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.