Friction and wear characteristics of UHMWPE against Co-Cr alloy under the wide range of contact pressures in lumbar total disc replacement

Ultra-high molecular weight polyethylene (UHMWPE) and Co-Cr alloy are still commonly used bearing materials for total disc replacement (TDR) as well as total joint replacement (TJR). Compared to TJR, in TDR, there are different articulating conditions such as higher contact pressure, shorter sliding distance, and less lubricant, unlike hip and knee joints. In this study, friction and wear phenomena between UHMWPE and Co-Cr alloy were characterized under the wide range of contact pressures including the extremely high contact pressure with limited amount of lubricant in lumbar TDR. All friction and wear tests were conducted by using a Pin-on-Disk type tribo-tester under normal contact pressures of 5, 10, 20, 40, 60 and 80MPa in the repeat pass rotational and the linear reciprocal sliding motions, respectively. UHMWPE cylindrical pins slid against Co-Cr alloy disks in three different kinds of dry, rarely and fully immersed lubricated conditions for friction tests and in a fully immersed lubricated condition for wear tests. For all lubricated conditions, mean coefficients of friction decreased as contact pressure increased. There were statistically significant differences in the mean coefficients of friction among six levels of contact pressures and among three kinds of lubricated conditions. The amount of wear linearly increased as number of sliding cycles increased for all contact pressures, and also it increased as the contact pressure increased. There were statistically significant differences in the wear rates of UHMWPE among six levels of contact pressures. Coefficients of friction of UHMWPE against Co-Cr alloy under very high contact pressure such as 40–80MPa in the lubricated condition were so low as to induce no clinical problem at the bearing surface. For reducing wear of UHMWPE component in TDR, it is necessary to decrease the contact pressure in design as much as possible.