CONTACT PRESSURE-DEPENDENCE OF WEAR RATES OF ULTRA HIGH MOLECULAR-WEIGHT POLYETHYLENE

While contact stresses in hip joint prostheses rarely exceed 1000 psi (6. 9 MPa), the pressures on articulating surfaces in knee joints are very possibly several times higher and one might expect a substantial increase in wear rate. Wear rates of orthopedic polyethylene in 37 degree C water are not very contact stress dependent below 1000 psi (6. 9 MPa) but above that level they accelerate substantially. The pressure dependence overall follows an exponential function. Creep in the contact pressure range of 3-17 MPa and above is a much larger factor than wear in indentation effects. For accurate measurement of wear rates by depth measurements it is necessary to permit creep to proceed under static load until it ceases before beginning the wear test.