Brushite cements from polyphosphoric acid, calcium phosphate systems

Brushite cement is more soluble than apatitic cement under physiological conditions. Thus, brushite cement may be resorbable in vivo when used as a bone substitute material. Our group has previously reported the formation of brushite cement from orthophosphoric acid and several calcium phosphates such as beta-tricalcium phosphate or nanocrystalline hydroxyapatite. In this study, polyphosphoric acid was investigated as a calcium phosphate cement component. We found that brushite cement was not formed when polyphosphoric acid was mixed with beta-tricalcium phosphate but instead monocalcium phosphate monohydrate (MCPM) was formed. Although stronger in compression when tested dry, MCPM is readily soluble under physiological conditions and cement loses structural integrity within hours of aging in water. However, by varying the solid component phases, we have discovered a new route to the formation of brushite cement. The mixture of polyphosphoric acid, water, and tetracalcium phosphate yielded cement composed predominantly of brushite. The mechanical performance, microstructure, and setting time of this cement appeared to be dependent on the composition of the set cement, which in turn was determined by the composition of cement reactants in the cement slurry.