Phase composition, mechanical performance and in vitro biocompatibility of hydraulic setting calcium magnesium phosphate cement

Brushite (CaHPO4·2H2O)-forming calcium phosphate cements are of great interest as bone replacement materials because they are resorbable in physiological conditions. However, their short setting times and low mechanical strengths limit broad clinical application. In this study, we showed that a significant improvement of these properties of brushite cement could be achieved by the use of magnesium-substituted β-tricalcium phosphate with the general formula MgxCa(3–x)(PO4)2 with 0 < x < 3 as cement reactants. The incorporation of magnesium ions increased the setting times of cements from 2 min for a magnesium-free matrix to 8–11 min for Mg2.25Ca0.75(PO4)2 as reactant. At the same time, the compressive strength of set cements was doubled from 19 MPa to more than 40 MPa after 24 h wet storage. Magnesium ions were not only retarding the setting reaction to brushite but were also forming newberyite (MgHPO4·3H2O) as a second setting product. The biocompatibility of the material was investigated in vitro using the osteoblast-like cell line MC3T3-E1. A considerable increase of cell proliferation and expression of alkaline phosphatase, indicating an osteoblastic differentiation, could be noticed. Scanning electron microscopy analysis revealed an obvious cell growth on the surface of the scaffolds. Analysis of the culture medium showed minor alterations of pH value within the physiological range. The concentrations of free calcium, magnesium and phosphate ions were altered markedly due to the chemical solubility of the scaffolds. We conclude that the calcium magnesium phosphate (newberyite) cements have a promising potential for their use as bone replacement material since they provide a suitable biocompatibility, an extended workability and improved mechanical performance compared with brushite cements.