Regulation of functional muscle transfer outcome by means of an interposed nerve graft

Facial palsy is a devastating condition. The gold standard treatment for chronic unilateral facial palsy is a functional muscle transfer reinnervated by a branch of the contralateral facial nerve via a cross-facial nerve graft. However, the few published long-term clinical series indicate that the outcome is unpredictable. Approximately half of all patients achieve a clinically satisfactory result, the rest experiencing underactivity or overactivity of the transferred muscle. Previous work using an animal model of functional muscle transfer has established that altering donor motor input to heterotopically transferred muscle correlates with reinnervation and force production, the corollary being that standardising donor motor input should standardise outcome. However there is presently no way of delivering a consistent motor input to a functional muscle transfer. The interposed nerve graft has been identified as potentially able to regulate motor input reaching a functional muscle transfer, ultimately controlling force production and clinical outcome. This project tests the hypothesis in an animal model that an interposed nerve graft of low axon regenerate transmitting capacity can standardise the outcome of functional muscle transfers despite variability in motor input, by comparing reinnervation of and force production by functional muscle transfers reinnervated with different sized motor inputs via high and low capacity interposed nerve grafts. Force production is measured in vivo after recovery of transplanted muscle, and reinnervation is measured by nerve morphometry of both the interposed nerve graft and the neural pedicle to transplanted muscle. In support of the study hypothesis, both reinnervation and force production data suggest that outcome correlates with donor nerve magnitude only when a high capacity interposed nerve graft is used, this correlation being lost when using a low capacity interposed nerve graft. The implications for human facial reanimation surgery are discussed