Origin of regenerated axons in nerve bypass grafts

It has been shown that end-to-side coaptation and nerve bypass grafting, which are essentially two sequential end-to-side coaptations, induce axonal outgrowth in peripheral nerve injuries. However, it is unknown whether the axons regenerated after end-to-side coaptation originate by collateral sprouting at the suture site or by elongation from the spinal neuronal pool. Also unknown is the extent of functional recovery that can be expected after bypass grafting for the reconstruction of injured peripheral nerves. We conducted a study to evaluate the origin of regenerated axons after end-to-side coaptation and the utility of nerve bypass grafting for peripheral nerve injury. For this purpose, we performed electrophysiological studies using bypass grafting (end-to-side coaptation) and conventional cable grafting (end-to-end coaptation) to treat complete paralysis of the peroneal nerve in rabbit models, and compared the recovery time and extent of functional recovery achieved with the two techniques. We assessed, by electromyography, the time to appearance of reinnervation potentials from the tibialis anterior muscle on the affected side. These times were not significantly different in the two study groups of animals (p = 0.5390). After a 12-week recovery period, electrophysiological findings and histological assessment showed similar recovery in both groups of animals. It is known that collateral sprouting of axons from the nodes of Ranvier proximal to the transected nerve stump occurs in cable grafting, and that axon elongation from the spinal cord requires more time. Our findings in the present study strongly suggest that collateral sprouting across end-to-side sutures is the chief means of axonal outgrowth in nerve bypass grafts, and that functional recovery can be expected in bypass grafting to nearly the same extent as in cable grafting.