The effect of cyclic true strain on the morphology, structure, and relaxation behavior of ultra high molecular weight polyethylene

Cyclic loading of total joint replacement bearing materials made of ultra h igh molecular weight polyethylene (UHMWPE) can lead to fatigue failures and the generation of wear particles resulting from the accumulation of plasti c strain. Susceptibility to damage processes can be further complicated by choice of sterilization method and shelf aging prior to implantation. The o bjective of this study was to characterize the effects of cyclic true tensi le strain on the morphological evolution and mechanical relaxation behavior of both non-sterile and radiation sterilized medical grade UHMWPE. A gamma -radiation sterilization process with several years of shelf aging was uti lized in order to discern morphological changes and mechanical behavior res ulting from oxidative degradation coupled with cyclic loading. A video-base d true stress-strain measurement system was developed and validated to char acterize the true stress-strain behavior of the UHMWPE. Non-sterile and ste rilized specimens were each subjected to a range of true strain over a numb er of loading cycles. Following each test, strain recovery data was collect ed and final residual plastic strain was determined. Density gradient colum n measurements were conducted to provide evidence of structural changes tha t resulted from the cyclic strain tests. Scanning electron and field emissi on microscopy were utilized to provide further evidence of the morphologica l evolution of the UHMWPE. This study showed that the morphological evoluti on of both the non-sterile and sterile material groups could be correlated with the amount of true strain and the number of cycles. Furthermore, these differences in the behavior of the two materials could be attributed to th eir distinct initial microstructure. The results of this study have importa nt implications for the development of constitutive and phenomenological mo dels that may incorporate morphological evolution in UHMWPE and other semi- crystalline polymers. (C) 2001 Elsevier Science Ltd. All rights reserved.