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Ultrahigh-molecular-weight polyethylene (UHMWPE) is used extensively in total joint replacements, particularly in hip and knee joints. At present, there is extensive concern with the long-term consequences of wear of this material, specifically osteolytic reactions to wear debris. The relationship of wear behavior to polymer morphology is almost completely unknown. Information on the effects of radiation sterilization on crystallinity is available, but gross conflicts exist. A central problem is the release, over long time periods, of huge numbers of submicron particles by wear. It has not been generally recognized that short-term, severe wear and long-term wear of UHMWPE in total joint replacements are very different in character and results. When the long-term wear regime is correctly defined, it is clear that knowledge of the influence of structure and properties on this specific phenomenon is almost completely absent.

Ultra high molecular weight polyethylene (UHMPWE) is now the material of choice for total joint replacement prostheses, in combination with a metal surface against which the polymer articulates. As this material has now been in use in this application for approximately three decades and other limiting factors (e.g. loosening of the prosthesis) have been improved upon, it is appropriate to attempt a long-term prognosis.

Scanning electron microscope examination of the polyethylene components of 8 total hip and 16 total knee prostheses which had been implanted 4--96 months revealed that in many cases severe wear may not necessarily be caused by the presence of acrylic cement debris or other abrasives. The craters and cracks observed on severely worn surfaces are associated with fusion defects in the plastic itself. The fusion defects were seen to occur as a result of the low temperature necessary to mold ultrahigh molecular weight polyethylene and are (at least at present) difficult to avoid. As a consequence of the above and variability of molecular weight in this material, relatively wide variations in wear rate should be expected even in the absence of acrylic debris.

The static friction, or "stiction-friction", in McKee-Farrar and Charnley-Müller prostheses in a hip joint simulator was compared with dynamic friction determined while the prostheses were oscillated. Under physiological conditions stiction-friction differed little from dynamic friction in both metal-on-metal and metal-on-plastic prostheses, and was affected very little by the lubricant as long as some fluid was present. Stiction-friction increased significantly only after relatively long stationary periods and high loads. However, the frictional forces generated in total hip-replacement prostheses were at least forty times higher than those generated in normal joints and may well be enough to cause late loosening of acetabular components by fatigue failure.