Cellular response to calcium phosphate ceramics implanted in rabbit bone

Two hydroxyapatite ceramics, synthesized by sintering from bovine bone and from a mixture of phosphate tricalcium and natural hydroxyapatite, were implanted in bone sites in rabbits. From day 7 after implantation, osteoblast-like cells were visible with thin layers of new bone on both biomaterials. Histomorphometry showed progressive increase in volume and surface of newly formed bone. Signs of cell-dependent resorption were visible at the surface of biomaterials and newly formed bone. There was a progressive decrease in relative volume and trabecular thickness of the biomaterials. Resorption of biomaterials appears to involve two cell types: multinucleated giant cells and osteoclast-like cells. The multinucleated giant cells observed had neither tartrate resistant acid phosphatase activity (TRAP) nor a ruffled border. Vesicles and vacuoles containing crystals observed in these cells suggest phagocytosis of biomaterials. The number of these cells decreased after day 14 following implantation. The osteoclast-like cells were TRAP positive. The structured modification and the TRAP activity demonstrated in the subjacent biomaterial suggest that the dissolution of the implant may be associated to an extracellular enzymatic activity of these cells. Electron microscopy revealed a clear zone and cytoplasmic membrane infolding in these cells, suggesting a ruffled border differentiation. The number of these cells increased with delay after implantation. It was concluded that the implantation of calcium phosphate ceramics in bone leads to new bone formation as well as to resorption of the biomaterials. The mechanism of resorption appears to associate crystal endocytosis by multinucleated giant cells and more classical resorption by osteoclast-like cells.