Cement from nanocrystalline hydroxyapatite: effect of calcium phosphate ratio

Nanocrystalline hydroxyapatite (nHA) can be mixed with phosphoric acid to form a brushite cement; a degradable inorganic bone filling material. nHA was precipitated from reactants of calcium to phosphate (Ca/P) ratio 0.8 to 2.0 and mixed with phosphoric acid, which resulted in the formation of a brushite cement. Cement was also formed by mixing microcrystalline calcium phosphates, beta-tricalcium phosphate, hydroxyapatite and tetracalcium phosphate with phosphoric acid solution. Cement produced with nHA was stronger in compression than that formed with crystalline calcium phosphate phases. Setting time, strength and composition of cement produced with nHA was dependant on both the Ca/P ratio of nHA and the concentration of phosphoric acid in cement slurry. Increasing phosphoric acid concentration increased compressive strength whilst reducing the initial setting time of cement. Reducing the Ca/P ratio of nHA precipitation reactants retarded the setting and increased the extent of reaction of cements. This finding was unexpected and suggests that Ca/P ratio may strongly affect dissolution behaviour and this parameter is more important than stoichiometry in determining extent of reaction in this system. This study demonstrated that the wide variation in stoichiometry that may be attained in nanocrystalline apatite may be utilised to change cement performance and setting behaviour.