Mechanical properties of calcium sulphate/hydroxyapatite cement

Setting times, volume after setting, injectability and hardness (at 37 degrees C in contact with Ringer's solution) were determined for cements made of mixtures of calcium sulphate hemihydrate (CS) and hydroxyapatite (HA) with a range of compositions. The purpose of these experiments was to determine the behaviour of a mixture that could be used as an injectable cement for orthopaedic applications, including spinal fusion. A suitable mixture consisted of 60% CS and 40% HA by mass; a slurry was made by mixing solid (36 g) with water (15 cm(3)). The slurry had initial and final setting times of 5.7 +/- 1.3 min and 19.6 +/- 0.7 min (mean +/- standard deviation), respectively. The hardness of the cement did not systematically increase or decrease in the 72 h following the final setting time. The volume of the cement was 99.8 +/- 0.4% of the volume of the initial slurry, i.e. there was negligible shrinkage on setting. It was able to withstand a pressure of 7.3 +/- 1.2 MPa, applied by a hemispherical indenter before the onset of permanent damage, indicating adequate strength for spinal fusion.