Elevated Crosslinking Alone Does Not Explain Polyethylene Wear Resistance

Abstract

New ultra high molecular weight polyethylene (UHMWPE) materials have been introduced that utilize elevated radiation doses combined with post-irradiation heat treatments. The elevated radiation dose creates higher levels of crosslinking and is reported to cause improved abrasive wear resistance. The heat treatment quenches free radicals that result from irradiation, thus preventing oxidative degradation. Although abrasive wear occurs between conforming bearing surfaces, macroscopic wear damage is more commonly cited than abrasive wear as a limiting factor with non-conforming surfaces such as occur in knee replacements. Though these new materials demonstrate reduced mechanical properties compared to conventional UHMWPE gamma irradiated in air, recent studies revealed that some of these materials performed well under non-conforming in vitro wear conditions. Our aim was to understand why materials with reduced fracture toughness and yield strength would be more resistant to wear damage. Polyethylene materials with varying radiation doses and heat treatments were examined. We compared modulus, yield strength, ultimate stress, and ductility, J-integral fracture toughness, density and wear. Crystalline morphology was evaluated using transmission electron microscopy. A relation was found between modulus, morphology and wear behavior. Materials with modulus equal to or less than 850 MPa and the majority of lamellae measuring less than 200 nm demonstrated no pitting, delamination or cracking in wear tests, regardless of the level of crosslinking. Even though elevated crosslinked materials showed reduced toughness, they demonstrated good damage resistance, but elevated crosslinking alone does not explain wear damage resistance.