Role of Age and Donor Nerve Injury as Determinants of Regeneration across End-to-Side Nerve Repairs

End-to-side neurorrhaphy is indicated when a proximal nerve stump is unavailable due to trauma, surgical resection, or disease. A growing body of literature supports the use of end-to-side nerve repairs for the treatment of peripheral nerve injuries. However, the key determinants of functional outcome remain largely unknown. It was hypothesized that donor nerve injury and younger age would both favor enhanced nerve regeneration. The reported experiment tested these hypotheses across a spectrum of donor nerve injury and animal age. Twenty-eight rats were randomized to four end-to-side experimental groups that corresponded to progressively greater donor nerve injury: 1) donor nerve with perineurial window alone; 2) donor nerve with crush; 3) donor nerve with partial neurotomy; and 4) complete nerve transection. In this experiment, the terminal limb of a transected peroneal nerve was sutured to the lateral aspect of a tibial nerve. Animals were then allowed to regenerate for 12 weeks. Seventy-two hours prior to harvest, peroneal nerves were injected with the retrograde tracers fast blue and fluorogold. Spinal cords, peripheral nerves, and extensor digitorum longus muscles were harvested. Motor neuron cell body counts and peripheral nerve regeneration were quantified by fluorescence microscopy and histomorphometry. Normalized set muscle masses were determined for functional assessment. Physiological and ultrastructural features of the perineurial window were also characterized. The effect of age on neuroregenerative capacity was investigated by analysis of separate animal cohorts reconstructed at ages 2 weeks, 3 months, and 1 year. More severe donor nerve injury was strongly associated with increased motor neuron counts, enhanced peripheral nerve regeneration, and progressive increase in normalized wet muscle mass (p