Chemical changes in hydroxyapatite biomaterial under in vivo and in vitro biological conditions.

The introduction of a synthetic calcium phosphate into a biological environment is likely to result in surface-mediated chemical events. On the basis of such an assessment, we studied the chemical changes occurring in the mineral after exposure of a synthetic hydroxyapatite ceramic to both in vivo (implantation in human) and in vitro (cell culture) conditions. A small amount of the material was phagocytized but the major remaining part behaved as a secondary nucleator as evidenced by the appearance of a newly formed mineral. Morphologically, the newly formed mineral appeared as tiny crystals precipitated and grown from the surface of the initial synthetic crystals. The density of the additional mineral increased from the periphery to the core of each biomaterial aggregate. Chemically, it was identified by IR spectroscopy as a carbonated apatitic mineral. We propose that the adsorption of biomolecules could inhibit precipitation, accounting for the increasing amount of precipitate from the periphery to the core of the aggregates.