Commentary & Perspective

Commentary & Perspective on
"Effect of Terminal Sterilization with Gas Plasma or Gamma Radiation on Wear of Polyethylene Liners"
by Robert H. Hopper Jr., et al.

Commentary & Perspective by
John P. Collier, DE*,
Thayer School of Engineering, Dartmouth College, Hanover, NH

In this issue of The Journal, Hopper et al. provide an excellent clinical study of the effect of the method of sterilization on the wear rates of polyethylene liners. They concluded that components sterilized with gamma radiation in air had an average wear rate that was 50% less than that of components sterilized with gas plasma. The study methods appear sound and the size of the series (124 hips) provided sufficient power. The authors stated that they discouraged readers from "…attempting to extrapolate our results to the new generation of highly cross-linked polyethylene liners." While this caveat is helpful, there are other extrapolations which, if made, would be of greater concern.

Hopper et al. compared the wear rate of acetabular components sterilized with gamma radiation in air at a mean of six years after sterilization (a mean shelf life of one year plus a mean follow-up of 5.2 years after implantation) with the wear rate of gas-plasma-sterilized liners (determined at a mean follow-up of 3.9 years). Their finding—that acetabular components sterilized with gamma radiation in air demonstrated a lower wear rate after 5.2 years in vivo (six years after sterilization) than did gas-plasma-sterilized components—is not surprising. Gamma radiation induces cross-linking in polyethylene, which has been shown, in both clinical and wear-simulation studies, to reduce wear rates. In an earlier study, members of this research group¹ found that the wear rate of cups sterilized with ethylene oxide (0.17 mm/yr) was higher than that of cups sterilized with gamma radiation in air (0.085 mm/yr). One would expect that the amount of cross-linking in components sterilized with gas plasma would be similar to that in cups sterilized with ethylene oxide². Thus, one might extrapolate that sterilization with gamma radiation in air would be associated with orthopaedic bearings that would perform well over time. This extrapolation, although not discussed by the authors, deserves attention.

Sterilization with gamma radiation in air generates free radicals that, in the presence of oxygen, will oxidize the polyethylene over time. This oxidation eventually produces a substantial degradation of the mechanical properties of the polyethylene³. The liners in this study by Hopper et al. were sterilized with gamma radiation in air, a technique that provides sufficient oxygen at the time of sterilization to ensure that future oxidation will occur. Oxygen is available to the components as they sit on the shelf, and the amount of oxidation increases with longer shelf life. The rate of subsequent oxidation of these components in vivo is not as well documented, but many years after sterilization, free radicals remain in sufficient numbers to produce further oxidation in vivo⁴. The amount of oxygen available in vivo may be less than that available in air, but it is not negligible. Therefore, one may predict that oxidation will continue in vivo for the life of the component and that, as a result, the mechanical properties of the polyethylene may become sufficiently reduced at some point to cause the component to fracture. Cracking has been identified in 19% of retrieved acetabular components that had been sterilized with gamma radiation in air⁵.

Examination of acetabular components that had been sterilized with gamma radiation in air indicates that the region of high oxidation and reduced mechanical properties lies below the surface of the component at a depth of approximately 1 mm, referred to as the ‘clear zone.’ This observation is consistent, independent of the length of shelf life after sterilization, the implant type, or the manufacturer. The clear zone retains much of its original strength and ductility and would be expected to exhibit good wear resistance. While Hopper et al. did not present data on polyethylene oxidation, it is reasonable to assume that the oxidation of these components sterilized with gamma radiation in air would be consistent with that of other material sterilized with that technique (with the exception of highly cross-linked polyethylene that was produced with very high doses of radiation)⁶. These components could be expected to have little oxidation in the surface material to a depth of approximately a millimeter. The graph in Figure 1 shows that the average head penetration of the liners sterilized with gamma radiation in air was less than 0.8 mm at a follow-up of 5.5 years, suggesting that the head of the femoral component had not penetrated into the highly oxidized polyethylene material. Fisher et al.⁷ showed that the wear rate of oxidized polyethylene that had been sterilized with gamma radiation in air was greater than that of unoxidized material that had been sterilized with the same technique. Continued follow-up of the components that were sterilized with gamma radiation in air, with a duration of more than eight to nine years (when the head might be expected to penetrate the oxidized polyethylene), should provide data that can be used to determine whether oxidation has a measurable effect on the wear rate of these components.

Of greater concern, however, is the possibility that readers of this article may conclude that because sterilization with gamma radiation in air resulted in lower polyethylene wear rates in these acetabular components, this same sterilization technique might be satisfactorily applied to components used in knee arthroplasty. While rates of abrasive two-body wear in the knee may be quite low, the potential for fatigue damage is much higher, as evidenced by the 65% frequency of cracking or delamination found in retrieved knee components that had been sterilized with gamma radiation in air and had been in vivo longer than four years⁸. In the knee, oxidation over time, both on the shelf and in the body, results in a reduction in the mechanical properties of the polyethylene, leading to its inability to withstand the stresses caused by the rolling and sliding of the femoral component so that fatigue wear is very common. In this environment, components that have been sterilized with ethylene oxide or gas plasma, which are not subject to subsurface oxidation, may be expected to perform better than components sterilized with gamma radiation in air⁹.

In summary, Hopper et al. have provided sufficient clinical data to suggest that acetabular polyethylene liners that have been cross-linked as a result of sterilization with radiation, in the absence of substantial oxidation, will demonstrate lower wear rates than will non-cross-linked polyethylene at a follow-up of 5.2 years in vivo. However, extrapolation of these wear rates beyond a follow-up of eight years (when the depth of penetration might be expected to enter the oxidized polyethylene) is problematical. Continued follow-up of the hips in this series would be very helpful to determine what effect higher oxidation has on the longer-term wear rate of polyethylene components. Finally, for the reasons discussed, my colleagues and I strongly discourage extrapolation of these results to the performance of polyethylene in total knee arthroplasty.

*In support of the research or preparation of this manuscript, the author received grants or outside funding from Johnson & Johnson and Zimmer, Inc. In addition, the author received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity (Johnson & Johnson). Also, a commercial entity (Johnson & Johnson, Zimmer, Inc.) paid or directed, or agreed to pay or direct, benefits to a research fund, foundation, educational institution, or other charitable or nonprofit organization with which the author is affiliated or associated.

References

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