Neuromuscular reinnervation efficacy after nerve repair or nerve graft in YFP mice

INTRODUCTION: The gold standard reconstruction for facial reanimation is the functional muscle transfer. The reinnervation of a muscle is never complete and clinical results are variable with 20% not achieving a satisfactory outcome (1). This study uses transgenic YFP mice to investigate the patterns of reinnervation through a nerve repair and a nerve graft. METHOD: 81 YFP-16 mice were studied in three intervention groups (nerve repair, nerve graft and nerve cut) over three time periods (four, six and twelve weeks post intervention). Two parameters were investigated: the number and surface area of reinnervated neuromuscular junctions and the number and calibre of regenerating axons. An assessment was made of motor unit proportions. 14 YFP-H mice were used to further investigate the patterns of axon regeneration through a nerve repair and graft. RESULTS: In the nerve cut group there was no reinnervation of the muscle at any time period. In all cases of nerve repair and nerve graft the muscles were completely reinnervated by regenerating axons with no significant difference between either intervention group and controls. The number and calibre of the regenerating axons was significantly different to controls for both intervention groups. The motor units were smaller in both intervention groups, but when the axons were thresholded to within two standard deviations of the control axon populations, the motor units were larger. DISCUSSION: Reinnervation after nerve repair or graft appeared adequate, however the neuromuscular junctions showed some evidence of morphological change that persisted. The number and type of axons after repair or graft differed from the control groups. There were more small calibre axons, suggesting the arbor had been reinnervated by a smaller number of axons that branched to increase their influence. These axons showed evidence of remodeling in the repair group, but not in the graft group