199. Apatite-Based Coating for Improving Epidermal Integration With Percutaneous Osseointegrated Implants

PURPOSE: Percutaneous osseointegrated (OI) prosthetic docking devices have been developed as alternatives to traditional socket suspension systems. They consist of two components: 1) an endoprosthesis inserted in the medullary canal. 2) a linking percutaneous post that traverses the skin barrier, providing a platform for exoprosthesis attachment. Clinically, these systems have demonstrated their effectiveness across multiple amputee cohorts, offering faster donning and doffing, obviating pain and abraded skin from poor socket fit, and allowing extended wear-time. However, these devices create an open wound at the implant exit site, known as the “stoma.” As titanium is not a biologically active material, thus post-implantation these devices elicit both a foreign body and delayed wound healing response at the stoma. Because of this failure of the epithelium to recognize a biological signal at the surface, inevitably, a soft tissue lining is formed distally around the device but with no biological seal. Achieving an epidermal seal at the implant surface is important for ensuring the conclusion of wound healing at the interface and preventing infection. No known solutions exist to date. Drawing from established knowledge of gingival mucosal integration with the enamel surface via “hemidesmosomes,” implant surfaces with bioactive attributes that stimulate specific cellular responses are considered the path forward. Therefore, we tested the hypothesis that implant surfaces coated with fluorapatite (FA)—the major component of enamel— would elicit epidermal cells to induce hemidesmosome expression, integrate with the implant surface, and limit down-growth. METHODS: To study this hypothesis, we investigated the capacity of FA surfaces to induce keratinocyte differentiation in vitro by evaluating the presence of hemidesmosome expression using immunofluorescence and quantitative reverse transcription polymerase chain reaction (RT-qPCR). After confirming the material’s bioactivity, low-heat coating techniques were applied and tested using in vivo models. First, coated and uncoated percutaneous implants were studied in a non-loading rat back model for 4 weeks. Downgrowth and granulation tissue area were measured using histology, and protein and gene expression were confirmed using immunohistochemistry (IHC) and RT-qPCR, respectively. Second, a load-bearing sheep implant is being used for confirmation in a 12-week amputation model. Histology will be used to quantify epithelial downgrowth and areas of granulation tissue for statistical comparison. RESULTS AND DISCUSSION: Using immunofluorescence, the in vitro studies qualitatively showed surface-induced differentiation and hemidesmosome expression on FA compared to Ti surfaces. The RT-qPCR revealed quantitative increases in gene expression markers of differentiation (Keratin 10 and involucrin) and cell-adhesion hemidesmosome (BP180) markers. Using the rat-back model, we confirmed that epidermal downgrowth and granulation tissue were significantly limited around FA-coated implants when compared to uncoated implants. The load-bearing sheep model is ongoing, with all devices implanted.The in vitro methods outlined here show their ability to predict surface-induced differentiation and hemidesmosome expression. With further in vivo validation of our test material FA, we aim to confirm the feasibility of our cost-effective in vitro assays to identify materials suitable for percutaneous application. Our results support the tested hypothesis, highlighting the potential implication of FA coatings for improving the longevity of percutaneous devices.