Abstract
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. © 2003 Wiley Periodicals, Inc. J Biomed Mater Res 67A: 599–608, 2003