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Comparison of the Wear Rates of Twenty-eight and Thirty-two-Millimeter Femoral Heads on Cross-Linked Polyethylene Acetabular Cups in a Wear Simulator

Hermida, Juan C. MD; Bergula, Arnie BS; Chen, Peter PhD; Colwell, Clifford W. Jr. MD; D'Lima, Darryl D. MD

Journal of Bone & Joint Surgery - American Volume: December 2003 - Volume 85 - Issue 12 - p 2325–2331
Scientific Articles

Background: The use of larger femoral head sizes in total hip arthroplasty has been shown to reduce the rate of dislocation and to increase the range of motion; however, such components have been associated with unacceptably high polyethylene wear rates. Studies have shown dramatic differences in wear rates between nominally cross-linked polyethylene (i.e., polyethylene that is cross-linked during radiation sterilization) and elevated cross-linked polyethylene (i.e., polyethylene that is cross-linked to a higher degree than that obtained by radiation sterilization alone). The aim of this study was to test the effect of increased cross-linking and of increased head size on polyethylene wear rates.

Methods: Four groups of acetabular liners obtained from a single manufacturer, including 28-mm-diameter nominally cross-linked, 32-mm-diameter nominally cross-linked, 28-mm-diameter elevated cross-linked, and 32-mm-diameter elevated cross-linked polyethylene liners, were tested. Three implants from each group were tested in a twelve-station hip wear simulator with use of 90% bovine serum as a lubricant. The liners were articulated with the appropriately sized cobalt-chromium femoral head. Additional liners from each design were subjected only to the same load without motion to serve as load-soak controls to account for any weight gain due to fluid absorption. Gravimetric analysis was performed every 500,000 cycles for a total of five million cycles.

Results: Nominally cross-linked liners demonstrated mean wear rates of 14.97 and 16.92 mg per million cycles for the 28-mm and 32-mm head sizes, respectively. Both of the elevated cross-linked liners had significantly lower wear rates than the nominally cross-linked liners, with a mean of 1.51 and 2.57 mg per million cycles for the 28-mm and 32-mm head sizes, respectively (p < 0.001).

Conclusion and Clinical Relevance: The dramatic reduction in wear rates with polyethylene cross-linking, even with the larger head size, may increase the potential for use of 32-mm head components in total hip arthroplasty.

1 Scripps Clinic, 11025 North Torrey Pines Road, Suite 140, La Jolla, CA 92037. E-mail address for C.W. Colwell Jr.: colwell@scripps.edu

Copyright 2003 by The Journal of Bone and Joint Surgery, Incorporated
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