Introduction: Joint replacement procedures such as revision impaction grafting and spinal fusion interbody operations are stretching allograft bone stocks to their limits. The need for synthetic alternatives that offer a structural and biological matrix for graft incorporation are paramount for future bone regeneration procedures. Synthetic bone graft alternatives that offer biocompatibility to the host bone (i.e. a biological response) such as hydroxyapatite/tricalcium phosphate (HA/TCP), in addition to possessing an interconnected porosity network have been shown to have a strong influence on the osteoinductive potential of these materials. The current method allows the production of calcium phosphate ceramic components (CPC) that possess an interconnected open porous network in the required size range for osteoid growth and revascularisation.

Materials and Methods: The method can be described as the reticulated foam technique, whereby two grades of calcium phosphate powder are blended together to form a HA/TCP ceramic slip. The slip is then ball milled for 24hrs with zirconia milling media. This slip is used to impregnate polyurethane (PU) foam via a mechanical plunging procedure. The impregnated foam is then held above the slip bath in order for the slip to flow and coat the struts of the foam. The impregnated foam is then dried on tissue paper and treated with high velocity compressed air to avoid the formation of any closed cells. Samples are dried at 120°C for 15hrs. The PU foams are graded as 30 and 45ppi (pores per inch). The slip viscosity ranges from 6000 – 8000 cps (measured with a Brookfield Viscometer, spindle no. 5 and at 10rpm). Samples are sintered slowly until 600°C to ensure PU burnout is complete. Sintering continues up to 1280°C to ensure densification. Image analysis was performed using optical microscopy, digital photography and SEM analysis. Mechanical testing was performed by 3 point bending using an 1122 Instron.

Results: Macroporosity in the samples varied from 40 – 70%. Typical pore sizes far exceeded 300μm (the pore size acknowledged as that needed for osteogenesis). Approx. 79% of all pores were between 150 – 450μm in area equivalent diameter. Typical strut thicknesses ranging from 100 – 500μm were also reported, as was a strut thickness-pore size-mechanical strength relationship. One hundred and twenty samples possessed a breaking stress with a 95% confidence level of 0.30MPa±0.01MPa. The low strengths reported are due to the formation of blow-out holes at triple point junctions on the interconnected struts.

Conclusions: Major requirements for replacement bone materials have been met including a wide range of interconnected porosity from 50 – 1000μm. Bioactivity combined with an excellent porosity size range suggests excellent possibility of osteogenesis. In addition, this fabrication procedure offers consistency and reliability. Future work will focus on improving the strength of these open porous calcium phosphate ceramics.

Introduction: Calcium phosphate based ceramics with a porous configuration are attraction for use as synthetic bone grafts as the porous network allows tissue ingrowth, which further enhances the implant-tissue attachment. The degree of interconnectivity and the nominal pore size are the critical factors that determine the success of the implants. It is generally accepted that a minimum pore size of 100 μm is necessary for the porous implant materials to function well and a pore size greater than 200 μm is an essential requirement for osteo-conduction. However, research has suggested that the degree of interconnectivity is more critical than the pore size. In this study, porous Hydroxyapatite/Tricalcium phosphate (HA/TCP) bioceramics with interconnected porosity and controlled pore sizes were fabricated by a novel technique involving vacuum impregnation of reticulated polymeric foams with ceramic slip. HA/TCP samples with a range of pore sizes and functionally gradient materials (FGM) with porosity gradients were made.

Materials and Methods: Two grades of calcium phosphate powder, TCP 118 and TCP 130, were used. Varying the blend ratios could change the ratios of HA and TCP in the sintered samples. The foams used comprised polyurethane (PU) which had one of three different porosities 20, 30 and 45 pores per inch (ppi). In order to make a FGM with porosity gradients mimicking the bimodal structure of cortical and cancellous bone, two different foams were either joined together by sewing or pressfitting together. The foams were substantially impregnated with slip by vacuum impregnation. The impregnated foams were removed from the vacuum chamber and dried on tissue for at least 24 hours then sintered at temperatures of up to 1280°C.

Results and Discussion: Using a slip with the appropriate viscosity, porous HA/TCP bioceramics having interconnecting pores and a range of pore sizes can be produced successfully. By joining different ppi foams together, it is possible to develop functional gradient materials in which the porosity varies through the thickness of the samples. No weakness could be seen at the interface between the two different structures. This demonstrated that porous HA/TCP with two or more different levels of porosity could be produced in a single block. Image analysis shows the porosity measured for the three different foams was similar. The area equivalent diameters of the pore structure are 197–254 μm with 20ppi foam, 143–183 μm with 30ppi foam and 105–127 μm with 45ppi foam. The compressive strengths of the HA/TCP samples are in the range of 30–170 MPa and the apparent densities were 2.34–2.76 g/cm3. The technique developed for fabricating porous bioceramics can be extended to produce a range of bone substitute materials with properties tailored to specific clinical applications.