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In Vivo Degradation of Polyethylene Liners After Gamma Sterilization in Air

¹ Implant Research Center, School of Biomedical Engineering, Science, and Health Systems and Department of Materials Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104. E-mail address for S.M. Kurtz: skurtz{at}drexel.edu
² Musculoskeletal Mechanics and Materials Laboratories, Departments of Mechanical and Aerospace Engineering and Orthopaedics, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106-7222
³ Department of Orthopaedic Surgery, University Hospitals of Cleveland, 11100 Euclid Avenue, Cleveland, OH 44106
⁴ Rothman Institute, 925 Chestnut Street, Philadelphia, PA 19107

Investigation performed at the School of Biomedical Engineering, Science, and Health Systems, Drexel University, Philadelphia, Pennsylvania

In support of their research or preparation of this manuscript, one or more of the authors received National Institutes of Health Grant R01 AR 47904. None of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. 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 authors are affiliated or associated.

NOTE: The authors thank Gina Bissett, Rothman Institute, and Ke Zhou, Exponent Inc., for their contributions to this study.

Methods: Fourteen modular cementless acetabular liners were revised at an average of 10.3 years (range, 5.9 to 13.5 years) after implantation. All liners, which had been machined from GUR 415 resin, had been gamma-sterilized in air; the average shelf life was 0.3 year (range, 0.0 to 0.8 year). After removal, the components were expeditiously frozen to minimize ex vivo changes to the polyethylene prior to characterization. The average duration between freezing and testing was 0.6 year. Mechanical properties and oxidation were measured with use of the small-punch test and Fourier transform infrared spectroscopy, respectively, in the loaded and unloaded regions of the liners.

Results: There was substantial regional variation in the mechanical properties and oxidation of the retrieved liners. The ultimate load was observed to vary by >90% near the surface. On the average, the rim and the unloaded bearing showed evidence of severe oxidation near the surface after long-term in vivo aging, but these trends were not typically observed on the loaded bearing surface or near the backside of the liners.

Conclusions: The mechanical properties of polyethylene that has been gamma-sterilized in air may decrease substantially in vivo, depending on the location in the liner. The most severe oxidation was observed at the rim, suggesting that the femoral head inhibits access of oxygen-containing body fluids to the bearing surface. This is perhaps why in vivo oxidation has not been associated with clinical performance to date.

Clinical Relevance: In vivo degradation of polyethylene liners that have been gamma-sterilized in air may be substantial, but the results of this study suggest that it is most frequently localized near the unworn surface regions of the liners. The performance of gamma-sterilized polyethylene liners should continue to be studied to determine whether the in vivo changes in properties have long-term clinical relevance.

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