Georgiou, George; (2002) Development of glass reinforced hydroxyapatite for hard tissue surgery. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

Hydroxyapatite (HA) is of great interest as a material for surgical implants due to its biocompatibility with living tissue, however, synthetic HA is limited in its use as a biomaterial, this is primarily due to its low load bearing capacity and is reflected by the relatively low mechanical properties compared to bone. The aim of this project is to improve the mechanical properties of synthetic hydroxyapatite by optimising the processing method and also by incorporating a phosphate glass as a sintering aid to form Glass Reinforced Hydroxyapatite (GR-HA). Phosphate glasses were introduced into hydroxyapatite at 2.5wt% and 5wt% additions, during the milling process and subsequently sintered to 1200°C or above. The materials were characterized in terms of density, phase analysis, porosity, grain size, flexural strength, hardness, flexural modulus and fracture toughness. Mean values ranging from 96.43 to 142.59MPa were obtained for the flexural strength of a GR-HA containing a 2.5wt% calcium phosphate glass with a 15mol% composition of CaF2. This composite therefore exhibits strength values that are comparable to those recorded for cortical bone (50-150MPa). HA alone, however, exhibited much lower flexural strength values ranging from 42 to 53MPa. The increased mechanical properties were attributed to the presence of residual stress introduced due to HA decomposing to -tricalcium phosphate ( -TCP) and in some cases a- tricalcium phosphate (a-TCP). Hot pressing was also employed as an alternative processing method that could potentially enhance the mechanical properties. This method gave flexural strength values of 79 and 92MPa for HA and GR-HA respectively at a sintering temperature of 1100°C. Sintering at temperatures above 1100°C did not improve the mechanical properties any further even though densification occurred at a lower temperature. This could be attributed to other factors such as grain growth. In vitro studies examined the effects of surface chemistry on cell attachment, cell spreading and proliferation, differentiation and function of an osteoblast-like cell line cultured on the surface of HA and GR-HA. The results suggest that the presence of phosphate glasses provides a suitable chemical composition which up-regulates the expression of proteins such as bone sialoprotein (BSP), osteonectin (ON), fibronectin (FN) and collagen I (COL I) compared to HA alone. Compared to the control, both HA and GR-HA did not show a high level of cell proliferation.

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