ASSESSMENT OF POROUS HYDROXYAPATITE FOR BONE REPLACEMENT
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Hydroxyapatite (HA) is a highly biocompatible calcium phosphate which closely resembles the mineral component of bone. Porous structures composed of biocompatible materials are believed to enhance fixation (and integration) as they encourage the ingrowth of bone into the implant. Consequently there is great interest in the potential of porous HA as an alternative to bone homo grafts and auto grafts. However, despite the interest in the field, there has been no thorough investigation into the physical and structural properties of porous HA and their effects on bone ingrowth. The material used in this study was a carbonated apatite (containing < 0.9 % levels of trace element impurities) with a trabecular macrostructure, that had been converted from bovine cancellous bone. The apparent density of the material ranged from 0.35 -1.45 g.cm-3 and the macrostructural morphology varied from an open equiaxed foam to a columnar honeycomb-like structure. The ultimate compressive stress was strongly related (r = 0.9) to the square of the apparent density, while compressive modulus was influenced by both apparent density and macrostructural morphology. Transmission electron microscopy of human osteoblast-like cells cultured on the material demonstrated that cells were closely associated with the surface. Specimens with densities of 0.6.0.9 and 1.2 g.cm-3 were then implanted in a lapine cancellous site for periods of 10 days, 3, 5, 13 and 26 weeks. After implantation all specimens elicited a highly biocompatible response, with active areas of bone deposition, remodelling and revascularization and no fibrous encapsulation. The amount of bone ingrowth within the implant (25-10%) after.5 weeks was found to vary with apparent density (0.6-1.2 g.cm-3) indicating that osseointegration was a function of macrostructural morphology. Pushout testing of retrieved spedmens indicated that all implants were securely fixed by 5 weeks (2-3 MPa). Compression testing demonstrated that after 5 weeks low density implants were sufficiently reinforced by bone ingrowth to equal the compressive strength of the host tissue (6 MPa) which increased to approximately 20 MPa at 3 and 6 months.
AuthorsHing, Karin A
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