Phase transformation on bone cement: Monocalcium phosphate monohydrate into calcium-deficient hydroxyapatite during setting

This study evaluated the phase transformation of calcium phosphate cement (CPC) using a mixture of monocalcium phosphate monohydrate (MCPM) and CaCO ₃ as the solid phase and either water or a sodium phosphate buffer (SPB) solution (pH=7.0) as the liquid phase. The synthetic CPC was characterized by X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM). The setting reaction in the SPB solution involved three phase transformations. Firstly, MCPM and CaCO₃ reacted with sodium phosphate immediately to form dicalcium phosphate dehydrate (DCPD) which continued to dissolve. Secondly, meanwhile, an intermediate amorphous calcium phosphate (ACP) was formed. Finally, ACP transformed into calcium-deficient hydroxyapatite (CDHA). In contrast, the reaction stopped at the first stage in water. Consequently, the SPB solution not only caused the dissolution of DCPD but also provided the buffering capacity to induce the conversion of the starting materials to CDHA.