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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Creep behaviour of hydroxyapatite reinforced polyethylene composites.

Author: SUWANPRATEEB, Jintamai

Year: 1995

Supervisor(s): Liz Tanner, Bill Bonfield

Hydroxyapatite reinforced polyethylene composites have been developed as a bone-substitute material for clinical applications. A particular advantage over existing implant materials is that their mechanical properties can be varied to suit the application. In addition, the ductility of the material allows final shaping by the surgeon prior to implantation. The use of hydroxyapatite as an reinforcing phase has produced excellent biocompatibility, and favourable bioactivity, for the composites, inducing bone ongrowth onto the materials after implantation. Current applications of these materials are in reconstruction of orbital floor fractures or orbital volume augmentation using 0.40 volume fraction composite and as a middle ear implant.
In this study, a long-term mechanical property, creep, of polyethylene and of hydroxyapatite reinforced polyethylene composites at filler volume fractions of 0.20 and 0.40 has been evaluated. Isochronous stress-strain experiments, creep experiments and recovery experiments were performed. The creep experiments were performed on a special three-station tensile creep machine developed for the project and the calibration of the machine function is reported. The effect of hydroxyapatite content, gamma-irradiation for sterilisation, long-term immersion in Ringer's solution and thermal annealing upon the properties of the test materials were investigated. Physical properties such as density, molecular weight distribution and thermal properties of the polyethylene and hydroxyapatite-polyethylene composites were also studied.

It was found that increasing hydroxyapatite content resulted in an increasing creep resistance. For a given volume fraction, both sterilisation and thermal annealing further increased the creep resistance. (DXN005,217)