ROLE OF PHYSIOCHEMICAL PARAMETERS IN THE OSTEOGENIC POTENTIAL OF CALCIUM PHOSPHATE BIOMATERIALS
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The number of clinical procedures performed in the USA using bone graft substitutes was estimated at 1.1 million in 2010 and is projected to reach 1.3 million in 2015. This increasing demand for bone graft substitutes is a result of an ever-ageing population coupled with recent reports in the clinical literature of concerns regarding the safety of allograft and recombinant bone morphogenetic proteins such as rh- BMP-2 and the supply of autograft, which has led to an increased clinical interest in synthetic alternatives to allograft; autograft; and recombinant growth factors. One such synthetic material is silicate-substituted hydroxyapatite (SiCaP). Mechanical testing revealed SiCaP to have similar mechanical behaviour to morcellised cancellous bone. In computated spinal and hip models the simulated stresses in SiCaP were determined to be low when in situ, indicating a stressshielding effect from the implanted metalwork and surrounding bone. We also found an inverse relationship between porosity and Young’s Modulus. Our results indicated that the strut-porosity of a material substrate should be increased to maximise the potential for formation of a precursor to bone-like apatite after implantation in osseous defects and further confirmed previous reports that betatricalcium phosphate is less bioactive than hydroxyapatite. We demonstrated a direct link between the amount of strut-porosity and the osteoinductivity of SiCaP. We learned that adding a resorbable carrier phase did not impair the osteoinductive potential of SiCaP, suggesting that osteoinductivity is not necessarily determined in the first 24-48 hours post implantation. Most notably from our studies we determined that the osteoinductivity of SiCaP correlated with its performance in orthotopic defects. Our research confirmed our hypothesis that modifying the micron-scale physical structure of a hierarchical porous SiCaP based biomaterial influences its functional performance in vitro and such modifications can be applied to improve its performance outcomes in ectopic and orthotopic treatment sites in vivo.
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