Functional muscle transfer and the variance of reinnervating axonal load: Part II. Peripheral nerves

Background: Peripheral limb reconstruction using functional muscle transfer following injury or tumor resection has been widely reported in the literature. Such procedures often fail to deliver the physiologic contractile strength that might be hoped for in relation to the size of the transplanted muscle. Pure motor nerves of differing sizes were used to reinnervate a constant-sized muscle flap to see whether functional results could be improved in an experimental model analogous to clinical peripheral limb reconstruction. Methods: Twenty-four New Zealand White rabbits were divided into two groups of 12 animals each. The rectus femoris muscle was raised as a free flap and transplanted back to the donor site orthotopically (coapted back to its original motor nerve) in the rectus femoris group or heterotopically (coapted to the larger motor nerve to the vastus lateralis-double the axonal load of the indigenous nerve) in the vastus lateralis group. Half the animals from each group were euthanized at 6 months, and the remaining animals were at 9 months. In all cases, the contralateral rectus femoris muscle served as a control. Results: Physiologic assessment of the transplants for maximal tetanic contractural force was undertaken, together with immunohistochemical and histologic analysis of muscle and nerve tissue. Result analysis demonstrated that the double-sized nerve failed to produce significant improvements in physiologic outcome between the two groups at either the 6- or 9-month time point. The results were further endorsed by the histologic findings. Conclusion: Increasing the axonal load to a functional muscle transplant beyond that of its indigenous motor nerve does not improve outcome.