Use of Silk Scaffolding to Modulate the Interface Between Skin and Percutaneous Implants of Osseointegrated Prostheses for Patients With Limb Amputations

Abstract

Background: Loss of function from traumatic limb loss can be uniquely disruptive for young healthy patients. This active cohort is frequently ill-suited for traditional socket prostheses which apply pressure and friction on the skin of the residual limb, resulting in open sores, pain, and poor proprioception that limit overall mobility. In contrast, these patients may be ideal candidates for surgically implanted osseointegrated prostheses which provide better quality of life, proprioception, and mobility. While recent research has demonstrated some success in limiting the high infection rate of this method, no technique has been able to produce an efficacious seal between skin and implant that could effectively normalize infection risk. The leading contributors for this faulty seal are differences in mechanical properties between the elastic skin and rigid metal implant combined with the inability of skin cells to adhere to the implant surface, resulting in a down growth of epithelium along the implant which allows seeding of the bone with surface bacteria. This initial study was conducted to investigate the use of silk proteins as a subcutaneous scaffolding material to address this skin implant seal. Methods: Six adult male Sprague-Dawley rats (aged 13 weeks) were subjected to surgery in the form of subcutaneous insertion of a silk implant under the right dorsal skin. Animals were euthanized 10 weeks after the surgery. Harvested skin from the right side was used as the intervention specimens while skin from the left side was used as the control. Histology studies included staining with hematoxylin and eosin and alpha-smooth muscle actin. Images of the H&E stained skin sections were captured and used for quantitative evaluation of skin thickness. For mechanical testing, specimens were tensioned to 1 N and then loaded at a rate of 1mm/sec until failure. Results: H&E staining revealed qualitative differences with the intervention group containing areas of denser connective tissue and higher ratios of nuclei. The mean skin thickness of the intervention samples was 3.89 mm with a standard deviation of 1.19 compared to the mean skin thickness of the controls of 2.63 mm with a standard deviation of 0.391; this difference was statistically significant (p = .049). Alpha-smooth muscle actin staining of six samples yielded high concentrations of stain in the areas surrounding the silk implants which contrasted with sparse staining in the control group. The differences in the mean tensile stiffness and mean tensile maximum load between intervention and control groups were not statistically significant. Conclusion: The silk implants were biocompatible and demonstrated minimal degradation over the course of 10 weeks suggesting their suitability for long-term subcutaneous enhancement. Histologic studies provided strong evidence of a robust fibrotic reaction in the areas surrounding the implant which portends a possible mechanism for increasing subcutaneous attachment to a percutaneous prosthesis. Measurement of the epidermal, dermal, and subcutaneous layers showed skin thickening that could aid the longevity of the skin implant interface.