Radioisotope tracing of knee implant wear

The incidence of knee replacement surgery in Australia (∼30,000 per year) is rising annually by about 10%. Due to increasing life expectancy, and due to medical complications caused by wear debris, enhanced wear resistance of knee implants will reduce overall health costs and will benefit patients' quality of life. Particulate wear debris from the UHMWPE component of the implant, typically causes the inflammatory reaction that breaks down bone around the prosthesis and limits its lifespan. Previous measurement attempts of wear in surgical implants using mass loss or geometrical techniques have drastically been lacking in sensitivity. We propose a new concept which promises to be more sensitive and may allow the characterisation of the wear debris shedding. Radioisotope tracers are introduced into knee joint materials and wear rates are derived by detecting the radioactivity of the prosthesis lubricant during laboratory simulation. The radioisotope tracer chromium-51 with a half-life of∼30 days is well suited for this work. It is aimed to precisely place the tracer atoms at a certain material depth (∼50 nm) using ion implantation, at minimum atomic concentration and with negligible material modification, however, with detectable activity. This technique may allow the detailed measurement of early wear phenomena and debris tagging. Alternative ways of introducing the tracer into the material are also studied. In initial work a constant load knee simulator has been constructed to establish correlations between wear debris shedding and measured radioactivity. In order to test the efficacy of the simulator a wear rate curve has been measured for a test-polymer. The morphology of wear debris from the test-polymer has been characterised using Atomic Force Microscopy.