Functional muscle transfer and the variance of reinnervating axonal load: Part I. The facial nerve

Background: Results of functional muscle transfer for the treatment of facial palsy vary. Surgical technique in such cases remains constant, with only the selected ramus of the buccal branch of the facial nerve changing. In this study, different-sized branches of the facial nerve were used to reinnervate a constant-sized muscle flap to see whether this might explain the spectrum of clinical results seen. Methods: Twenty-four New Zealand White rabbits were divided into two groups of 12 animals. The rectus femoris muscle was raised as a free flap and transplanted to the cheek, after being coapted to a large (buccal) or small (marginal mandibular) branch of the facial nerve. Physiological testing of the flaps and histological examination of the muscle and motor nerve were undertaken. Half the animals from each group were euthanized at 6 months and the remaining at 9 months. In all cases, the contralateral rectus femoris muscle served as the control. Results: Improved physiological results were noted in the groups reinnervated by the larger nerve at both time points. Significant improvement in force production was also noted between time points in the large nerve group. These results were confirmed by histological studies. Conclusion: Varying sizes of reinnervating axonal load may be responsible for the varied outcomes observed clinically and may also explain the phenomenon of late-onset tightening.