Comparison of the wear rates of twenty-eight and thirty-two-millimeter femoral heads on cross-linked polyethylene acetabular cups in a wear simulator.
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).


CONCLUSIONS
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.
Hermida
Juan C
JC
Scripps Clinic, 11025 North Torrey Pines Road, Suite 140, La Jolla, CA 92037, USA.
Bergula
Arnie
A
Chen
Peter
P
Colwell
Clifford W
CW
D'Lima
Darryl D
DD
D000077:Acetabulum; D000704:Analysis of Variance; D001696:Biomechanical Phenomena; D019245:Compressive Strength; D019544:Equipment Failure Analysis / Q000295:instrumentation / Q000379:methods; D005270:Femur Head; D006622:Hip Prosthesis; D006801:Humans; D008422:Materials Testing; D016544:Patient Simulation; D020959:Polyethylene / Q000737:chemistry*; D011336:Probability; D011474:Prosthesis Design; D012680:Sensitivity and Specificity; D013314:Stress, Mechanical; D013499:Surface Properties; D014894:Weights and Measures
D020959:Polyethylene