Friction Measurement in the Biaxial Rocking Motion Hip Joint Simulator

The established biaxial rocking motion (BRM) hip joint simulator was complemented by a novel friction measurement accessory. This simple and practical system, which was easily added to an existing BRM design, is described in detail and shown to perform well in long-term wear tests and in comparative tests of 24 h duration involving several different bearing couples. The system was based on the measurement of frictional torque about the leaning axis of the lower component, the femoral head. In the 28 mm CoCr-on-ultrahigh molecular weight polyethylene articulation with diluted calf serum lubricant at body temperature and 1 kN static load, the maximum value of frictional torque during a cycle was 1.2 N m on the average. The alternative system based on the measurement of torque about the vertical loading axis was shown to be less sensitive and highly insensitive when contact area was small, as was the case with alumina-on-alumina.