BACKGROUND:
A higher degree of cross-linking has been shown to improve the tribological properties of ultra-high molecular weight polyethylene in laboratory studies; however, its effect on in vivo behavior has not been well established. We investigated in vivo wear mechanisms in retrieved highly cross-linked polyethylene acetabular liners in order to determine if early in vivo wear behavior is accurately predicted by hip-simulator studies.

METHODS:
A total of twenty-four liners (twenty-one explanted and one unimplanted highly cross-linked liners and two explanted ethylene-oxide-sterilized non-cross-linked liners) were examined for this study. The average age of the patients was 59.9 years, and the average time in vivo was 10.1 months. Articular surface damage on the front and back sides of the liners was assessed with an optical scoring system. Surface quadrants were assigned a grade from 0 to 3 according to the observed wear mechanisms and the percentage of surface affected. The micromechanisms of liner damage were evaluated with use of scanning electron microscopy.

RESULTS:
The average front and back-side explant damage scores were 11 (range, 2 to 26.5) and 6.7 (range, 3.7 to 13.3), respectively. There was consistent evidence of early surface deformation and cracking. All explants exhibited some form of surface change, including surface cracking, abrasion, pitting, or scratching. The original machining marks on the liner surface were observed to be either unaltered, drastically distorted, or absent.

CONCLUSIONS:
Highly cross-linked ultra-high molecular weight polyethylene acetabular liners that were retrieved at an average of ten months after implantation exhibited signs of surface damage that had not been predicted by in vitro hip-simulator studies. These devices had not failed clinically as a result of wear. The discrepancy between in vitro and in vivo wear surfaces may be due to variability in terms of in vivo lubrication and cyclic loading or may represent early surface damage mechanisms that are not well demonstrated by long-term simulator studies.





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