Axonal regeneration through an autogenous nerve bypass: an experimental study in the rat

Neuroma-in-continuity can manifest itself not only as pain but also as incomplete return of motor and sensory nerve function. The mainstay of current treatment for peripheral neuromas employs neurolysis or segmental resection with interposition grafting. These techniques are complicated by the loss of the remaining conduction through intact fibers within the injured segment. Based on the recent finding that end-to-side neurorrhaphies demonstrate axonal growth, we studied the use of a nerve "bypass" graft as a possible alternative to neurolysis or segmental resection with interposition grafting. A sciatic nerve crush injury model was induced in the Sprague-Dawley rat by compression with a straight hemostat. Epineurial windows were created proximal and distal to the injury. An 8-mm segment of radial nerve was harvested and anastomosed to the sciatic nerve at the epineurial window sites proximal and distal to the compressed segment (bypass group). A sciatic nerve crush injury without bypass served as a control. Electrophysiological testing and gate studies were performed over an 8-week period. Sciatic nerves were then harvested en bloc and studied under transmission electron microscopy at 1250 times magnification. Myelinated and unmyelinated axon counts were obtained. Nerve conduction velocity in the bypass group was significantly faster than conduction velocity in the control group at 8 weeks (44.8 m per second vs. 36.4 m per second; p = 0.031). We found no difference in myelinated axon counts between the proximal and distal segments of the control sciatic nerve. In the experimental sciatic nerve, a 160% increase in the number of myelinated axons was noted in the distal segment. Significant axonal growth was noted in the bypass nerve segment itself. Gait analysis using the sciatic functional index revealed improved function of the bypass group compared to the control group, but this was not statistically significant. Nerve bypass may serve to augment peripheral axonal growth while avoiding further loss of the native nerve.