Development of bioresorbable sintered apatites

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Sintering of carbonate apatite, prepared at 100ﾟC and pH 9.0 for 3 days, was studied by therma analysis, x-ray diffraction and infrared spectroscopy. The sintering temperature, at which the linear thermal shrinkage of isostatically compacted specimens increased sharply, decreased in proportion to the amount of carbonate initially present in the apatite. For example, specimens with over 8 wt% carbonate could be sintered at a temperature (650ﾟC) which was nearly 400ﾟC lower than that needed for sintering a specimen with no carbonate. Amounts of carbonate lost at the end of sintering, estimated chemically and by infrared specroscopy, were approximately equal to sample weight losses estimatd thermogravimetrically. Dissolution behavior of sintered carbonate apatite was also investigated in a 10mM/L acetic acid solution adjusted to pH 5.0 at 37ﾟC and compared to that of sintered hydroxyapatite and bone apatite for the purpose of establishing some similarities between pysicochemical dissolution of apatite biomaterials in vitro and their ability to be resorbed by osteoclasts in vivo. Both the sintered carbonate apatite and the bone apatite dissolved to an appreciable extent. Their solution compositions changed in an almost identical manner until toward the end of the reaction. The solution compositions for sintered carbonate apatite at 30 seconds was comparable with that for sintered hydroxyapatite at 3.8 days with respect to the degree of supersaturation, indicating that the former specimen is much more soluble than the latter specimen. Osteoclasts, which were obtained from the long bones of 1-day-old neonatal rabbits, resorbed bone and sintered carbonate apatite, but they did not resorb sintered hydroxyapatite. These findings suggest that sintered carbonate apatites, which have characteristics that can be favorably compared with those of bone, especially with respect to its reactivity to acid media, would be useful as bioresorbable bone substitutes.