| Although ultrahigh molecular weight polyethylene (UHMW-PE) has been used in total joint reconstruction for more than 30 years, the wear, creep, and other long-term characteristics of the polymer have been concerning surgeons and bioengineers since its introduction. Wear and creep of UHMW-PE can lead to loosening, and eventually failure of the arthroplasty. Many factors, such as materials, processing, and sterilization can affect the extent of wear and creep of articulating materials.;This study investigates effects of reinforcement and gamma irradiation on mechanical and thermal properties of UHMW-PE, and the following questions are addressed: (1) Can creep resistance of UHMW-PE be improved? (2) How will gamma irradiation affect the properties of the polymer? (3) Can the long-term creep behavior of the material be predicted?;To achieve the research goals, fiber reinforcement was first used to improve the creep performance of UHMW-PE. Then, gamma irradiation was conducted at 1.25 to 5 Mrad in different irradiation environments, and post-irradiation effects were monitored over a period of one year. Time-temperature superposition and extrapolation methods were used to predict long-term creep behavior of UHMW-PE systems.;The research has lead to the following results. Firstly, a novel fiber self-reinforced UHMW-PE composite has been developed and shown to have higher creep resistance as compared to the non-reinforced polymer. No changes in wear performance for the self-reinforced composites were observed. Secondly, it has been found that gamma irradiation, types of irradiation environment and post-irradiation aging affect the thermal and tensile properties of the composites and their individual components. Finally, time-temperature superposition and extrapolation methods were successfully used to predict the long-term creep behavior of the materials. The shift factors associated with time-temperature superposition were found to increase with increasing temperature, as per the Arrhenius equation, and their differences could be explained by free volume theory. The study suggests that successful modification of UHMW-PE by self-reinforcement and minimizing undesirable radiation sterilization effects by using controlled environment may lead to a solution to the clinical problems associated with the currently-used material systems in total joint arthoplasty. |
