Pyrophosphate ions are both inhibitors of HA formation and substrates for phosphatase enzymes. Unlike polyphosphates their hydrolysis results simultaneously in the complete loss of mineral formation inhibition and a localised elevation in orthophosphate ion concentration. Despite recent advances in our knowledge of the role of the pyrophosphate ion, very little is known about the effects of pyrophosphate on bone formation and even less is known about its local delivery. In this work we first developed a self setting pyrophosphate based calcium cement system with appropriate handling properties and then compared its in vivo degradation properties with those of a non-pyrophosphate containing control. Contrary to expectation, the presence of the pyrophosphate phase in the cement matrix did not inhibit mineralisation of the healing bone around the implant, but actually appeared to stimulate it. In vitro evidence suggested that enzymatic action accelerated dissolution of the inorganic pyrophosphate ions, causing a simultaneous loss of their mineralisation inhibition and a localised rise in supersaturation with respect to HA. This is thought to be a rare example of a biologically responsive inorganic material and these materials seem to be worthy of further investigation. Bioceramics to date have mainly been limited to orthophosphate, silicate and carbonate salts of calcium, here we report the successful application of a pyrophosphate material as a degradable osteoconductive bone repair cement.

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 b-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 c-calcium pyrophosphate occurred at 400uC. Increasing sintering temperature from 400 to 1000uC resulted in an increase in compressive strength from 6?9 to 10?2 MPa and allowed the conversion of the c-calcium pyrophosphate to the b 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.

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How to cite:Mehrban, N.; Paxton, J. Z.; Bowen, J.; Bolarinwa, A.; Vorndran, E.; Gbureck, U. and Grover, L. M. (2011 AbstractIn order to combat the low regenerative capabilities of ligaments full 'bone to bone' replacements are required, which will integrate with bone while providing a smooth transition to the replacement soft tissue (tissues surrounding organs in the body, not being bone). This study investigated the use of 3D powder printing technology to form calcium phosphate brackets, previously used for forming bespoke scaffold geometries, to 95% ± 0.1% accuracy of their original CAD design. The surface and internal structure of the printed samples was characterised both chemically and morphologically and compared with hand-moulded cements in the dry state and after 3 days of immersion in phosphate buffered saline. X-Ray Diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM) all showed the presence of brushite in the hand-moulded samples and brushite and monetite within the printed samples. Furthermore, the printed structures have a higher level of porosity in the dry state in comparison to the hand-moulded (36 ± 2.2% compared to 24 ± 0.74%) despite exhibiting a compressive strength of almost double the hand cast material. Although the compressive strength of the printed cements decreases after the 3-day immersion, there was no significant difference between the printed and hand-moulded cements under the same conditions. 3D powder printing technology has enabled the manufacture of bespoke calcium phosphate brackets with properties similar to those reported for hand-moulded cements.