Fabrication of Porous Calcium Phosphate Bioceramics as Synthetic Cortical Bone Graft

Abstract

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.