Abstract
Introduction: Irradiation crosslinking of UHMWPE has been developed as an improvement over conventional UHMWPE to provide improved wear resistance, demonstrated during in-vitro studies as well as in early clinical results [1, 2]. The analysis of explants made of highly crosslinked UHMWPE yields valuable information about their clinical utility.
Material: 26 explanted highly crosslinked UHMWPE inlays (Durasul, Zimmer) from various sources were surgically retrieved and analyzed. All inlays articulated with 28 mm CoCr heads except one (32 mm) and had an implantation time from 3 to 43 months (mean 14 months). The reasons for revision were: 9 for infection, 2 for ossification, 4 for luxation, 2 for pain, 4 for bone fracture and 5 for aseptic loosening.
Results: On all explants, scratches on the articulation area as well as machining marks were visible; the latter have sometimes been smoothed out in the loaded area. There were no signs of delamination or oxidation. The loaded area, analyzed by SEM, exhibits microscopic changes in the morphology which have been reported to be potential microcracks [3]. When analyzed by TEM, it was seen that these surface features are ripples and folds, which are induced by normal adhesive-abrasive wear behavior and have a maximum size of 5 m [4]. Mechanisms like folding are reported also from conventional UHMWPE [5]. In order to separate wear from deformation, the shape memory behavior of UHMWPE was employed. After thermal treatment, smoothed machining marks were observed to recover. After about two years in-vivo, some of the machining marks in the loaded area do not recover, which indicates that minimal wear in the range of the height of the machining marks (approx. 10 m) has occurred. In comparison, after this time, 200–600 m of wear would be expected for conventional UHMWPE [6].
Conclusions: The findings from the retrievals showed that there is no adverse wear or material failures due to delamination or cracks at 3 to 43 months. The accumulated scratches are due to the fact that the wear of this highly crosslinked polyethylene is very small. The scratches do not polish out over time as with conventional UHMWPE and do not show adverse effects on the long-term behaviour of the implant. These results are consistent with the performed in-vitro studies.
Theses abstracts were prepared by Professor Roger Lemaire. Correspondence should be addressed to EFORT Central Office, Freihofstrasse 22, CH-8700 Küsnacht, Switzerland.
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