Potential thermal artifacts in hip joint wear simulators

Frictional heat was monitored during wear tests of ultrahigh molecular weight polyethylene acetabular cups bearing against femoral balls of metal or ceramic in a hip simulator, using bovine serum as a lubricant. About 1 to 2 h of continuous cycling were required for the temperature in the zone of contact between the cup and ball to rise to its maximum steady value, and this equilibrium temperature was markedly higher with increased load and/or cycling rate. Frictional heating caused substantial precipitation of the proteins from the serum and, in some of the tests running at 1.5 or 2 Hz, an adherent proteinaceous layer was observed attached to the surface of the balls. The maximum temperature was also substantially higher in tests run with the cup mounted above the ball rather than below. Surprisingly, the tests running at higher frictional torque and temperature (i.e., those with the most protein precipitation and/or adherent layers) produced the least wear of the polyethylene. This might have been due to the solid proteins that formed a protective layer between the ball and cup. Because patients with hip prostheses typically do not walk for hours without rest, the maximum temperatures in vivo are likely to be much lower than those reached in the hip simulator. Therefore, the affects of protein precipitation on the resultant wear properties of the materials should be considered potential artifacts of the hip simulator tests. Increasing the volume of the lubricant bath reduced the maximum temperatures for tests running at 1.5 Hz but had little affect at 2 Hz. Reducing the cycling rate is an effective way to avoid overheating of the specimens, but this necessarily extends the time required to complete a test.