Use of the interposed nerve graft to reduce differences in functional muscle transfer outcome arising from variability in motor input.

Regulation of functional muscle transfer outcome by means of an interposed nerve graft: The gold standard treatment of chronic unilateral facial palsy is a functional muscle transfer (FMT) reinnervated by a branch of the contralateral facial nerve via a cross-facial nerve graft, but clinical outcome varies from underactivity to overactivity of the transferred muscle. Work in an animal model has established that the magnitude of donor motor input received by heterotopically transferred muscle correlates with reinnervation and force production, the corollary being that standardising donor motor input could standardise outcome. We hypothesise that a 'low-capacity' interposed nerve graft could deliver a fixed motor input to an FMT despite variable motor input, thereby standardising force production and clinical outcome. To test this hypothesis, we compared reinnervation and force production by FMTs reinnervated with different-sized motor inputs via high- and low-capacity interposed nerve grafts. Force production was measured in vivo after recovery of transplanted muscle, and reinnervation quantified by both nerve morphometry and intramuscular immunohistochemical staining for protein gene product 9.5. Both reinnervation and force production data suggest that outcome correlates with donor motor 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.