Consequences of Oversizing: Nerve-to-Nerve Tube Diameter Mismatch

Background: Although commercially available nerve conduits are an accepted tool for overcoming short gaps in peripheral nerve repair, unexplained inconsistencies in clinical outcomes are not uncommon. Although exceeding the critical gap size and nerve caliber are most frequently cited as the source of these failures, oversizing of the nerve conduit in relation to the nerve diameter may be a previously unrecognized factor as well. Methods: Sixty female Sprague-Dawley rats underwent excision of a 10-mm section midway between the sciatic notch and the sciatic nerve division of one hindlimb. The defect was immediately repaired by reversing the resected nerve section and suturing it back in place (reverse autograft) (group A, n = 13) or it was repaired with a 14-mm nerve tube of 3mm in diameter (group B, n = 12), 2mm in diameter (group C, n = 15), or 1.5mm in diameter (group D, n = 15). At twelve weeks, the rodents underwent muscle strength testing before harvest of muscle and nerve (including the conduit) for histomorphologic assessment. Results: Most conduits from group B were collapsed at final inspection and demonstrated diminished nerve regenerate. Muscle atrophy was most pronounced in groups B and C (p < 0.05), although normalized muscle contraction force was weakest in group B (p < 0.05), indicating inferior reinnervation. The axon counts, axon diameter, and G-ratios at the midpart of the conduit or graft demonstrated more axons and lower G-ratios in the autologous graft group. Among the conduit groups, the axon counts were lowest in group B (p < 0.05) followed by group C (p < 0.05). The G-ratio was highest in group B (p < 0.05), although the axon diameter was highest in group B (p < 0.05) as well. Conclusions: Repair of a 10-mm gap in a rodent nerve with an oversized, poorly fitted nerve conduit resulted in tube collapse, poor nerve regenerate, and decreased muscle reinnervation compared with the findings in the animals treated with more accurately fitted nerve conduits.