Commentary and Perspective - The Journal of Bone & Joint Surgery

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COPYRIGHT Ó 2010 BY THE JOURNAL OF BONE AND JOINT SURGERY, INCORPORATED
Commentary and Perspective
Commentary on an Article by B.H. Currier, MChE, et al.: ‘‘In Vivo Oxidation in Remelted Highly Cross-Linked
Retrievals’’
By Jevan Furmanski, PhD, and Victor M. Goldberg, MD
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 associates 1
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.
Jevan Furmanski, PhD
Department of Mechanical and Aerospace Engineering,
Case Western Reserve University,
Cleveland, Ohio
J Bone Joint Surg Am. 2010;92:e23(1-2) d
doi:10.2106/JBJS.J.00981
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T HE J OURNAL OF B ONE &JOINT SURGERY JBJS. ORG
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VOLUME 92-A 14 CTOBER 20, 2010
COMMENTARY AND P ERSPECTIVE
Victor M. Goldberg, MD*
Department of Orthopaedics,
Case Western Reserve University School of Medicine,
Cleveland, Ohio
*In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants of less than $10,000 from
the National Institutes of Health. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to
provide such benefits from a commercial entity.
Reference
1. Currier BH, Currier JH, Mayor MB, Lyford KA, Van Citters DW, Collier JP. In vivo oxidation of gamma-barrier-sterilized ultra-high-molecular-weight polyethylene bearings.
J Arthroplasty. 2007;22:721-31.
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