Resorption of sintered synthetic hydroxyapatite by osteoclasts in vitro

Primary rat bone marrow cells were cultured for periods of 8 d on hydroxyapatite discs produced by sintering compressed powder at 1130 degrees C. The disc surfaces were roughened using silicon carbide paper to create three groups of samples (n = 10) of differing surface topography. The culture conditions permitted both the differentiation and fusion of cells of the osteoclast lineage. Following culture, the cells were stained in situ for tartrate-resistant acid phosphatase activity, and the samples were prepared for scanning electron microscopy. Evidence of cellular resorption of the hydroxyapatite discs was seen on all samples. Small tartrate-resistant acid phosphatase positive cells created resorption pits of 15-25 microns diameter in the ceramic surface, which were morphologically similar to those found in natural bone tissue, while multinucleate cells caused erosion of the ceramic surface without pit formation. Statistical analyses showed that the total numbers of cells, tartrate-resistant acid phosphatase positive cells, and multinucleated cells were all higher on the roughened surfaces, although the resorption pits were more easily visualized on the smooth surfaces. The results clearly demonstrate that not only are osteoclasts capable of resorbing sintered hydroxyapatite but that the rugosity of the hydroxyapatite influences the fusion of osteoclast mononuclear precursors.