Time-dependent mechanical properties of HA/TCP particles in relation to morsellized bone grafts for use in impaction grafting.

In reconstructive surgery human bone defects are sometimes filled with the use of the impaction bone grafting technique. Currently different types of biomaterial particles are being developed as bone-substitute materials. Before these biomaterials can be applied their mechanical and biological behavior should be characterized. In this study the time-dependent mechanical behavior of biomaterial particles with different tri-calcium-phosphate/hydroxy-apatite (TCP:HA) ratios, particle sizes, and porosities is determined and compared to the behavior of human bone grafts, the latter being the standard material currently used to augment bone defects. The mechanical properties were assessed with the use of dynamic confined compression creep tests with a loading and unloading phase. Different graft material groups were tested, consisting of 100% human bone grafts, 100% biomaterial particles, and 50:50 weight mixtures of human grafts and biomaterial particles. No damage to the particles was observed by the impaction in the test chamber or by the dynamic load. Relative to the human graft material, the biomaterial particles hardly deformed under loading, were much stiffer, and showed almost no viscoelastic behavior. The mixtures showed intermediate results. Particle size and porosity influenced the behavior of the biomaterial particles. TCP:HA ratio did not have a great effect. The conclusion is that the application of these particles should be done with great care, as their mechanical behavior is drastically different than that of the human graft material. Mixing it with human bone grafts gave the material some biphasic, viscoelastic behavior that may be important for its biological response.