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Surface Analysis of Early Retrieved Acetabular Polyethylene Liners: A Comparison of Conventional and Highly Crosslinked Polyethylenes
Muratoglu, O. K., Greenbaum, E. S., Bragdon, C. R., Jasty, M., Freiberg, A. A., Harris, W. H.
Abstract
Sixteen highly crosslinked and 19 conventional UHMWPE acetabular liners were removed at revision surgery for reasons not related to polyethylene wear. The average in vivo duration was 7±6 months (range, 5 days - 18 months) and 5±3 months (range, 14 days - 10 months), respectively. The articular surfaces of these explants were analyzed using optical and scanning electron microscopy for the presence of machining marks, fine and heavy scratching, and polishing. In the highly crosslinked group, optical microscopy revealed reduced incidences of machining marks concomitant with extensive scratching and some polishing of the articulating surfaces. The conventional group showed higher frequency of polishing, scratching, and loss of machining marks. Due to the markedly improved wear resistance exhibited in highly crosslinked UHMWPE liners as shown in previous in vitro simulator studies, we hypothesized that these alterations to the initial surface morphology were due primarily to plastic deformation of the surface with minimal if any, removal of material. This hypothesis was tested by exploiting the shape memory phenomenon exhibited by UHMWPE, through melt-recovery experiments. Melting a plastically deformed UHMWPE triggers the shape memory effect, enabling the recovery of the initial geometry. The melt-recovery experiments consistently resulted in the disappearance of surface scratches and the restoration of the original machining marks in the highly crosslinked explants. In contrast, in the conventional group few of the surface scratches were eliminated and only limited restoration of the machining marks was apparent. These observations support the hypothesis that the changes in surface morphology of the highly crosslinked explants were due to recoverable deformation of the surface of the plastic and not adhesive/abrasive wear resulting in loss of material.
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