Temperature dependent setting kinetics and mechanical properties of β-TCP–pyrophosphoric acid bone cement
Brushite (CaHPO4·2H2O) is the product of acidic calcium phosphate cement forming reactions. It has a higher solubility than hydroxyapatite under physiological conditions and is a promising resorbable bone cement candidate. It reacts extremely fast so dilute mixes are required to form cements having setting times with surgical utility, which in turn compromises strength and produces a highly porous material. Pyrophosphate ions have a strong affinity for calcium orthophosphate surfaces, can inhibit their mineralisation and are thought to hydrolyze, forming orthophosphate during bone formation. The effect of replacing the acid orthophosphate component of a brushite cement with pyrophosphoric acid on the cement setting reaction time at temperatures of between 24 °C and 70 °C was determined. The substitution of pyrophosphate for orthophosphate retarded setting and improved mechanical performance of the cement. Pyrophosphate did not inhibit the extent of reaction but did influence microstructure of the brushite crystals. Temperature was found to have a significant (p < 0.01) influence on mechanical performance, and this was attributed to the formation of monetite (CaHPO4) rather than brushite at temperatures of ≥55 °C.