Hydroxyapatite-coated femoral stems. Histological analysis of components retrieved at autopsy

Plasma-sprayed coating of hydroxyapatite are biocompatible and, because of their osteoconductive properties, may contribute to the early fixation of total joint prostheses. To evaluate this interface, we histologically analyzed five hydroxyapatite-coated femoral stems which, along with the surrounding bone, were retrieved from three humans at autopsy. The five femoral components had been in situ for a mean duration of twelve months (range, almost five to twenty-five months) and had been inserted for osteonecrosis (two), osteoarthrosis (two), and as an uncermented revision for failure of a cemented stem. The three patients had had a good or excellent clinical result and had died of causes unrelated to the joint arthroplasty. A coating of hydroxyapatite was identified on each stem. There was a variable amount of apposition of bone (32 to 78 per cent of available surface per section). The deposition of bone was most prominent on the surface of the prosthesis that was close to the endosteal surface of the bone, especially in areas that are predicted by Wolff's law (anterior and medial aspects of the implant, and at lateral-oblique corners). There were occasional foci of bone-remodeling around the implant, including osteoclast-mediated removal of the coating of hydroxyapatite along with adjacent bone. Occasional particles of ceramic were present within macrophages in the adjacent bone marrow. Other areas showed formation of new bone with a few areas of bone directly against the metal substrate. The over-all histological features suggest mechanically stable implants with bone-remodeling at the surface of the bone-implant interface.