By: 1 September 2011

For many years surgeons have been debating the balance between hip bearing stability and polyethylene wear. Advancements in polyethylene have been made over the years from UHMWPE to highly cross-linked polyethylene (HXLPE) and more recently vitamin E polyethylenes.

Dr Orhun Muratoglu

Corin has had the unique opportunity to collaborate closely with Massachusetts General Hospital (MGH) in the development and commercialisation of a next-generation vitamin E polyethylene. ECiMa™ is the most advanced polyethylene of its kind, and is only available with Corin's Trinity advanced bearing acetabular system. David Simpson (Research Manager, Corin) talks to Dr Orhun Muratoglu about the latest advancements in this technology.

Dr Muratoglu received his PhD from MIT in 1995 and joined the Harris Orthopaedic Laboratory at MGH in the same year, and has been the co-director of the lab since 2004. His research interests are primarily in the area of understanding the failure mechanisms of polyethylenes and developing technologies to address these failure modes.

What has driven you and your institution to re-visit HXLPE and further improve on existing technology?
We are pleased with the success of the first generation, irradiated and remelted HXLPE. It helped to alleviate a major problem with conventional PE components - that is, osteolysis due to excessive wear necessitating revision surgery. For our laboratory the natural next step was to further improve on the existing technology so as to obtain higher strength and fatigue resistance along with low wear and low oxidation.

What are the key failure modes of the previous generations of HXLPE?
So far there haven't been any failure modes for the previous generations of HXLPE, with the exception of a handful of rim fractures with malpositioned (high inclination angle) acetabular liners. It is important to note that these types of rim fractures also happen with conventional polyethylene.

Cross-linking is achieved by radiation, which also results in residual free radicals. These free radicals are responsible for oxidation and embrittlement of polyethylene components both on the shelf and in the patients. First generation highly crosslinked polyethylenes were either remelted or annealed below the melt temperature to improve oxidative stability. Remelting was successful in quenching all of the detectable free radicals. On the other hand, annealing only reduced the concentration of the free radicals and as a result irradiated and annealed polyethylene components showed very high levels of oxidation in vivo in less than five years. Irradiated and remelted polyethylene components are showing no detectable oxidation up to 10 years of in vivo service; beyond 10 years there appears to be some early signs of potential oxidation. While remelting was thought to mostly eliminate the oxidation caused by the residual free radicals, this late onset oxidation is puzzling. We hypothesise that cyclic loading and/or absorbed lipids initiate and progress oxidation in these components.

What are the key features of ECiMa™ and how do they overcome previous polyethylene technology that have claimed to get rid of free radicals?
ECiMa™ material is irradiated for cross-linking and reduction in wear, followed by mechanical deformation to anneal or quench the free radicals responsible for oxidation. In addition ECiMa™ contains an antioxidant, vitamin E, to protect the polymer against any oxidative attack in vivo. ECIMa™ is not remelted after irradiation, therefore retains its mechanical properties.

What is so important about vitamin E and mechanical annealing?
The contained vitamin E actively protects the component from oxidation. This active protection is unlike the first generation technology, which relied on initially reducing/eliminating the free radical burden to impart oxidative stability. The joint fluid contains potential oxidisers, such as lipids, that can be absorbed over time during in vivo use, challenging the oxidative stability of the component. The contained vitamin E will continuously combat these oxidisers and keep the component oxidatively stable. Mechanical annealing helps to reduce residual free radicals in the irradiated material without remelting it, making it oxidatively stable while retaining mechanical properties.

How can the surgeon be sure that the body can tolerate extra vitamin E in the body?
We have done multiple in vitro tests assessing the possible release of vitamin E from a component under various adverse conditions - at elevated temperatures and in aqueous environments - and under simulated gait. The calculated worst-case values of vitamin E elution in these tests appear to be very low - substantially less than natural plasma vitamin E levels in most adults. Plus, at a nominal concentration of 0.1 wt.% of vitamin E, a relatively large, 50g component would only contain 50mg of vitamin E. Vitamin E toxicity has been associated with large doses (>400 IU/day) over a prolonged period of time; even if all 50mg of vitamin E from the component were somehow lost in a single day, it would still equate to less than 400 IU. In addition the animal studies that we carried out have shown that any vitamin E that might elute out of the components will not adversely affect bone ingrowth and will not cause any adverse tissue reaction.

Will vitamin E simply elute out of the device over time and then leave the liner susceptible to future oxidation?
We have very strong laboratory data that indicates that vitamin E elution will be minimal. During irradiation, some of the vitamin E becomes chemically linked or grafted to the polyethylene molecules. The grafted vitamin E still protects the polyethylene against oxidation based on our laboratory studies.

ECiMa demonstrates an alternative to hard bearings allowing ultra low wear with larger bearings. Will hypersensitivity be an issue for ECiMa?
The primary reaction to PE particles is an immune response that may lead to osteolysis if there is a large number of particles present. There have been no documented cases of hypersensitivity to first generation highly cross-linked or conventional polyethylene particles - analogous to metal ion sensitivity - and there is no reason to believe that ECiMa will be any different.

How do you think vitamin-E stabilised crosslinked UHMWPE would compare to X3?
X3 is sequentially irradiated and annealed, as a result it contains residual free radicals. The oxidative stability of this material is debatable. While some argue that the free radical account is very low and that therefore the material is stable against oxidation, others have shown in laboratory aging experiments that this material does oxidise in other aggressive aging environments. In addition, explanted X3 acetabular liners and tibial inserts are showing in vivo oxidation. Our longest duration X3 explant is a tibial insert showing high levels of oxidation and embrittlement (white banding). The observations from explanted components would indicate that X3 is not resistant to oxidation in vivo. In contrast, vitamin E stabilised irradiated UHMWPE has shown resistance toward oxidation in laboratory experiments under aggressive aging environments. We have analysed a small number of vitamin E stabilised bearings retrieved at revision surgery (the reason for revision was not related to the polyethylene bearing) and they showed no detectable oxidation up to two years in-vivo.