Improved friction and wear performance in polymeric components of orthopaedic bearings.

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Hip and knee replacements are most often designed using specialized components made of metal (cobalt chrome) and plastic (ultrahigh molecular weight polyethylene). These devices are the treatment of choice for severe degenerative joint disease, and they are now being implanted in younger more active patients. As a result, there is an increased need to improve their durability, particularly with respect to wear in the plastic component. Of concern is the phenomenon of osteolysis, in which the bone surrounding an artificial joint reacts to plastic wear particles and dies. Consequently, the implant loosens and needs to be replaced. There have been improvements in the design of polyethylene for use in hips, but this has not been the case for highly stressed joints such as the knee. ******This project examines a novel way of treating the plastic to reduce wear. It is thought that indentations in the polyethylene surface will entrain joint fluid, thus developing locally pressurized regions that reduce the stress on the plastic itself. In a previous NSERC-funded study, it was discovered that specific patterns of indentations reduce plastic stress and also reduce friction in these systems. However, research in other laboratories has shown that one of the conditions leading to excessive wear is not the continuous motion of walking, but the discontinuous motion of stopping and starting. It is thought that during these episodes, the fluid that lubricates the surfaces seeps out when stopping, thus leaving the plastic unprotected from wear when motion starts again. ******The objective of this project is to understand this mechanism of wear and to determine whether the initial positive findings of texturing will result in reduced plastic wear during intermittent motion. Samples of plastic will be textured and tested in a device that simulates the stop-start motion. If successful, the texturing process will be evaluated on artificial joints in a wear simulator that produces the same loads and motion that occur in real life.******Nearly 25,000 Canadians and 600,000 worldwide are implanted with an artificial knee each year. By improving their durability beyond the 15 years currently expected, researchers hope to provide a growing number of our aging population with a lifetime of activity well past what is now possible.