Ca/P ratio effects on the degradation of hydroxyapatite in vitro

Phase purity is a well-recognized but not well-understood variable affecting the biological integration of hydroxyapatite (HA)-based biomaterials. Minor amounts of specific, relevant impurities--calcium oxide (CaO) and tricalcium phosphate (TCP)--may often be present either as deliberate additions or as a result of decomposition during sintering. We investigated the influence of these two impurities in terms of their effects on surface morphology, weight loss/gain, and microstructural-level degradation. Phase purity variations were deliberately introduced into an otherwise-standardized HA matrix--the parent HA grain size and bulk density were relatively constant--produced using identical fabrication conditions. Stability varied markedly during exposure to mildly acidic, neutral, and pH 7.4 phosphate-buffered saline. Equivalent molar variations in the Ca/P ratio (1.62 vs 1.72) on either side of the stoichiometric ratio produce relatively small volumetric amounts of CaO (1.6 vol%) versus TCP (27 vol%) in HA. However, the relatively small amounts of CaO render the bulk more susceptible to degradation and more likely to have negative effects on a biological milieu. Interestingly, the presence of CaO is also a potent nucleating agent for the precipitation of new surface phases and detectable weight gain. The TCP-containing ceramic, in contrast, paradoxically exhibited slightly greater resistance to degradation than HA.