Highly cross-linked UHMWPE materials were clinically introduced in 1998 and have rapidly become the standard of care for total hip replacements, at least in the United States. These new materials are cross-linked with gamma or electron beam radiation (50–105 kGy) and then thermally processed to improve their oxidation resistance. Five-year clinical data, from several centers, are now available demonstrating their superiority relative to conventional UHMWPE for total hip replacement (see arthroplasty). Clinical studies are still underway to investigate the performance of highly cross-linked UHMWPE for knee replacement.