Sterilization and Polyethylene Wear: Clinical Studies to Support Laboratory Data.

BACKGROUND:
Recently, in vitro studies have linked gamma radiation sterilization in air and subsequent oxidation to decreasing mechanical properties and rapid polyethylene wear. As a result of these studies, orthopaedic device manufacturers have developed alternate sterilization strategies designed to limit or to prevent oxidative degradation. We compared how the in vivo wear performance of conventional and highly-crystalline polyethylene acetabular liners was affected by two sterilization strategies (gas plasma sterilization and gamma radiation sterilization in vacuum-barrier packaging) as compared with gamma radiation sterilization in air.

METHODS:
Using multivariate linear regression analysis, we analyzed radiographic wear data on 385 total hip implants after a mean duration of follow-up of 6.2 years (range, four to eleven years). Two hundred and twenty-seven components had been sterilized with gamma radiation in air, seventy-seven had been sterilized with gas plasma, and eighty-one had been sterilized with gamma radiation in vacuum-barrier packaging. The liners had a mean shelf life of 0.65 +/- 0.85 year. Two hundred and fifty-two components were made from conventional ultra-high molecular weight polyethylene (Enduron), and 133 were made from highly crystalline ultra-high molecular weight polyethylene (Hylamer).

RESULTS:
In the conventional polyethylene group, the clinical wear performance of the components that had been sterilized with gamma radiation, either in air or in vacuum-barrier packaging, was superior to that of the nonirradiated components that had been sterilized with gas plasma. However, in the highly-crystalline polyethylene group, only the components that had been sterilized with radiation in vacuum-barrier packaging had a reduced rate of in vivo wear; the components that had been sterilized with gamma radiation in air had essentially the same high wear rate as did the liners that had been sterilized with gas plasma.

CONCLUSIONS:
The radiographic wear data on conventional polyethylene are consistent with the findings of laboratory studies that have demonstrated that radiation-induced cross-linking has a beneficial effect on polyethylene wear resistance. The conventional and highly-crystalline polyethylene liners in the present study, however, responded differently to gamma radiation sterilization in air despite the fact that the implants in both groups had very short shelf lives. This finding probably was due to the difference in the morphological structures of the two polyethylenes and the resulting greater susceptibility of highly-crystalline polyethylene to oxidative degradation. Clinical Relevance: These findings provide clinical information that will help surgeons to make informed decisions concerning which components to implant. The data on Hylamer polyethylene liners provide insight into the expected performance of liners of this type that have already been implanted as well as potential insight into the performance of new polyethylenes. In particular, this information demonstrates that differences in the structure of polyethylene, as seen with the different highly cross-linked polyethylenes currently on the market, can affect clinical wear behavior.