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Ex Vivo Stability Loss of Irradiated and Melted Ultra-High Molecular Weight Polyethylene
Orhun K. Muratoglu, PhD1; Keith K. Wannomae, BS1; Shannon L. Rowell, BS1; Brad R. Micheli, BS1; Henrik Malchau, MD, PhD1
1 Harris Orthopaedic Laboratory, Massachusetts General Hospital, 55 Fruit Street, GRJ-1206, Boston, MA 02114. E-mail address for O.K. Muratoglu: omuratoglu@partners.org
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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 Zimmer, Inc. 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, Inc.).

Investigation performed at Massachusetts General Hospital, Boston, Massachusetts

Copyright © 2010 by The Journal of Bone and Joint Surgery, Inc.
J Bone Joint Surg Am, 2010 Dec 01;92(17):2809-2816. doi: 10.2106/JBJS.I.01017
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Abstract

Background: 

Radiation crosslinking reduces wear of ultra-high molecular weight polyethylene (UHMWPE), and subsequent annealing or melting increases oxidative stability. Little is known about the oxidative stability of polyethylene total joint components after in vivo service and subsequent shelf storage in air.

Methods: 

We analyzed thirty-four surgically retrieved, radiation crosslinked acetabular liners to determine their oxidative stability after in vivo service (range, 0.5 to 84.0 months). Oxidation was determined at the time of explantation. After shelf storage in air (range, 7.0 to 72.0 months), oxidation, crosslink density, and thermal properties were determined. Oxidation of one control liner that was shelf-aged in air (for eighty-four months) was also determined.

Results: 

At the time of explantation, all components showed minimal oxidation; however, oxidation levels increased during shelf storage, with a concomitant decrease in crosslink density and increase in crystallinity. Increasing oxidation, increasing crystallinity, and decreasing crosslink density correlated with the duration of ex vivo storage. The shelf-aged control liner showed no detectable oxidation.

Conclusions: 

The oxidation and loss of crosslink density of the irradiated and melted UHMWPE was surprising. Two potential mechanisms that might alter the oxidative stability of UHMWPE in vivo are cyclic loading and absorption of lipids. Both of these mechanisms can generate new free radicals in UHMWPE and can initiate and propagate its oxidation.

Clinical Relevance: 

It is not known if the ex vivo instability reported here will occur in vivo in the long term and, therefore, further investigation of surgical explants is warranted.

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    These activities have been planned and implemented in accordance with the Essential Areas and policies of the Accreditation Council for Continuing Medical Education (ACCME) through the joint sponsorship of the American Academy of Orthopaedic Surgeons and The Journal of Bone and Joint Surgery, Inc. The American Academy of Orthopaedic Surgeons is accredited by the ACCME to provide continuing medical education for physicians.
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