A three-axis hip joint simulator for wear and friction studies on total hip prostheses

A three-axial, single-station hip joint simulator was designed and built for wear and friction studies on total hip prostheses. The design of the apparatus is described in detail. Continuous level walking is simulated. All three motion components, flexion-extension, abduction-adduction and internal-external rotation, are included. The motions are implemented electromechanically and the uniaxial load pneumatically. The load is measured continuously. For accurate measurement of wear, the apparatus has a loaded control joint, which also renders both the test and control joints self-centering, as they are loaded in series. The frictional torque of the test joint can be measured continuously throughout the wear test, which is an exceptional feature. Four tests of five million cycles each were completed using 32 mm diameter Co-Cr-Mo femoral heads and 5.6 mm thick, metal-backed, ultra-high molecular weight polyethylene acetabular cups as test specimens. Their wear and friction behaviour is described and discussed in relation to previous simulator studies and clinical observations. The lubricant was distilled water, maintained at body temperature. The wear of the cups was measured gravimetrically at intervals. The average wear rate was 3.9 mg/one million cycles, corresponding to 0.03 mm/year, and the average coefficient of friction was 0.01.