Stability of osteosynthesis with bicortical screws placed in a triangular shape in mandibular sagittal split 5 mm advancement osteotomy: biomechanical tests.

Sagittal split ramus osteotomy (SSRO) is common in orthognathic surgery, and osteosynthesis with bicortical titanium screws placed in a triangular shape is used for stabilisation. We biomechanically tested the stability of this configuration in 5 mm advancement SSRO with condylar replacement in the initial position using six fresh frozen human mandibles raised subperiosteally for surgical simulation. Osteotomy was done after marking the initial positions of the condyles and the symphysis on graph paper. The condyle was set in its initial position and the symphysis advanced exactly 5mm; this position was maintained during osteosynthesis using 2.7 mm bicortical screws placed in a triangular shape. The mandible was then placed horizontally on the testing machine and a continuous static force was applied perpendicularly at a displacement speed of 5 mm/min. A stainless steel plate was used to transmit the forces independently of the teeth. A mean force of 129.6 Newtons (N) was applied at the elastic/plastic limit on an effort/displacement curve, which is well beyond the maximal mean (SD) chewing forces of 21 N (14) measured after bilateral sagittal split ramus osteotomy (BSSRO). A linear equation relating force to displacement was worked out from this experiment. Osteosynthesis using three bicortical screws mounted triangularly after advancement SSRO of 5 mm is experimentally stable, and our results could be used experimentally to compare two osteosyntheses using the equation.