ROLE OF TIMING IN ASSESSMENT OF NERVE REGENERATION

Small animal models are indispensable for research on nerve injury and reconstruction, but their superlative regenerative potential may confound experimental interpretation. This study investigated time-dependent neuroregenerative phenomena in rodents. Forty-six Lewis rats were randomized to three nerve allograft groups treated with 2 mg/(kg day) tacrolimus; 5 mg/(kg day) Cyclosporine A; or placebo injection. Nerves were subjected to histomorphometric and walking track analysis at serial time points. Tacrolimus increased fiber density, percent neural tissue, and nerve fiber count and accelerated functional recovery at 40 days, but these differences were undetectable by 70 days. Serial walking track analysis showed a similar pattern of recovery. A ‘’ effect is observed in rodents whereby an advancing nerve front overcomes an experimental defect given sufficient time, rendering experimental groups indistinguishable at late time points. Selection of validated time points and corroboration in higher animal models are essential prerequisites for the clinical application of basic research on nerve regeneration. V C 2008 Wiley-Liss, Inc. Microsurgery 28:265–272, 2008. Much of our understanding of nerve regeneration is derived from experiments in small animal models. This research is premised upon the notion that observations in mice or rats have direct implications for related biological phenomena in human patients. While rodent models offer advantages of small size and suitability for performing experimental neurorrhaphy, data from these models must be evaluated critically. In the rodent, exceptional regenerative capacity, short limb length, and biological variability among animals all conspire to obscure key differences between experimental groups. As a result, type II errors—in which a difference exists between groups but fails to be detected— may be more common than generally recognized. It was hypothesized that, due to superior regenerative potential observed in rodents, there exists only a finite window of opportunity for making accurate assessments of nerve regeneration in this model. The present study employed an established nerve allograft model of neuroenhancement using the agent tacrolimus (FK506), which possesses immunosuppressive properties and is the most well studied agent for neuroenhancement in small animals. 1 Allografted animals that were treated with cyclosporine A (CsA) or that were treated with no agent served as negative controls. In this model, the nerve allograft is a scaffold for nerve regeneration, and allogeneic Schwann cells are the antigenic target for rejection. Under the influence of immunosuppression, host Schwann cells migrate into allograft nerve and support regenerating nerve fibers as they extend towards target end-organs. Previously validated histological 2 and functional assessments 3–5 of nerve regeneration