Total joint arthroplasty is a highly successful procedure; however, the one major issue that remains is wear of the bearing surfaces and the resultant adverse biological response to wear particles. Many studies of ultra-high molecular weight polyethylene have indicated that residual free radicals combined with oxygen result in increased brittleness and reduction of mechanical properties. The development of highly cross-linked ultra-high molecular weight polyethylene with use of irradiation in inert environments, and subsequent free-radical stabilization by either annealing or remelting, provided a bearing surface with the potential for significant reduction of wear. There have been several clinical studies that have confirmed, with use of radiographic techniques, this reduction in wear. However, in the recent radiographic study by Currier et al., an unexpected increase in wear was seen in this ultra-high molecular weight polyethylene after seven years in vivo. This troubling observation requires further study.
Currier et al. investigated clinically retrieved components of several designs, manufactured from distinct formulations of ultra-high molecular weight polyethylene, to provide additional data on their in vivo performance. Several important observations were made in both retrieved acetabular cups and retrieved tibial inserts. The range of radiation doses employed is of interest, as this essentially determines the extent of cross-linking and free-radical generation in the unstabilized material. It is notable that only one of twenty-six moderately irradiated (5-Mrad) ultra-high molecular weight polyethylene acetabular liners had an oxidation index of >0.1, while eight of twenty-four highly irradiated (>9-Mrad) acetabular liners were found to have a peak oxidation index of >0.1. The oxidation index results for the tibial components were more mixed for the various materials, with six of the nineteen components exhibiting an oxidation index of >0.1; however, nearly all of these had been moderately irradiated. These initial results seem to indicate that, if there is a secondary mechanism for in vivo oxidation of ultra-high molecular weight polyethylene that does not depend on an initial population of free radicals, it could depend on the irradiation dose or the severity of mechanical loading experienced during patient use.
While it is very interesting that substantial amounts of oxidation were observed in remelted ultra-high molecular weight polyethylene retrievals, the clinical relevance of marginal amounts of oxidation is not clear. The authors found the oxidation index to exceed 0.1 in fifteen of sixty-nine cases, and these cases had been in vivo approximately one to five years. To interpret the meaning of this level of oxidation, some context is needed to compare these levels of oxidation with those that have been observed to result in substantial material degradation and loss of mechanical integrity. The oxidation threshold, beyond which substantial and precipitous changes in mechanical strength and ductility are expected, is generally given as an oxidation index of >1. This threshold was clearly documented in tensile experiments on two radiation-sterilized conventional materials aged in air by Currier and associates1 in 2007. However, the mechanical and clinical consequences of marginal or subcritical oxidation are not well understood, and this is an area of active research. It is not known, for instance, if the critical amount of oxidation resulting in compromised structural performance is a function of cross-link density, which would directly impact the expected clinical performance of marginally oxidized cross-linked formulations. It stands to reason that clinical oxidation performance is a function of the density of molecular entanglements between the crystalline lamellae, which is in turn modulated by cross-link density, heat treatment, and crystallinity.
Currier et al. reported that the oxidation index correlated with the time for which the implant had been in the patient, but it is unknown whether the rate of oxidation accelerates or decelerates, particularly since the mechanism for oxidation in cross-linked and remelted ultra-high molecular weight polyethylene formulations is obscure. We agree with Currier et al. that further study is warranted to understand the long-term oxidation of moderately and highly cross-linked and remelted ultra-high molecular weight polyethylene formulations, with attention to the effect of both marginal and severe oxidation on mechanical performance.