The Effects of Sliding, Rolling, and Rotation on Cross-Shear and Wear of UHMWPE in Knee Replacements

Wear debris from the ultra-high molecular weight polyethylene (UHMWPE) tibial bearing surface leading to loosening is still the main cause for revisions after 5 years for total knee replacements (TKR) [1]. Wear of UHMWPE is greatly increased under cross-shear motion, such as in the hip joint, due to how the molecules and crystalline regions can align in one direction and make the material weaker in the other [2]. The motion in the knee joint is mainly linear rolling and sliding, but there is rotation and medial-lateral sliding that introduce cross-shear [3]. Wear tests are typically performed with basic motion parameters and simplified geometry (pin-on-disk tests) or under gait simulation to test specific designs, but little is known about the effects of the different motions in the knee on UHMWPE wear. There is also disagreement over how to best quantify cross-shear and model how much wear it will cause [3–5]. This study investigates the effect on wear of the different individual and combined motions in TKR: sliding, rolling, and rotation, for a total of eight separate kinematic conditions. The different measures of cross-shear present in each case were calculated in a computer model and compared to the wear test results.