Annually, over 18 million extremity injuries are reported in the United States, resulting in a substantial number of peripheral nerve injuries. Nerve defect management includes primary repair, nerve grafting, and nerve conduits. Clinically, nerve conduit use has been restricted to smaller defects because of limited functional recovery with larger nerve gaps. It was hypothesized that a tissue engineering approach employing a nerve conduit filled with an optimized scaffold would accelerate regeneration. Keratins extracted from human hair fiber act as cell-binding scaffolds and provide an alternative to other nerve conduit fillers. Human hair has been identified as a “depot” of growth factors including nerve growth factor. The study goal was to determine the effects of a keratin hydrogel matrix on nerve regeneration through a conduit.
Swiss Webster mice were assigned to two groups. Each animal underwent transection of the left tibial nerve, 5 mm above neural insertion into the gastrocnemius. A 7-mm Silastic conduit was secured using 10-0 microsuture, creating a 4-mm gap between the proximal and distal nerve ends. In Group 1, a keratin hydrogel was injected into the 4-mm gap. The gap was left empty in Group 2. After 6 weeks, the regenerating nerve and control nerve were exposed and evaluated using 1) electrophysiology (amplitude and latency); 2) muscle force generation (twitch and tetanus); and 3) histologic examination.
At 6 weeks, substantial axonal regeneration had occurred with visible axon fibers crossing the conduits in both groups. In Group 1 (keratin), the amplitude was 34% of the control (13.99 mV vs. 40.65 mV), with Group 2 (empty) achieving only 11% (3.44 mV vs. 30.24 mV). The latency revealed an 18% conduction delay compared to control in Group 1 (1.3 msec vs. 1.1 msec) and a 77% delay in Group 2 (2.3 msec vs. 1.3 msec). Muscle force generation data were similar to electrophysiologic data. Cross-sectional histology demonstrated generating myelinated axon fibers in both groups, with increased neovascularization in Group 1.
These data suggested that a keratin matrix may facilitate nerve regeneration through a conduit. Certain keratin preparations have the ability to self-assemble into porous, fibrous morphologies acting as scaffolds for regenerating axons. Axon regeneration may be enhanced by the inherent growth factors present in the conduit matrix. This tissue-engineering approach may allow use of nerve conduits in correcting larger nerve defects, with enhanced regeneration and return of function. Keratin is a promising conduit filler with potential future clinical application.
