Paper II (i) Mechanisms of the Generation of Wear Particles of Ultra-High Molecular Weight Polyethylene

Abstract

Abstract The generation of ultra-high molecular weight polyethylene (UHMWPE) wear particles from bearing surfaces in patients remains a major concern with long term implants. Both the volume and morphology of the wear debris are important factors in determining the response of the body to particles and subsequent effects on secure fixation. Uni-directional tri-pin on disc tests of UHMWPE sliding on a smooth stainless steel counterface have shown a low underlying wear rate with periodic transient increases in wear rate of the UHMWPE pins. Monitoring of the topography of the wear pins revealed the formation of ridges in the surface parallel to the direction of travel. The periodic increases in wear rate corresponded to the loss of UHMWPE from the tops of the ridges subjected to the largest deformations. Birefringent micrographs of sections of UHMWPE pins cut perpendicular to the surface showed residual sub-surface shear strains under these ridges. The sub-surface strains were much higher in the sections cut parallel to the direction of sliding than in those perpendicular to the sliding direction. Wear debris collected from the fluid consisted of two types, the first having a large aspect ratio and the second being more granular. Wear debris was also attached to the trailing edge of the pin in the form of an extruded ribbon of few microns thick. The observations indicated that a sub-surface failure mechanism operating on a macroscopic scale caused the incremental increases in the wear rate. The intermittent higher wear rates were associated with the removal of UHMWPE that had been subjected to the highest deformations. After the removal of the highly strained material the wear rate returned once more to the underlying lower wear rate assocaited with a particular level of counterface asperity interactions with the UHMWPE.