Effect of Sterilization Method and Other Modifications on the Wear Resistance of Acetabular Cups Made of Ultra-High Molecular Weight Polyethylene

Background: Wear of ultra-high molecular weight polyethylene acetabular cups in hip prostheses produces billions of submicrometer wear particles annually that can cause osteolysis and loosening of the components. Thus, substantial improvement of the wear resistance of ultra-high molecular weight polyethylene could extend the clinical life span of total hip prostheses. It has become apparent that the conditions under which ultra-high molecular weight polyethylene cups have been sterilized can markedly affect their long-term wear properties, and new sterilization methods and other modifications have been developed to minimize the negative effects.

Methods: In the present study, a hip-joint simulator was used to assess whether it is preferable to sterilize ultra-high molecular weight polyethylene cups without gamma irraSdiation, to avoid radiation-induced oxidative degradation, or to sterilize with gamma irradiation while the cups are packaged in a suitable low-oxygen atmosphere to minimize oxidation while retaining the increased wear resistance conferred by the radiation-induced cross-linking. Ion-implanted cups and cups made of a highly crystalline polyethylene (Hylamer) also were investigated. Cups made of each material were subjected to wear-testing prior to and after artificial thermal aging to accelerate oxidative degradation.

Results: The results of the present study demonstrated that the cross-linking induced by gamma irradiation improves the wear resistance of ultra-high molecular weight polyethylene, while oxidation reduces it. Without thermal aging, the two types of cups that were sterilized with gamma irradiation while in low-oxygen packaging exhibited about a 50 percent lower rate of wear than did either the nonsterilized cups or the nonirradiated cups sterilized with gas plasma. There was a comparable advantage in the rate of wear after fourteen days of thermal aging. However, after thirty days of aging, the cups sterilized with gamma irradiation in low-oxygen packaging wore several times faster than did the nonirradiated cups. Ion-implanting improved the wear resistance without thermal aging, but after extensive thermal aging the oxidation and wear were greater than those of the controls. Hylamer cups (that is, those that were sterilized with gas plasma) exhibited wear properties very close to those of the nonsterilized ultra-high molecular weight polyethylene cups (the controls) with or without aging.

Conclusions: Sterilizing an ultra-high molecular weight polyethylene acetabular cup without radiation (for example, with ethylene oxide or gas plasma) avoids immediate and long-term oxidative degradation of the implant but does not improve the inherent wear resistance of the polyethylene. Sterilizing with use of gamma irradiation with the implant packaged in a low-oxygen atmosphere avoids immediate oxidation and cross-links the polyethylene, thereby increasing its wear resistance, but long-term oxidation of the residual free radicals may markedly reduce the wear resistance. Ideally, cross-linking with gamma irradiation to reduce wear should be done in a manner that avoids both immediate and long-term oxidation.

Clinical Relevance: The present study demonstrated how the fabrication and sterilization processes influence the resistance to oxidation and wear of the various types of ultra-high molecular weight polyethylene that are currently available. As an exact quantitative relationship between days of thermal aging and years of real-time aging (on the shelf and/or in vivo) has not yet been established, it is not possible to predict precisely when, if ever, the in vivo wear rate of cups sterilized with gamma irradiation while in low-oxygen packaging would exceed that of nonirradiated cups. Nevertheless, the results of these wear tests with use of a hip simulator suggest that, for at least ten years of clinical use, the in vivo wear rate of cups sterilized with gamma irradiation while in low-oxygen packaging will be substantially lower than that of cups sterilized without irradiation. The fundamental interactions among radiation, cross-linking, and oxidation exhibited by the specific materials included in the present study may also apply to acetabular cups of other types of polyethylene. Understanding these fundamental interactions will assist the surgeon in making an informed choice among the materials examined in the present study and among other types of modified polyethylene already in clinical use, including those sterilized with ethylene oxide, those sterilized with gamma irradiation in other forms of low-oxygen packaging, and the various new cross-linked and thermally stabilized polyethylenes.