ENGINEERING FAILURE ANALYSIS OF RETRIEVED ACETABULAR CUPS

We conducted an engineering failure analysis of retrieved acetabular cups. From one centre, 37 properly-marked components were retrieved. The details of the patients were noted. Of the 37 components, 27 were brought to the laboratory for an engineering investigation of the cause of failure. A further 10 components were also taken to the biochemistry laboratory within an hour of retrieval, with tissue removed from the patients. The purpose of the investigation was to determine whether any proteins were deposited inside the cups and to see whether there was wear debris on either the retrieved component or in the tissue surrounding the prosthesis. We used visual inspection, colour-dye penetrant, stereo-microscopes, scanning electron microscopes, and mass-spectometric analysis to examine the cups. Debris was captured using 0.4-um filters. We found mechanical failure in vivo is mainly caused by plastic flow. Fretting is the second most likely cause of failure. Both of these are indicative of localised overheating between the acetabular component and the ceramic femoral head. The most likely cause of the overheating is lack of lubrication. With electrophoresis it became evident that at some time in their in-vivo service these cups had reached temperatures exceeding 60°C. These temperatures were confirmed on a five-poster hip simulator. We suggest that an in-depth study be undertaken to establish the method of in-vivo lubrication and the lubricity of the available lubricant.