Functional stability of sagittal split ramus osteotomies: effects of positional screw size and placement configuration.

PURPOSE

This study examined the effects of positional screw size and placement configuration on osteotomy site stability in a sagittal split ramal osteotomy model.

MATERIALS AND METHODS

A reproducible analog of a sagittal split ramus osteotomy was developed to facilitate equitable comparisons of fixation stability. The distal segment of each analog was advanced by 7 mm and fixed bilaterally with 2.0-mm or 2.4-mm cortical screws applied in either a linear or a triangular configuration. Reduced analogs were placed in a straining frame, and simulated masticatory loads wee applied alternatively to the mandibular first molars. Ensuring osteotomy site displacements were acquired and registered by displacement transducers attached to a computer-based data acquisition program. A coordinate transformation procedure was used to convert the component displacements captured by the individual transducers into a common "instability factor" to reflect fixation stability for each construct and loading condition.

RESULTS

Between configurations, the magnitude of interfragmentary displacement was less when the screws were applied in a triangular than in a linear configuration (P < or = .0001). Within configurations, 2.4-mm cortical screws provided greater stability than 2.0-mm screws. The stability profiles of the 2.0-mm screws, applied in a linear configuration at the superior border, were very similar regardless of whether two or three screws were used. The loads at which the linear systems became unstable were significantly higher (P < or = .0001) than for the triangular systems. Despite the twofold or higher difference in the mean instability factor of the individual test constructs, the SDs for each construct were only one tenth to one fifth of the corresponding means.

CONCLUSIONS

None of the internal screw systems were grossly unstable when used to stabilize ramal osteotomies. Other conditions, including quality and strength of investing bone, being equal, cortical screws placed in a triangular configuration provide greater stability than screws placed in a linear configuration. Osteotomy sites stabilized by 2.4-mm screws are more stable than those stabilized by 2.0-mm screws. It does not matter whether two or three screws are applied at the superior border in a linear configuration, provided that the outer screws are an inch apart. The model system has a very good measurement precision and has important implications in optimizing fixation device design and placement configuration.