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Introduction: Peripheral nerve injuries resulting in an unrepairable nerve gap are difficult to treat. Currently available biomaterial solutions are hollow nerve conduits, but these conduits are effective only for nerve gaps of < 3 centimeters. We propose that bioresorbable magnesium metal microfilaments (Mg), axially aligned inside well-characterized biodegradable nerve conduits will act as “cables” to provide physical support to guide cells across the gaps and the metal will completely and safely resorb, allowing further natural recovery. Mg metal resorption releases Mg++ ions, which have been shown to improve neurorecovery after both sciatic nerve crush wounds and brain damage from stroke. In this study we test the hypothesis that Mg metal microfilaments are biocompatible and may enhance early recovery following nerve injury.

Materials & Methods: 6 mm nerve gaps were created in the sciatic nerves of adult male Lewis rats. Poly(caprolactone) nerve conduits were sutured into the gaps and were filled with Mg microfilaments (99.9% pure, 250µm diameter) and/or saline or keratin hydrogel as either: 1) empty conduit with saline filler (n=3), 2) Mg plus saline (n=3), 3) empty conduit with keratin hydrogel (n=8), or 4) Mg plus keratin hydrogel (n=8). After sacrifice at 6 weeks, gastrocnemius muscles were removed and cross-sectional area determined. Nerves were removed, fixed, imaged by micro computed tomography (microCT) to determine Mg resorption, embedded, sectioned and stained with H&E, or immunostained with antibodies labeling neurons (neurofilaments, NF), Schwann cells (S100 protein), macrophages (ED-1, labels CD68) and an intact perineurial fibroblast capsule (GLUT1).

Results: Muscle cross-sectional area was significantly improved by Mg in saline-filled conduits. By microCT imaging, Mg microfilaments were almost completely intact after 6 weeks in saline-filled conduits, but major gaps appeared in the microfilaments with the acidic keratin filler. Keratin alone improved muscle size compared to saline alone, but keratin+Mg did not, presumably due to the greater resorption of Mg with keratin. By immunostaining, well-defined mini-fascicles of nerves containing axons occurred almost immediately adjacent to the cavity left by the Mg microfiliaments. A thin layer of 1-3 macrophages (ED-1+) surrounded the Mg and very few were in adjacent tissues, with no differences between conditions, confirming excellent biocompatibility of the Mg.

Conclusions: Mg microfilaments provoked a minimal immune response and muscle size suggests that the Mg filaments improve nerve regeneration if they are not quickly resorbed. Future studies will address longer nerve gaps, late results from this configuration, and mechanisms to prolong Mg bioresorption.