Hydroxyapatite Coating Modifies Implant Membrane Formation. Controlled Micromotion Studied in Dogs

We studied the influence of controlled micromovements between bone and porous titanium alloy implants with and without hydroxyapatite coating. A dynamically loaded unstable device producing approximately 150-microns axial translation of knee implants during each gait cycle was developed. Stable implants served as controls. Matched stable and unstable implants with either porous titanium (Ti) or hydroxyapatite (HA) coating surrounded by a gap of 0.75 mm were inserted into the weight-bearing regions of the medial femoral condyles in 14 mature dogs. Histologic analysis after 4 weeks showed a fibrous membrane surrounding both types of implants subjected to micromovements, whereas various amounts of bone ingrowth was obtained in the stable implants. The membrane around unstable HA implants was thinner than that around unstable Ti implants. Islands of fibrocartilaginous tissue characterized the membrane around unstable HA implants, whereas fibrous connective tissue surrounded unstable Ti implants. The collagen concentration of the fibrous membranes was higher around unstable HA implants compared with Ti implants. Instability reduced the shear strength of the implants. However, the shear strength of unstable HA implants exceeded that of the Ti implants, both unstable and stable. The greatest shear strength was obtained by stable HA implants, i.e., tenfold greater than that of stable Ti implants. The gap-healing capacity around stable HA implants increased toward the HA surface, and was greater than that around Ti implants. Our study demonstrates that micromovements between bone and implant inhibit bone ingrowth and lead to the development of a fibrous membrane. The superior fixation of unstable HA implants compared with unstable Ti implants may be ascribed to the presence of fibrocartilage, a higher collagen concentration, and radiating orientation of collagen fibers in the membrane. The strongest mechanical anchorage and the greatest amount of bone ingrowth was obtained by stable implants coated with hydroxyapatite.