Background: Studies have suggested that cross-linked polyethylene
bearings reduce wear rates from 40% to 100% compared with conventional
polyethylene. However, the reduced mechanical properties of highly
cross-linked polyethylene have the potential to be a limiting factor in device
performance. We reviewed a series of retrieved acetabular liners with a
fracture of the superior rim to assess the factors that played a role in their
failure.
Methods: Four Longevity acetabular bearings, which had been
retrieved from two patients after seven to twenty-seven months in vivo, were
visually examined for clinical damage, were assessed with use of Fourier
transform infrared spectroscopy to determine the level of oxidation, and were
analyzed for mechanical properties and fracture surface characterization.
Control data were obtained from never-implanted devices and from global
reference ultrahigh molecular weight polyethylene bar stock as an industry
calibration material.
Results: All four retrieved liners demonstrated articular surface
wear modes, which in most cases were rated as moderate, and none were rated as
severe. All showed cracking or rim failure of the liner at the superior aspect
along the groove in the polyethylene that engages the locking ring of the
shell. The retrieved liners had no measurable oxidation, and the mechanical
properties were comparable with those of never-implanted material.
Conclusions: There was no notable in vivo degradation of the
retrieved liners. Important factors related to failure appear to be thin
polyethylene at the cup rim, relatively vertical cup alignment, and the
material properties of the highly cross-linked polyethylene that are decreased
relative to conventional polyethylene. The critical dimension with respect to
rim failure in modular liners appears to be the minimum thickness at the
equatorial region.
Clinical Relevance: For a given implant design and loading, highly
cross-linked polyethylene may be more susceptible to fatigue damage, such as
rim cracking, than is conventional polyethylene. The potential for excess rim
loading on thin polyethylene should be assessed carefully when the use of a
cross-linked liner and a large femoral head is being considered, particularly
in a cup with a more vertical abduction angle.