Hydrolysis of tetracalcium phosphate under a near-constant-composition condition—effects of pH and particle size

Tetracalcium phosphate (TTCP) is a component of a number of calcium phosphate cements used clinically for bone defect repairs. The strength, phase composition, and solubility of the set cement are highly dependent on the reactions of the cement components during setting. This study investigated hydrolysis reactions of TTCP under solution compositions chosen to mimic the compositions of the cement liquid during setting. The study utilized a pseudo-constant-composition technique that allowed both the rate and stoichiometry of the reaction to be determined while the reaction proceeded under a specific, constantly held solution pH, thereby keeping a constant calcium-to-phosphate ratio in solution. The hydrolysis experiments were conducted using either a fine (median particle size 3.5 μm) or coarse (median particle size 13.2 μm) TTCP powder at pH 7, 8 and 10. Low crystalline calcium (Ca)-deficient hydroxyapatite (HA) was the product in all experiments. Both the solution pH and TTCP particle size produced significant effects on all aspects of the hydrolysis reaction. At a given pH, the fine TTCP produced a HA product with a greater Ca deficiency than did the coarse TTCP. For a given particle size, the Ca deficiency generally decreased with increasing pH. Hydrolysis reaction rate generally decreased with increasing pH or TTCP particle size. At pH 7 and 8, the solution was undersaturated with respect to TTCP and supersaturated with respect to HA, suggesting that the reaction rate was limited by TTCP dissolution. In contrast, at pH 10, the solution was approximately saturated with respect to TTCP and highly supersaturated with respect to HA, suggesting that HA formation was the rate-determining step of the reaction. The findings provided useful insights into the setting reaction mechanisms of TTCP-containing calcium phosphate cements.