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Friday, March 2, 2001

Cross-linking, radiation reduces wear

Cross-linking of polyethylene with gamma radiation can significantly reduce wear. James B. Stiehl, MD, clinical associate professor of orthopaedic surgery, Medical College of Wisconsin, Milwaukee, Wis., evaluated the wear and material properties of two cross-linked direct compression molded (DCM) polyethylene materials.

The study in scientific exhibit 38 found hip wear was substantially decreased via moderate levels of cross-linking, while minimally affecting material properties, said Dr. Stiehl, who also is director, Midwest Orthopaedic Biomechanical Research Laboratory. These results are attributed to the high level of consolidation, quality and degradation resistance of the direct molded polyethylene and not overly cross-linking the polyethylene.

Acetabular liners machined from DCM GLJR 1020 (does not contain calcium stearate) were gamma irradiated to 30 kGy (group 1) and also 60 kGy with a simultaneous thermal process (group 2) to further promote cross-linking and eliminate free radicals. Hip wear was performed with 28 mm liners and chrome-cobalt heads at 1Hz for 5 million cycles, and a Paul-curve profile with 674 lb. maximum load. Stiffness, ductility, strength, swell ratio and crystallinity were determined for both materials.

The researcher found that average head penetration (1 year = 1 million cycles) was 0.032 mm/year for standard cross-linked DCM polyethylene (n=3) and 0.004 mm/year for moderately cross-linked DCM polyethylene (n=4); or a 90 percent wear reduction (one-tail t-test, p = 7.9 x 10-7, a=0.005). The material properties of standard and moderately cross-linked polyethylene, respectively, were: stiffness of 84 +/- 4 ksi and 79 +/- 7 ksi (n=5); yield strength of 3110 +/- 75 psi (n=7) and 3070 +/- 10 psi (n=3); ultimate strength of 5976 +/- 770 psi (n=7) and 5580 +/- 70 (n=3); ductility of 376 percent (n=7) and 333 percent (n=3); swell ratio of 4.8 and 3.6; and crystallinity of 52 and 54 percent.

Robert A. Poggie, PhD, Implex Corp., and Kenneth St. John, MS, University of Mississippi Medical Center, were coauthors of the study

Funding was provided by Implex Corp.

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