The effect of frictional heating and forced cooling on the serum lubricant and wear of UHMW polyethylene cups against cobalt–chromium and zirconia balls

Hip simulator tests of femoral balls of cobalt–chromium alloy or zirconia against acetabular cups of UHMW polyethylene were run with and without a coolant circulated inside the femoral balls. Without cooling, the wear of polyethylene against zirconia was about 48% lower than with cobalt–chromium alloy, but the steady-state temperature of the zirconia ball was higher (55°C vs. 41°C), and there was more precipitation of protein from the serum, which sometimes formed an adherent layer on the surface of the zirconia. Circulating coolant at 1–20°C markedly reduced the bearing temperatures and the protein precipitation. With coolant at 4°C, wear of the polyethylene against cobalt–chromium alloy was about 26% lower than against zirconia, but the macroscopic and microscopic appearance of the worn polyethylene surfaces were unlike that typically generated in vivo. With or without coolant, the morphology of the polyethylene wear debris was comparable to that generated in vivo, but the ratio of fibrillar to granular debris was higher at the reduced temperature. These results suggested that circulating coolant at an appropriate temperature could avoid overheating (due to non-stop running of the simulator), preventing excessive protein precipitation while providing wear surfaces and wear debris with morphologies closely comparable to those generated in vivo.