Fluorapatite-Coated Percutaneous Devices Promote Wound Healing and Limit Epithelial Downgrowth at the Skin-Device Interface

A percutaneous osseointegrated device becomes deeply ingrown by endosteal bone and traverses the overlying soft tissues of the residual limb, providing a direct link to the bone-anchored artificial limb. Continuous wound healing around these devices can result in the formation of sinus tracts as "down-growing" epithelial cells are unable to recognize and adhere to the "nonbiological" implant surface. Such sinus tracts provide paths for bacterial colonization and deep infection. In order to limit adverse outcomes and provide a robust seal, it was hypothesized that by coating the titanium surface of the percutaneous post with the mineral component of dental enamel, down-growing epidermal cells might recognize the coating as "biological" and adhere to this nonliving surface. To test this hypothesis, sintered partially and fully fluoridated hydroxyapatite (HA) was chosen as coatings. Using an established surgical protocol, fluorapatite (FA), hydroxyfluorapatite (FHA), HA-coated percutaneous posts, and titanium controls were surgically placed under the dorsal skin in 20 CD hairless rats. The animals were sacrificed at four weeks, and implants and surrounding tissues were harvested and subjected to further analyses. Downgrowth and granulation tissue area data showed statistically significant reductions around the FA-coated devices. Moreover, compared to the control group, the FA- and HA-coated groups showed downregulation of mRNA for EGFr, EGF, and FGF-10. Interestingly, the FA-coated group had upregulation of TGF-alpha. These data suggest that FA could become an ideal coating material for preventing downgrowth, assuming the long-term stability of these coated surfaces can be verified in a clinically relevant animal model.