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The Journal of Bone and Joint Surgery (American) 86:1271-1282 (2004)
© 2004 The Journal of Bone and Joint Surgery, Inc.

Wear and Surface Cracking in Early Retrieved Highly Cross-linked Polyethylene Acetabular Liners

Letitia Bradford, MD1, David A. Baker, PhD2, Jove Graham, PhD2, Arun Chawan, MS2, Michael D. Ries, MD1 and Lisa A. Pruitt, PhD2

1 Department of Orthopaedics, 500 Parnassus Avenue, MU-320 West, University of California, San Francisco, San Francisco, CA 94143. E-mail address for L. Bradford: letitiabradford{at}yahoo.com
2 Department of Mechanical Engineering-Bioengineering, 2121 Etcheverry Hall, University of California, Berkeley, Berkeley, CA 94720

Investigation performed at the Department of Mechanical Engineering-Bioengineering, University of California, Berkeley, Berkeley, California

The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. They did not receive 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.

A commentary is available with the electronic versions of this article, on our web site (www.jbjs.org) and on our quarterly CD-ROM (call our subscription department, at 781-449-9780, to order the CD-ROM).

Background: A higher degree of cross-linking has been shown to improve the tribological properties of ultra-high molecular weight polyethylene in laboratory studies; however, its effect on in vivo behavior has not been well established. We investigated in vivo wear mechanisms in retrieved highly cross-linked polyethylene acetabular liners in order to determine if early in vivo wear behavior is accurately predicted by hip-simulator studies.

Methods: A total of twenty-four liners (twenty-one explanted and one unimplanted highly cross-linked liners and two explanted ethylene-oxide-sterilized non-cross-linked liners) were examined for this study. The average age of the patients was 59.9 years, and the average time in vivo was 10.1 months. Articular surface damage on the front and back sides of the liners was assessed with an optical scoring system. Surface quadrants were assigned a grade from 0 to 3 according to the observed wear mechanisms and the percentage of surface affected. The micromechanisms of liner damage were evaluated with use of scanning electron microscopy.

Results: The average front and back-side explant damage scores were 11 (range, 2 to 26.5) and 6.7 (range, 3.7 to 13.3), respectively. There was consistent evidence of early surface deformation and cracking. All explants exhibited some form of surface change, including surface cracking, abrasion, pitting, or scratching. The original machining marks on the liner surface were observed to be either unaltered, drastically distorted, or absent.

Conclusions: Highly cross-linked ultra-high molecular weight polyethylene acetabular liners that were retrieved at an average of ten months after implantation exhibited signs of surface damage that had not been predicted by in vitro hip-simulator studies. These devices had not failed clinically as a result of wear. The discrepancy between in vitro and in vivo wear surfaces may be due to variability in terms of in vivo lubrication and cyclic loading or may represent early surface damage mechanisms that are not well demonstrated by long-term simulator studies.


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