The Journal of Bone and Joint Surgery

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Commentary & Perspective

Commentary & Perspective on
"Short-Term in Vivo Wear of Cross-Linked Polyethylene"
by Christian Heisel, MD, et al.

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

Many new techniques, designs, and materials have been introduced in an effort to improve the outcome of total hip replacement over the years. Not all have lived up to the expectations of the developers, and many highly touted innovations have proved to be less satisfactory than what they were meant to replace. Thus, we have learned to expect that short-term results may not predict long-term outcomes. This has led to the conventional wisdom that changes should be carefully observed over at least five years and 100 or more patients before concluding that an innovation is actually an improvement.

This article statistically compares the two-to-three year results of a series of thirty-four cross-linked polyethylene bearings with a series of twenty-four conventional bearings that were sterilized with gamma irradiation in air. The major conclusion of the study is summarized in the statement, given without caveat, that the amount of wear reduction in the hips with cross-linked polyethylene was "…81% lower than in the conventional group (p < 0.00001)."

This level of confidence in data that were collected in a study with a duration of less than three years is notable. The basis of the comparison is a series of wear measurements made with what is described as very high accuracy and little measurement error. However, we would note that the overall average yearly wear values of both conventional (0.13 mm) and cross-linked polyethylene (0.02 mm) are in the range of the measurement sensitivity of the technique (0.1 mm). Normal "bedding-in" of the bearings and creep of the machine marks may be included in these measurements and, if different for the two systems, would tend to confound the early data. One would expect that, with increasing use of the components, the amount of wear would increase and the sensitivity of the measurements to the error inherent in the technique would decrease.

The authors did not provide any data regarding linear wear penetration but only yearly wear averages. For the conventional polyethylene, the average was 0.13 mm per year, with a highest sample wear of 0.4 mm per year, or about three times the average. Although the average wear of the cross-linked components is lower, it is not without variability. The cross-linked series had an average wear of 0.02 mm per year, with a highest sample wear rate of 0.2 mm per year, or ten times the average. No note of this variability or an identification of the possible sources of this relatively high wear was made, but the observation remains that the cross-linked polyethylene sample with an average wear of 0.2 mm per year is apparently undergoing wear at a rate that exceeds the average for the series of components that were sterilized with use of gamma irradiation in air by about 50%. While the authors' statistical analysis stated that an 81% reduction in wear is more than 99% probable, the patient with the component that is undergoing wear at 0.2 mm per year is doing no better than the average of the patients in the series with the components that were sterilized with use of the now-discontinued gamma-irradiation-in-air technique. A caveat that states that "early results are not necessarily a guarantee of future performance" and a second one that states that "individual results may vary" would be appropriate with any wear study, including this one.

There is at least one issue in this study that is a cause for concern: the apparent recent implantation of polyethylene that was sterilized with use of gamma irradiation in air. Additionally, the authors state that "The mean shelf life of these components was 14.5 ± 13.4 months (range, 0.8 to 59.5 months)." Our research on the rapid oxidation of shelf-aged and implanted components that have been sterilized with use of gamma irradiation in air demonstrated that most of the toughness of the material was lost with sixty months of shelf life¹. On the basis of this research and corroborative studies^2-5, the industry has generally abandoned the use of gamma irradiation in air as a sterilization method. Yet the authors have used this potentially highly oxidized material as the benchmark with which to compare the new, crosslinked material; why?

This paper raises a number of other, simpler questions, which, if answered, may shed light on the extent to which these data might be valuable predictors.

It has been demonstrated in many previous studies that such factors as head size are directly correlated with linear penetration rates, yet no factor other than polyethylene type was found to correlate with wear in this study. Is this a function of the short duration, the small cohort size, or some other factor?
In both the conventional and the cross-linked polyethylene, the authors found no correlation between wear and time, yet they provide all of the comparisons between the two on the basis of a yearly, volumetric wear rate; why?
The variation in the average wear data of both series indicates that some "negative wear" was recorded. How was this handled in the determination of wear volumes?
Both Student t and ranked sum tests were applied selectively to the data. How was the appropriate test determined?

Overall, this paper has made use of well-documented techniques to assess wear, and of statistically valid methods to analyze the wear data. These early results appear to validate the benefits of cross-linking as a wear-reduction method, but the variation of a factor of ten in wear rates of components in the series with the cross-linked polymer was not addressed and will require attention.

Note: The author thanks Jon Kittredge, MD, Doug van Citters, MS, John Currier, MS, and William Shields, AB, for their contributions to the analysis of statistics related to this commentary.

*In support of his research or preparation of this manuscript, the author received grants or outside funding from Johnson and Johnson and Zimmer. In addition, the author received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity (Johnson and Johnson). No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the author is affiliated or associated.

References

1. McGovern TF, Ammeen DJ, Collier JP, Currier BH, Engh GA. Rapid polyethylene failure of unicondylar tibial components sterilized with gamma irradiation in air and implanted after a long shelf life. J Bone Joint Surg Am. 2002;84:901-6.
2. Currier BH, Currier JH, Collier JP, Mayor MB. Effect of fabrication method and resin type on performance of tibial bearings. J Biomed Mater Res. 2000;53:143-51.
3. Collier JP, Sutula LC, Currier BH, Currier JH, Wooding RE, Williams IR, Farber KB, Mayor MB. Overview of polyethylene as a bearing material: comparison of sterilization methods. Clin Orthop. 1996;333:76-86.
4. Williams IR, Mayor MB, Collier JP. The impact of sterilization method on wear in knee arthroplasty. Clin Orthop. 1998;356:170-80.
5. Bohl JR, Bohl WR, Postak PD, Greenwald AS. The effects of shelf life on clinical outcome for gamma sterilized polyethylene tibial components. Clin Orthop. 1999;367:28-38.