Unified wear model for highly crosslinked ultra-high molecular weight polyethylenes (UHMWPE)

Crosslinking has been shown to improve the wear resistance of ultra-high mo lecular weight polyethylene in both in vitro and clinical in vivo studies. The molecular mechanisms and material properties that are responsible for t his marked improvement in wear resistance are still not well understood. In fact, following crosslinking a number of mechanical properties of UHMWPE a re decreased including toughness, modulus, ultimate tensile strength, yield strength, and hardness. In general, these changes would be expected to con stitute a precursor for lower wear resistance, presenting a paradox in that wear resistance increases with crosslinking. In order to understand better and to analyze this paradoxical behaviour of crosslinked UHMWPE, we invest igated the wear behavior of (i) radiation-crosslinked GUR 1050 resin, (ii) peroxide-crosslinked GUR 1050 resin and (iii) peroxide-crosslinked Himont 1 900 resin using a bi-directional pin-on-disk (POD) machine. Wear behavior w as analyzed as a function of crystallinity, ultimate tensile strength (UTS) , yield strength (YS), and molecular weight between crosslinks (M-c). The c rosslink density increased with increasing radiation dose level and initial peroxide content. The UTS, YS, and crystallinity decreased with increasing crosslink density. While these variations followed the same trend, the abs olute changes as a function of crosslink density were different for the thr ee types of crosslinked UHMWPE studied. There was no unified correlation fo r the wear behavior of the three types of crosslinked UHMWPE with the cryst allinity, UTS and YS. However, the POD wear rate showed the identical linea r dependence on M-c with all three types of crosslinked UHMWPEs studied. Th erefore, we have strong evidence to propose that M-c or crosslink density i s a fundamental material property that governs the lubricated adhesive and abrasive wear mechanisms of crosslinked UHMWPEs, overriding the possible ef fects of other material properties such as UTS, US and crystallinity on the wear behavior. (C) 1999 Elsevier Science Ltd. All rights reserved.