Dual constant composition kinetics characterization of apatitic surfaces

The use of calcium phosphate phases as implant materials in forms such as ceramic hydroxyapatite (CHAP) and hydroxyapatite plasma-coated implants (HPCTI) as well as the synthetic phases is now quite well established. Although conventional physical chemical characterization methods such as X-ray diffraction may indicate the exclusive presence of hydroxyapatite (HAP), there is little doubt that other calcium-containing surface impurities play important roles in the initial reactions that take place when such materials are brought into contact with aqueous phases. These surface impurities usually dissolve rapidly with the release of excess calcium and hydroxide ions. Based on the constant composition (CC) method, the dual constant composition (DCC) approach has been developed for investigating the dissolution kinetics behavior of these apatitic materials in an attempt to characterize and modify the surfaces, and possibly to predict their behavior when placed in vivo. The results of this study confirm the presence of highly reactive heterogeneous calcium-containing phases on the surface of materials such as CHAP and HPCTI, as evidenced by the release of appreciable amounts of calcium and hydroxyl ions upon introduction of these surfaces in saline solutions. Furthermore, these calcium phosphate phases exhibited a unique dissolution behavior when compared with synthetically prepared phases such as HAP and beta-tricalcium phosphate (beta-TCP), as suggested by the observation that they dissolve in solutions supersaturated with respect to HAP.

Hydroxyapatite mineralization and demineralization in the presence of synthetic phosphorylated pentapeptides

A constant composition (CC) method was used to compare the influence of statherin-like N-terminal 5-residue fragments having different amino acids in the terminal position on hydroxyapatite (HAP) growth and dissolution. The CC experiments were done in solutions containing 4.00 x 10(-4) mol/l calcium and 2.40 x 10(-4) mol/l phosphate. The solutions used in the crystallization studies were supersaturated only with respect to HAP (pH = 7.40, sigma HAP = 3.60). The CC dissolution studies were done in solutions undersaturated with respect to HAP (pH = 6.00; sigma HAP = -0.39). The HAP mineralization and demineralization processes were markedly inhibited by the negatively charged pentapeptides. Those containing a phosphorylated terminal residue inhibited dissolution to a greater extent than the native statherin fragment having aspartate as the N-terminal residue. Strong dependencies of the degree of inhibition of growth and dissolution reaction rates on the extents of reaction were noted. As the reactions proceeded, the rate inhibition decreased in the case of crystal growth and increased for dissolution.