Changes in the inferior alveolar nerve following sagittal split ramus osteotomy in monkeys: a comparison of monocortical and bicortical fixation.

We aimed to observe the changes in the inferior alveolar nerve (IAN) after bilateral sagittal split ramus osteotomy (SSRO) using monocortical or bicortical screw fixation. Bilateral SSRO for setback of the mandible was done in 12 monkeys, and monocortical or bicortical fixation was applied on opposing sides of each mandible. Sensory nerve action potentials were tested before and immediately after operation, and at 2, 4, 8, and 12 weeks. Six animals selected randomly were killed at 4 and 12 weeks after the operation. Specimens of nerve were harvested and processed for histological examination and electron microscopic analysis. Obvious prolongation of latency and diminution of amplitude in the IAN were found postoperatively. At 4 weeks after operation, Wallerian degeneration was apparent, and there were signs of axonal regeneration in the nerves. The IAN had more abnormalities of evoked potentials and pathological changes after bicortical than after monocortical fixation. Although considerable recovery was found after both forms of fixation at 12 weeks, the function of the nerve after plate fixation was better than after screw fixation. Our results suggest that the nerve damage during SSRO could be temporary and reversible, and monocortical fixation may result in restoration of the nerve function sooner than bicortical fixation.