Compression and tension fatigue resistance of medical grade ultra high molecular weight polyethylene: the effect of morphology, sterilization, aging and temperature

It is shown in this study that polymer morphology, sterilization with or wi thout accelerated aging, and temperature affect the fatigue crack propagati on behavior of ultra high molecular weight polyethylene (UHMWPE). Changes i n UHMWPE orthopedic implants due to sterilization and shelf aging are of gr eat clinical interest and importance. In recent years, accelerated aging ha s been used to simulate shelf aging and post-sterilization oxidation in UHM WPE orthopedic components. In order to understand the role of sterilization and post-sterilization aging, two commercially available polymers (GUR4150 HP and Hylamer(R)-M) with distinct morphologies were subjected to one of fo ur sterilization techniques (gamma radiation in air, gamma radiation in vac uum, ethylene oxide gas, and gas plasma). Non-sterile samples were used as controls for all studies. Half the groups were tested in the unaged conditi on. Time remaining samples were aged under accelerated conditions via eleva ted temperature (70 degrees C) and oxygen pressure (5 atm) for 14 days to s imulate five years of shelf aging. Subsequent to sterilization and aging tr eatments, specimens were subjected to cyclic compression loading followed b y cyclic tensile loading in order to characterize the fatigue resistance. S canning electron microscopy was used to provide an understanding of fatigue fracture mechanisms. Fourier transform infrared spectroscopy and density g radient column methods were used to determine the structural changes brough t about by sterilization and aging. It is found in this work that accelerat ed aging causes a decrease in fatigue resistance regardless of sterilizatio n method. Fatigue degradation is most severe for gamma radiation in air cou pled with accelerated aging conditions. The results of this study have impo rtant implications for the design of fatigue resistant polymers used in med ical applications. (C) 1999 Elsevier Science Ltd. All rights reserved.