Modification of Calcium Phosphate Cement with α-Hydroxy Acids and Their Salts

Calcium phosphate cements (CPCs) are usually modified by organic/inorganic additives to improve their mechanical performance and to adjust their rheological and setting properties to clinical requirements. In this work we used several nontoxic and biocompatible α-hydroxylated organic acids (glycolic, lactic, malic, tartaric, and citric acids) and their calcium and sodium salts for the modification of CPC. The unmodified cement used in this study consisted of an equimolar powder mixture of tetracalcium phosphate (TTCP) and dicalcium phosphate anhydrous (DCPA) mixed with water at a powder mass/liquid volume ratio of 3.3 g/mL. It had a compressive strength of 38 MPa and an initial setting time of 8 min. The free acids as cement liquids had mainly a detrimental effect on the strength of the cement and led to a decreased setting time around 2−4 min, while the calcium salts did not significantly alter the cement properties. However, the sodium salts of the oligocarboxylic acids (malic, tartaric, and citric acids) resulted in a liquefying effect combined with a strong reinforcement of the mechanical strength, such that compressive strength increased to 78−99 MPa. The liquefying effect and prolonged setting time of these compounds was thought to derive from a strong increase in the surface charge of both reactants and the reaction product hydroxyapatite as determined by ζ potential, which increased from about −15 and −18 mV in pure water for TTCP and DCPA, respectively, to values around −40 to −50 mV. In contrast, the calcium salts did not alter ζ potentials due to the formation of neutral and stable complexes in aqueous solution.