FTIR-monitoring of a fast setting brushite bone cement: effect of intermediate phases

The setting reaction of an equimolar beta-tricalcium phosphate/monocalcium phosphate monohydrate (beta-TCP/MCPM) cement was monitored in real time with ATR-FTIR at 23 and 37 degrees C using powder to liquid ratios (PLRs) of 2.0 and 3.3 g ml(-1) and aqueous retardant citric acid concentrations of 800, 1000 and 1500 mM. The final set products, for PLRs of 2.0 to 3.3 g ml(-1) and citric acid concentrations of 300 to 1500 mM, were characterised with regard to phase composition, compressive strength, density and relative porosity. FTIR provided evidence for the formation of an intermediate dicalcium phosphate-citrate complex (DCPC). As the concentration of citric acid in solution increased so did the maximum level of citrate intermediate. Decreasing the PLR reduced the rate of citrate removal, but had no effect on its rate of formation or maximum level. FTIR also indicated a time delay before formation of any observable dicalcium phosphate (DCP) in solution. This delay increased as the citric acid solution concentration was raised or the temperature reduced, but was less affected by the PLR. There was then an additional delay between DCP formation in solution and its precipitation. Both Rietveld analysis of XRD patterns and density measurements 24 h after setting confirmed that the final product was primarily dicalcium phosphate dihydrate (DCPD or brushite) when the citric acid concentration was less than 1000 mM, irrespective of temperature or PLR. On the other hand, with 1500 mM citric acid significant levels of dicalcium phosphate anhydrous (DCPA or monetite) also present, this led to increased porosity and a dramatic decline in strength. As the levels of the intermediate phase increased, the final wet compressive strength of the resulting cements also deteriorated. It is therefore proposed that strength reduction may be due to formation of the intermediate at early stages of setting or DCPA formation in the final product, both causing increased material inhomogeneity. This study thereby illustrates that real time ATR-FTIR monitoring of a setting reaction clearly indicates that there is an upper limit to the use of citric acid as a setting retardant for a fast setting brushite-forming cement system, a limit that can also be expected for the use of other setting retardants, and that ATR-FTIR monitoring comprises a useful complement to the traditional before-after investigations.