Tetracalcium phosphate: Synthesis, properties and biomedical applications

Monoclinic tetracalcium phosphate (TTCP, Ca4(PO4)2O), also known by the mineral name hilgenstockite, is formed in the (CaO–P2O5) system at temperatures >1300 °C. TTCP is the only calcium phosphate with a Ca/P ratio greater than hydroxyapatite (HA). It appears as a by-product in plasma-sprayed HA coatings and shows moderate reactivity and concurrent solubility when combined with acidic calcium phosphates such as dicalcium phosphate anhydrous (DCPA, monetite) or dicalcium phosphate dihydrate (DCPD, brushite). Therefore it is widely used in self-setting calcium phosphate bone cements, which form HA under physiological conditions. This paper aims to review the synthesis and properties of TTCP in biomaterials applications such as cements, sintered ceramics and coatings on implant metals.