The Journal of Bone and Joint Surgery (American). 2009;91:839-849.
doi:10.2106/JBJS.H.00538
© 2009 The Journal of Bone and Joint Surgery, Inc.
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Gamma Inert Sterilization: A Solution to Polyethylene Oxidation?

Francisco J. Medel, PhD1, Steven M. Kurtz, PhD1, William J. Hozack, MD2, Javad Parvizi, MD2, James J. Purtill, MD2, Peter F. Sharkey, MD2, Daniel MacDonald, BS1, Matthew J. Kraay, MD3, Victor Goldberg, MD3 and Clare M. Rimnac, PhD4

1 Implant Research Center, School of Biomedical Engineering, Science and Health Systems, Drexel University, 3401 Market Street, Suite 300, Philadelphia, PA 19104. E-mail address for F.J. Medel: fjm33{at}drexel.edu
2 Rothman Institute at Jefferson, 925 Chestnut Street, Philadelphia, PA 19107
3 Case Western Reserve University, University Hospitals Case Medical Center, 11100 Euclid Avenue, Cleveland, OH 44106
4 Mechanical and Aerospace Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106

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

Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants in excess of $10,000 from the National Institutes of Health (NIH R01 AR47904), Stryker Orthopaedics, Zimmer, and Sulzer. In addition, one or more of the authors or a member of his or her immediate family received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (Zimmer). Also, commercial entities (Stryker and Zimmer) paid or directed in any one year, or agreed to pay or direct, benefits in excess of $10,000 to a research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which one or more of the authors, or a member of his or her immediate family, is affiliated or associated.

Background: In the 1990s, oxidation was found to occur in ultra-high molecular weight polyethylene total joint replacement components following gamma irradiation and prolonged shelf aging in air. Orthopaedic manufacturers developed barrier packaging to reduce oxidation during and after radiation sterilization. The present study explores the hypothesis that polyethylene components sterilized in a low-oxygen environment undergo similar in vivo oxidative mechanisms as inserts sterilized in air. In addition, the potential influence of the different sterilization processes on the wear performance of the polyethylene components was examined.

Methods: An analysis of oxidation, wear, and surface damage was performed for forty-eight acetabular liners and 123 tibial inserts. The mean implantation time was 12.3 ± 3.7 years for thirty-one acetabular liners that had been gamma sterilized in air and 4.0 ± 2.5 years for the seventeen acetabular liners that had been gamma sterilized in inert gas. The mean implantation time was 11.0 ± 3.2 years for the twenty-six tibial inserts that had been sterilized in air and 2.8 ± 2.2 years for the ninety-seven tibial inserts that had been gamma sterilized in inert gas. Oxidation and hydroperoxide levels were characterized in loaded and unloaded regions of the inserts.

Results: Measurable oxidation and oxidation potential were observed in all cohorts. The oxidation and hydroperoxide levels were regional. Surfaces with access to body fluids were more heavily oxidized than protected bearing surfaces were. This variation appeared to be greater in historical (gamma-in-air-sterilized) components. Regarding wear performance, historical and conventional acetabular liners showed similar wear penetration rates, whereas a low incidence of delamination was confirmed for the conventional tibial inserts in the first decade of implantation.

Conclusions: The present study explores the impact of industry-wide changes in sterilization practices for polyethylene. We found lower oxidation and oxidation potential in the conventional acetabular liners and tibial inserts that had been gamma sterilized in inert gas as compared with the historical components that had been gamma sterilized in air. However, we also found strong evidence that conventional components undergo mechanisms of in vivo oxidation similar to those observed following gamma irradiation in air. In addition, gamma sterilization in inert gas did not provide polyethylene with a significant improvement in terms of wear resistance as compared with gamma sterilization in air, except for a lower incidence of delamination in the first decade of implantation for tibial inserts.

Clinical Relevance: Our research demonstrates that gamma inert sterilization may have improved, but not completely solved, the problem of polyethylene oxidation for hip and knee arthroplasty.


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