Re-innervation of facial nerve territory using a composite hypoglossal nerve--muscle autograft--facial nerve bridge. An experimental model in sheep.

The hypoglossal nerve has been used both entirely and in part to repair the facial nerve. Using the partial technique it may be difficult to obtain sufficient length and a free interposed graft is then required to extend the hypoglossal element. In six sheep the facial nerve was excised between its emergence from the stylomastoid foramen and its bifurcation in the parotid gland. The hypoglossal nerve was exposed and split longitudinally producing a limb which was reflected towards the distal stump of the facial nerve. This left a gap of 4-5 cm which was bridged with a freeze-thawed coaxially aligned skeletal muscle autograft. The sheep were examined at 8 months. Laser doppler blood-flow studies showed the blood-flow distal to the graft to be about 25% of that at an equivalent site on the normal side. Peak nerve conduction velocities were also reduced on the repaired side but stimulation of the proximal hypoglossal nerve was nevertheless capable of causing adequate contraction of both facial and tongue muscles. Histological comparison of the repaired facial nerves with equivalent sites on the normal side showed a reduction in mean axon and fibre diameters with normal myelin sheath thickness for the regenerated axon sizes. All of these features are to be expected in a regenerated nerve and are consistent with a good level of recovery of function.