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.

Single-crystal hydroxyapatite and OH-carbonated hydroxyapatite have bending strength much higher than that of dense hydroxyapatite ceramic, indicating potential applicability to a load-bearing biomaterial. However, the effects of carbonate on the strength are less clear. The objective of the present study was to determine the bending strength and Young's modulus of these single crystals with carbonate contents from 0 to 0.62 in CO2 wt%. Three-point bending tests were performed by means of a modified ultra-microhardness tester with a span of 380 microm and a bending direction <210> of the single crystals. The crystals were broken in air, water, and air after immersion in a cell culture medium for 3 wks. The average Young's modulus of the single crystals was from 54 to 79 GPa. The average bending strength of the single crystals in air was 500+/-184, 468+/-205, 513+/-151, and 450+/-162 MPa for those with 0, 0.09, 0.37, and 0.62 wt% carbonate, respectively. No significant decrease in strength was found for hydroxyapatite single crystals both in water and after the immersion in the medium. However, the strength of OH-carbonated hydroxyapatite single crystals decreased significantly by 23 to 43% in water in proportion to the carbonate content. The strength of single crystals with 0.37 and 0.62 wt% carbonate decreased significantly, even after the immersion in the medium. Therefore, hydroxyapatite single crystals are superior to OH-carbonated hydroxyapatite single crystals as a biomaterial for a load-bearing purpose.