Species-related differences in percutaneous wound healing

Percutaneous devices permanently protrude through a surgically created defect in the skin. Usually they provide a connection for intracorporeal implants or organs with external devices. The skin penetration area presents unique medical problems. The interfacing tissue usually does not heal and seal to the implant but remains a focus of constant acute or chronic inflammation and eventually breaks down because of infection. This pathophysiological phenomenon has been studied previously with qualitative light microscopical methods. A large number of empirical studies have attempted to improve the implant-epidermal seal with various implant materials and designs. To allow systematic studies of the effect of biomaterials on implant-epidermal interface phenomena, quantitative histological parameters were evaluated. Test implants were made from Dacron velour and placed in dogs, goats, and rabbits for various preselected periods to determine time- and species-related histopathological variations. Results showed that the degree of connective tissue "maturity" within the pores of the implant appears to be related to the concentration of giant cells and polymorphonuclear granulocytes (histocompatibility). The process of epidermal proliferation around porous percutaneous implants appears to follow certain fixed patterns under different conditions that are accompanied by expelling forces, resulting in an outward movement of the implant until it is completely extruded. The presence of microhematomas throughout the implantation period indicates that mechanical forces disrupt interfacial tissue bridges. The basic histological processes are qualitatively the same in the three animal species studied. However, there are quantitative differences with regard to epidermal migration rate and connective tissue maturation within the implant pores, which may explain the different failure modes and times observed among species. The study indicated that percutaneous healing may be directly related to histocompatibility of the implant material, mechanical interfacial forces, and epidermal proliferative patterns. The first two may eventually be controlled by selection of optimal implant materials and device configurations. The control of epidermal migration, however, will be the key to prolonging percutaneous implant life.