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 implantati on, osteoblast-like cells were visible with thin layers of new bone on both biomaterials. Histomorphometry showed progressive increase in vo lume and surface of newly formed bone. Signs of cell-dependent resorpt ion were visible at the surface of biomaterials and newly formed bone. There was a progressive decrease in relative volume and trabecular th ickness of the biomaterials. Resorption of biomaterials appears to inv olve two cell types: multinucleated giant cells and osteoclast-like ce lls. The multinucleated giant cells observed had neither tartrate resi stant acid phosphatase activity (TRAP) nor a ruffled border. Vesicles and vacuoles containing crystals observed in these cells suggest phago cytosis of biomaterials. The number of these cells decreased after day 14 following implantation. The osteoclast-like cells were TRAP positi ve. 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 cell s. 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 bioma terials. The mechanism of resorption appears to associate crystal endo cytosis by multinucleated giant cells and more classical resorption by osteoclast-like cells.