Ultra high molecular weight polyethylene is not only fun to say, but it is also an incredibly versatile material commonly used as a bearing surface in joint replacements. Polyethylene has low friction, good fracture toughness and fatigue resistance. Despite its remarkable material properties, there was a pandemic of loosening with early generation polyethylene implants. Scientists discovered microscopic wear particles from the polyethylene generate an immune response which destroys surrounding bone, subsequently causing joint replacements to lose their fixation.
To combat this issue highly cross-linked polyethylene was developed. Exposing the plastic to radiation creates crosslinks which drastically decreases the amount of wear particles. Unfortunately, these implants suffered early catastrophic failure. It was discovered that letting the irradiated plastic interact with oxygen created free radicals, weakening the implant. Now these implants are stored in a vacuum or inert gas, which prevents the destructive oxidation.
And this brings us to present day. While oxidation of polyethylene implants is not as dire a problem as in the past, it is still a concern. Both when stored on the shelf and in the body, these implants suffer some amount of oxidative degeneration. A recent development is the concept of infusing an anti-oxidant in the polyethylene (such as vitamin E) to neutralize free radicals.
A new product in this relatively new field of anti-oxidant infused implants is DePuy‘s AOX antioxidant polyethylene for use in the Sigma rotating platform total knee replacement and LCS complete mobile bearing knee systems. DePuy has just announced that these devices have received FDA Premarket Approval (PMA). DePuy says their new antioxidant stabilizes free radicals while having no effect on the mechanical properties of the polyethylene. While these promising implants have been shown to resist oxidation in the laboratory, whether or not they improve joint replacement longevity remains to be seen.