Anomalous Wear Behavior of UHMWPE During Sliding Against CoCrMo Under Varying Cross-Shear and Contact Pressure.

Wear of ultra-high-molecular weight polyethylene (UHMWPE) in joint implant applications has been shown to increase with cross-shear (CS, i.e., multidirectional sliding) but decrease with higher contact pressure (CP). Moreover, structural changes, resulting in protrusions, are known to occur to the surface of the pin following multidirectional sliding. However, these phenomena are not yet fully understood. In this study, we simultaneously varied CP and CS to derive an empirical formula for the wear factor as a function of these parameters. The wear factor increased when going from unidirectional sliding to multidirectional sliding but decreased with increasing CP, as has been previously observed. Following these tests, the protrusions on the pin surface were chemically and mechanically characterized to gain insights into both their origin and influence on wear behavior. Micro-FT-IR confirmed that the structures consist of polyethylene, rather than adsorbed, denatured proteins. It also allowed the crystallinity of both the protrusions and unaffected UHMWPE to be estimated, showing a strong positive correlation with the hardness of these different areas on the surface. Time-of-flight secondary-ion mass spectrometry was used to probe the chemistry of the surface and near-surface region and indicated the presence of contamination from the test fluid within the structure. This suggests that the protrusions are formed by the folding of UHMWPE following plastic deformation. It is also suggested that the higher hardness of the protrusions affords some protection of the surface, leading to the observed anomalous behavior, whereby wear increases with decreasing CP. [ABSTRACT FROM AUTHOR]