Introduction: The use of commercially available nerve guides has shown success in repairing nerve defects.  Recently, a new silk fibroin microtube has been designed possessing qualities ideal for peripheral nerve repair. Our aim was to examine the cellular inflammatory response in rat sciatic nerve defects after the implantation of this unique nerve guide by using immunohistochemisty and histological techniques. In addition, we assessed the functional recovery of the rat by the gastrocnemius muscle ratio.
Methods:  A randomized control study utilizing 40 Lewis rats was performed.   In experimental animals, a sciatic nerve defect was bridged by a 1 cm silk guide, creating an 8 mm defect.  An 8 mm autograft served as a control for the native inflammatory response.  After 1, 4 and 8 weeks, the nerve guides or autografts were harvested, sectioned and, processed for histological analysis.  Macrophage recruitment, an indication of acute inflammation, was evaluated by counting the number of cells positive for ED1 antibody.  The thickness of the collagen capsule surrounding the graft or silk material, an indication of chronic inflammation was measured following treatment with Mason’s Trichrome stain.  Nerve regeneration was determined through counting myelinated axons as visualized through Toluidine Blue stain. Finally, functional recovery was identified by measuring the mass of the gastrocnemius muscle in comparison to the contralateral uninjured muscle.
Results: The number of ED1+ cells was greatest 1 week post-operatively in both the autograft and silk conduit groups. Following 8 weeks, the ED1+ cells in the autograft  and silk conduits decreased to less than 1% and 17% of the week 1 values, respectively. At the 8 week time point, ED1+ cells were localized to the center of the autograft, however in the silk conduits, ED1+ cells persisted throughout the material.  Analysis of collagen formation revealed no significant differences in collagen thickness at all measured timepoints (p >.05), suggesting a similar foreign body response between groups. Myelinated axons counts within the silk guide revealed the presence of fibers in the mid and distal portion only at 8 weeks, with the greatest number at the proximal end indicating initial axon sprouts. In the autograft group, myelinated axons began to appear at 4 weeks in the mid graft and increase by over 200% in number by week 8, indicating regeneration occurring quicker than in the silk conduits. Gastrocnemius weight ratio showed no difference between groups until week 8 at which point the silk conduit demonstrated a 27% decrease (p <.05) in muscle ratio as compared to autografts.
Discussion: Our study has shown that a silk fibroin microtube can be used as a peripheral nerve conduit. We have shown that the silk conduit has an immunogenicity profile that is comparable to that of a host autograft.  In addition, using histomorphometry we demonstrated substantial re-myelination of a nerve gap bridged by the silk conduits. After the course of the study, functional recovery paralleled the microscopic regeneration of the nerve defect.  In future studies, the silk conduit will be combined with neurotrophic factors to enhance axonal elongation.