Failure micromechanisms during uniaxial tensile fracture of conventional and highly crosslinked ultra-high molecular weight polyethylenes used in total joint replacements.

Abstract

Highly crosslinked UHMWPEs have demonstrated improved in vitro wear properties; however, there is concern regarding loss of fracture resistance and ductility. The goals of this study were to evaluate the micromechanisms of failure under uniaxial tension and to determine the effect of gamma radiation-induced crosslinking and post-irradiation thermal processing on the estimated fracture toughness (Kc) of UHMWPE. Kc was estimated for two conventional and two highly crosslinked UHMWPE materials from tensile tests. A 32% decrease in Kc was found following crosslinking at 100kGy. The highly crosslinked materials also exhibited less ductile fracture behavior. Kc was slightly dependent on displacement rate but was insensitive to changes in crystallinity (and thus, to thermal processing). The same basic failure mechanism, microvoid nucleation and slow coalescence followed by comparatively rapid fracture after the defect reached a critical size, was observed for all of the conventional and highly crosslinked UHMWPE specimens. These observations will be used in the development of a theoretical failure model for highly crosslinked UHMWPE, which, in conjunction with a validated constitutive model, will provide the tools for predicting the risk of failure in orthopaedic components, fabricated from these new orthopaedic bearing materials.