The interaction of supersaturated calcium phosphate solutions with apatitic substrates.

The in vitro formation of calcium phosphates from supersaturated solutions seeded with apatite was examined. Reaction kinetics were followed by monitoring solution Ca, Mg, and PO4. Seeds and reaction products were examined chemically and by X-ray diffraction and electron microscopy. Under conditions where amorphous calcium phosphate (ACP) normally precipitates spontaneously, apatite reduced the amount of free ACP formed with a considerable fraction of the Ca and PO4 initially removed from solution depositing directly onto the seeds. At sufficiently great seed concentrations all initial solution losses occurred by this latter route with no free ACP precipitating. These initial events were followed by seed-induced formation of new apatite crystals accompanied by further losses in solution Ca and PO4. Supersaturated solutions stable to spontaneous ACP formation underwent similar losses in solution Ca and PO4 when seeded with apatite. Initial decrease in these ions was instantaneous and proportional to the seed concentration. Subsequent ion removal exponentially decreased with time and correlated with the formation of new apatite crystals overlaying the original seeds. In seeding experiments using physiologically-balanced salt solutions, the initial deposit formed on the seed surface was a magnesium stabilized calcium carbonate phosphate phase. This stable surface phase did not prevent the secondary crystallization of apatite from occurring.