Wear of UHMWPE from sliding, rolling, and rotation in a multidirectional tribo-system

For total knee replacements (TKR), wear of the ultra-high molecular weight polyethylene (UHMWPE) tibial bearing surface is a frequent cause for long term revisions and failure. In order to better understand the link between cross-shear motions and wear in TKR, a custom multidirectional tribo-system was developed and used to investigate the individual and combined effects of the different motions in TKR: two-directional (2D) sliding, rolling, and rotation, for a total of eight separate kinematic conditions. The wear rates and wear factors for these different motions were measured and compared, and the surfaces of the wear scars were examined to elucidate wear mechanisms in UHMWPE for a range of kinematic conditions. To mimic the tribological conditions in a TKR, polished cobalt-chromium (CoCr) spheres were articulated against flat, polished UHMWPE disks with physiologically relevant loading, speed, and lubrication environments. Wear factors of UHMWPE were found to have a range between 0.3 and 8.7×10−6 mm3/(Nm). The higher values corresponded to increasing motion complexity and cross-shear, with the "figure 8 shape"sliding path having the greatest effect. These results indicate that knee kinematics may have a significant effect on the longevity of TKR and should not be neglected when designing TKR.