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Purpose: There are multiple tools available to bridge short nerve defects including bridging the nerve defect with a synthetic nerve tube. The theoretical basis by which these tubes are able to support axonal regeneration is well established after thirty years of research and explains many of the successes and shortcomings of this tool. Regardless, unexplained failures are reported both anecdotally and in the literature. Though acceptable bridging lengths and tube composition are still debated, the diameter of the nerve tube has received much less attention. The purpose of this study is to evaluate the nerve tube “fit” around the repaired nerve in a rodent model.

Methods: Under aseptic technique the left sciatic nerve of each rodent was exposed, and a 10 mm section midway between the sciatic notch and the sciatic nerve split was excised. The defect was immediately repaired by reversing the resected nerve and sewing it back in place (reverse autograft) (Group A, N=15) or repaired using a 14mm nerve tube of either 1.5mm diameter (Group B, N=15), 2mm diameter (Group C, N=15), or 3mm diameter (Group D, N=15). The diameter of the rat sciatic nerve was 1 to 1.5mm. At 3 months the rodents underwent force contraction, morphologic, and histiologic testing.

Results: When compared to Group A (1.20±0.30gm), Group B (0.71±0.13gm) and Group C (0.90±0.14gm) demonstrated a significant difference (p<0.05) in gastrocnemius muscle weight, but Group D was similar (1.07±0.19gm). The developed force measured in Group B (0.36±0.19N) significantly differed (p<0.05) from Group A (0.97±0.67N), however, Groups C (0.68±0.31) and D (0.94±0.42N) did not differ from Group A. The axon counts per nerve section were significantly less in Groups B (574±209), C (1437±594) and D (2180±781) when compared to Group A (3092±953). Significant differences (p<0.05) were also found in the G ratio for Groups B (0.7198±0.0095), C (0.6469±0.0138) and D (0.6692±0.0078) when compared to Group A (0.5457±0.0179).

Conclusion: The data suggests there is a detrimental effect to nerve regeneration if substantial size discrepancy between nerve diameter and nerve tube exists. Theoretically, an “oversized” nerve tube may fail to resist compression by surrounding tissues. The protective microenvironment essential for supporting axonal growth is compromised, and invading scar tissue may impede axon regeneration. In addition to gap size and conduit composition, tube “fit” is a critical factor that must be considered by the peripheral nerve surgeon when utilizing nerve conduits for nerve repair.