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Information on the solubility of OH-carbonated hydroxyapatite, Ca10(PO4)6(CO3)x(OH)2-2x, previously has not been available. In the present study the solubility product (Ksp) of OH-carbonated hydroxyapatite was measured in a 0.1 M acetic acid and sodium acetate buffer solution in a pH range of 4.0-5.8 at a CO2 partial pressure of 10(-3.52) atm. The equilibrium solubility increased with the increase of carbonate content. The Ksp values decreased with the decrease of pH. For example, Ksps were 10(-119), 10(-123), and 10(-130) for pure hydroxyapatite at pH 4.9, 4.5, and 4.1, respectively. The decrease of Ksp was not accounted for by calcium-carbonate complexation. Ksp measured at isoelectric points (L) was expressed as pL = 118.65 - 0.47316 x (CO2 wt%)2.4176. From this formula, the L values were calculated for pure and fully carbonated hydroxyapatite as 10(-118.7) and 10(102.8), respectively. The L value for pure hydroxyapatite agreed with values measured under carbonate-free conditions. Therefore, the L values were regarded as the Ksp for OH-carbonated hydroxyapatite excluding errors arising from carbonate contamination in the solution.

The growth rate of the hydroxyapatite (0001) face in pseudophysiological solution was measured directly for the first time by Moire phase shift interferometry. Common-path-type interferometry and a newly developed signal processor used for the phase shift technique enabled the precise and stable measurement of growth rates. It was determined that the growth rate was strongly time-dependent. The growth rate gradually decreased with time and finally ceased although the concentration of the solution remained constant. The thickness of the layer grown on the seed crystal was about 500−700 nm, while the growth rate during initial growth stages was of the order of 10-2 nm/s. respectively. Detailed surface morphology was observed by atomic force microscopy (AFM), indicating that the growth mode was multiple two-dimensional nucleation.

Using zinc-containing tricalcium phosphate (ZnTCP) as the zinc carrier for zinc-releasing calcium phosphate ceramic implants promoted bone formation around the implants. Because no quantitative information was available on the equilibrium solubility and resorbability of ZnTCP, in vitro equilibrium solubility and in vivo resorbability of ZnTCP were determined and compared quantitatively in this study. The solubility of ZnTCP decreased with increasing zinc content. The negative logarithm of the solubility product (K(sp)) of ZnTCP was expressed as pK(sp) = 28.686 + 1.7414C - 0.42239C(2) + 0.063911C(3) - 0.0051037C(4) + 0.0001595C(5) in air, where C is the zinc content in ZnTCP (mol %). The solubility of ZnTCP containing a nontoxic level of zinc (<0.63 wt %) decreased to 52-92% of the solubility of pure tricalcium phosphate (TCP) in the pH range 5.0-7.4. However, the in vivo resorbed volume of ZnTCP containing the same amount of zinc was much lower than that expected from the in vitro solubility, becoming as low as 26-20% of that of TCP. Cellular resorption of TCP is substantially a process of dissolution in a fluid with an acidic pH that is maintained by the activities of cells. Therefore, the reduction of the resorbability of ZnTCP could be attributable principally to its lowered cellular activation property relative to that associated with pure TCP.

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