Hydroxyapatite foams for bone replacement

Hydroxyapatite, often in the form of synthetic porous blocks, has been used in the repair of bone defects for over 20 years owing to its biocompatibility and osseoconductive behaviour. Bone ingrowth requires the existence of open and interconnected pores with diameters larger than 150 μm for proper circulation of nutrients. Hence, currently available materials are characterised by poor mechanical properties. Collapse of such products is therefore a major source of concern to surgeons using these weak materials in bone surgery. There is a need to develop stronger highly porous structures through adequate control over the size, shape and volume fraction of pores. In this work, highly porous open-cell hydroxyapatite foams were fabricated by the polymer foam replication process, where two types of polyurethane (PU) foams were infiltrated with optimised slurries containing appropriate binders and ceramic particles, followed by the removal of excess slurry, burning out of the polymer to leave a ceramic replica of the polyurethane and finally high temperature sintering. Open-cell HAP foams with porosities of about 80% were obtained, i.e. 30% higher than that determined for commercial ones (50%). Many of the commercial foam cells approach 500 μm in diameter whereas the developed foam cell size ranged from 300 up to 500 μm. The ultimate compressive strength of the developed foams (1-2 MPa) was found to be higher than that recorded for the commercial ones (0.7 MPa) indicating that these foams can more easily be modelled in theatre. Both the elastic moduli and the compressive strength of the developed foams were found to increase with increasing of the relative density, in accordance with the predictions of available micro-mechanical models.