulants for cortical and trabecular bones was fabricated. Corticotomies simulating a saggittal split osteotomy were performed on the left side of the mandiblar ramus. The model was mounted in a framework composed of anatomically reproduced mandibular fossae. The model was immersed in a oil bath to reduce surface tension and facilitate photoelastic evaluation. A flat osteototome was inserted into the osteotomy site and clockwise and counterclockwise rotational forces were applied. Resulting stresses in all parts of the mandible were observed and recorded photographically in the field of a circular polariscope. Results: Forces applied by the osteotome generated higher stress in cortical bone than within the trabecular bone. The magnitude of stresses observed on the proximal and distal segements were independent of the direction of rotation placed with the osteotome. However, the location of the stresses observed within the osteotomy were dependent upon the rotational direction. Clockwise rotation of the osteotome produced differential increase in stress on the distal segment while counterclockwise rotation had a similar effect on the proximal segment. Conclusion: The direction of rotation of the osteotome at the time of saggittal split directly effects the stresses placed on the mandibular segments. To avoid untoward fracture of the left proximal segment, counter clockwise rotation should be applied.