Natural History of Bone Response to Hydroxyapatite-Coated Hip Prostheses Implanted in Humans

A series of 15 autopsied femurs containing hydroxyapatite- coated (HA-coated) prostheses was analysed histologically. Their implantation time ranged from 5 days up to 3 years. The coating thickness of some prostheses and the percentage of the coating in contact with bone at different levels were evaluated using an image analysis device. After the newly formed bone tissue had became mature, several bone morphotypes were identified at the coating contact. From the proximal to the distal part of the prosthesis, bone morphotype was denser and the percentage of the coating surface in contact with bone increased. Several stages in the prosthesis osseointegration were evident. The early bone formation was characterized by a direct ossification forming a trabecular bone in the bone marrow cavity between the prosthesis and the endosteum. A few weeks after implantation, osteoblasts, differentiating from the loose connective tissue which invaded the bone marrow cavity, synthesized an osteoid matrix on the coating, forming an immature bone. During the maturation, several morphotypes appeared and the bone remodeling also involved the ceramic coating causing resorption and ingrowth inside the coating