Abstract
In this report we present our long-term developmental and clinical results with both highly cross-linked and extensively cross-linked polyethylene materials. Beginning in 1970s, we performed wear screening studies on UHMWPE (GUR412) sterilized by gamma- irradiation in air (range 0 to 10, 000 kGy) . From these scientific studies the 1, 000 kGy dose (100 Mrad) appeared optimal, and so we began clinical use in 1971, and that continued into 1978. The radiographic wear-rates in patients with 1, 000 kGy sockets, assessed by radiography, appeared 6-fold reduced compared to our standard UHMWPE sockets. Note also that we had not used any post-sterilization heat treatment for these pioneering extensively crosslinked polyethylene sockets. With clinical use now over 30 years, it was also clear that there was no adverse oxidation created by any free radicals present in our extensively cross-linked polyethylene sockets. With these encouraging clinical results, we further studied laboratory wear results with the modern UHMWPE resins, using the irradiation doses 1, 000, 5, 000, 10, 000 and 15, 000kGy and with both saline and serum lubricants in hip simulators. These more recent studies demonstrated that the wear in extensively cross-linked polyethylene sockets was undetectable, less even than the measurement errors in the simulator techniques. It was unfortunate that the physical properties of such extensively cross-linked polyethylene sockets did not meet the current ISO and ASTM standards. Thus, despite the excellent wear performance of these materials, we decided to investigate also the properties of the 60 kGy irradiated UHMWPE. The polyethylene sheet (GUR 1050) was first irradiated with 35 kGy under N2 and then heat treated to remove free radicals. The socket liners were then machined to shape and resterilized with 25 kGy under N2 gas. The mechanical and tribological properties were then measured for this moderately cross-linked polyethylene material. Our clinical studies are still very short and so much longer follow-up will be required for these moderately cross-linked polyethylene sockets. With continuing clinical success with moderately cross-linked, highly cross-linked and extensively cross-linked polyethylene sockets, it may be necessary to reconsider the criteria for ISO and ASTM standards.
