Cement casting of calcium pyrophosphate based bioceramics

An ideal bone graft replacement should degrade at the rate that new bone can be formed. Owing to the variability in the rate of bone turnover with age, diet, sex and health, this is almost impossible using traditional bioceramics such as hydroxyapatite and phosphate glasses. One way of achieving such a link is by coordinating implant dissolution with a specific biological stimulus associated with tissue formation. In the case of bone, one appropriate stimulus may be the enzyme alkaline phosphatase which is found in high concentrations attached to the membranes of cells responsible for bone formation (osteoblasts). It has been proposed that alkaline phosphatase functions by locally catalysing the hydrolysis of pyrophosphate ions, which are known inhibitors of hydroxyapatite formation thus enabling bone formation. It is possible, therefore, that the degradation of calcium pyrophosphate ceramics could be coordinated with bone formation. In the few studies that have been published on the use of this material as a bone replacement, the grafts were formed by pressing and sintering β-dicalcium pyrophosphate powders. This approach imposes limits on implant morphology and can be associated with significant material shrinkage. In this paper, the fabrication of calcium pyrophosphate ceramics by heating brushite based cement materials has been investigated. The results obtained in the study showed that complete conversion from brushite to γ-calcium pyrophosphate occurred at 400°C. Increasing sintering temperature from 400 to 1000°C resulted in an increase in compressive strength from 6·9 to 10·2 MPa and allowed the conversion of the γ-calcium pyrophosphate to the β form. The sintering process was associated with a considerable reduction in specific surface area which may limit the rate of resorption of the sintered material.