AN EVALUATION OF THE PENETRATION OF CERAMIC FEMORAL HEADS INTO POLYETHYLENE ACETABULAR CUPS

It is desirable to evaluate the performance of new designs of total replacement hip joints and new materials in joint simulators prior to clinical trials, yet few satisfactory hip joint simulators are available. In a previous paper (D. Dowson and B. Jobbins, Eng. Med., 17 (1988) 111-117) we have reported on the design and development of a versatile hip joint simulator and recorded an initial study of the performance of a standard 22 mm diameter Charnley prosthesis. There is now considerable interest in the use of ceramic femoral heads in total replacement hip joints and in the present paper we report the performance of larger diameter (32 mm) femoral heads made from high purity, fine grain alumina. A new procedure for the evaluation of penetration and wear volumes based upon the use of coordinate and roundness measuring machines is described. The results show a relatively rapid initial penetration of the femoral heads into the ultra-high molecular weight acetabular cups during the first 1.5 X 10(6) cycles of loading, followed by a very low penetration rate of about 27 mum per million cycles. This is shown to compare favourably with the previous findings for smaller, metallic Charnley prosthesis femoral heads of 22 mm diameter.