Orientation softening in the deformation and wear of ultra-high molecular weight polyethylene

Stress-induced anisotropy is an important phenomenon in the plastic deformation of semi-crystalline linear high polymers. The significance of this phenomenon in the wear of ultra-high molecular weight polyethylene (UHMWPE) bearing surfaces in total joint-replacement prostheses is studied in this investigation. Both linear and crosslinked UHMWPE materials were studied by means of a sequential biaxial tensile test and a hip-joint simulator experiment. The objective was to develop a wear model that focuses on the interactions between the molecular structure of the UHMWPE and the multi-directional stress field experienced on the articular surfaces of artificial joints. A plasma etching technique coupled with scanning electron microscopy was used to reveal the structural characteristics of wear surfaces and wear debris. This method revealed a significant degree of segmental orientation of molecular chains on the wear surfaces and within wear debris. Failure of the wear surfaces was in the form of transverse rupture between oriented molecules. This failure mechanism of the wear surfaces was further evidenced by the observation that the orientation of the molecular chain axis was always parallel to the longest dimension of the wear debris. Sequential biaxial tensile test results revealed that prestretching of UHMWPE in the longitudinal direction resulted in a significant softening in the transverse direction. The degree of transverse softening increased with increasing the stress of longitudinal stretching. Results obtained from the hip simulator test indicated that the wear resistance of UHMWPE can be significantly improved by radiation-induced cross-linking.