Hydroxyapatite particles are capable of inducing osteoclast formation

Hydroxyapatite (HA) coatings have been used to improve implant fixation by promoting bone formation around the prosthesis. A macrophage response to HA particulates has been noted around loosened HA-coated prostheses. As bioma terial wear particle-associated macrophages are known to be capable of diff erentiating into osteoclasts that are capable of bone resorption, we examin ed whether particulate HA could similarly induce macrophage-osteoclast diff erentiation. HA-associated macrophages were isolated from granulomas, forme d by subcutaneous implantation of HA, and co-cultured with UMR 106 osteobla st-like cells in the presence of 1,25-dihydroxyvitamin D-3 for up to 14 day s on glass coverslips and bone slices. HA-associated macrophage-osteoclast differentiation was evidenced by the formation of numerous multinucleated t artrate resistant acid phosphatase (TRAP)-positive cells which formed lacun ar resorption pits on bone slices. Polymethylmethacrylate (PMMA) particle-a ssociated macrophages, isolated from subcutaneous PMMA-containing granuloma s, caused significantly more osteoclast formation and bone resorption than HA-associated macrophages. These results indicate that macrophages respondi ng to HA particles are capable of osteoclast differntiation. They also sugg est that particles derived from uncemented (HA-coated) implants are likely to induce less osteoclast formation and osteolysis than cemented implants. (C) 2001 Kluwer Academic Publishers.