Fluid film lubricating ability of a total hip prosthesis depends on the profile accuracies including surface-roughness or the sphericity of a head or a cup. Therefore, surface polishing is important. It was, however, difficult to polish the central portion of a cup or head using the conventional rotating machine. In the present study, we developed a polishing method combining a pendulum machine and a robotic arm. The effect of the accuracy improvement by this method was evaluated by the friction measurements on some test specimens.

Nine balls and a cup of Co-Cr-Mo alloy that were polished by a conventional process using a rotating machine were prepared for the prototype. The average diameter of the balls was 31.9648 mm with the sphericity of 0.0028 μm. The inside diameter of the cup was 31.9850 mm with the sphericity of 0.0044 μm. We combined a robotic arm and a pendulum apparatus to enable the further polishing. The ability of both automatic centering and change in the sliding direction was accomplished by this system. The sliding direction has been changed 180 times every ten degrees. The total distance of polishing was 120 m under vertical load of 100 N in a bath of saline solution containing abrasive grains of silicate of the diameter of 2μm. The surface roughness of the central portion of the cup, which is important area for the fluid film lubrication decreased from Ra 20.2 μm before the polishing to Ra 18.7 μm after the polishing.

A pendulum type friction tester was used for the assessment of the improvement of the lubricating ability by the polishing. The measurement was run over at 10 times under the conditions of the load of 600 N in a bath of saline solution. As the result, the frictional coefficients decreased from 0.1456–0.1720 before polishing to 0.1250–0.1300 after polishing. The polishing effect was, however, observed only at the specimens that radial clearances did not exceed the value of 50 μm.

The present results indicated that the surface polishing of the central portion of hip prostheses must improve the lubrication ability and the radial clearance before the finishing process should be chinked as possible.