This article is more than 5 years old. Information may no longer be current.
Highly crosslinked polyethylene may provide increased longevity, durability of TJR
In this interview, John J. Callaghan, MD, says it is useful in patients with high risk for UHMWPE wear.
Douglas W. Jackson, MD: Dr. Callaghan, as orthopedists we have participated in the care and witnessed the incredible impact on our patients’ lives following joint replacement. The use of polyethylene is a major factor in that success. When orthopedists talk with their patients, they use many different ways to describe it. When we talk with each other, there can be some confusion in describing the advances that have been made in using this polymer. What is a clinically applicable definition for highly crosslinked UHMWPE? What is different about it?
|
|
John J. Callaghan, MD: Historically, polyethylene was sterilized by gamma irradiation in air. The typical dose of irradiation was 2.5 to 4 megarads. After research from the 1990s demonstrated the potential benefit of gamma irradiation in terms of UHMWPE wear resistance (by increasing the crosslinking in UHMWPE), research groups explored processes to increase crosslinking and decrease or eliminate free radicals, which can oxidize and decrease the wear resistance and fatigue strength of the material. Today, polyethylene that has been irradiated between 5 to 10 megarads would be considered highly crosslinked UHMWPE. In addition, today the material that is considered highly crosslinked polyethylene would have undergone a melting or annealing process to decrease or eliminate free radicals which develop during the radiation process. This process of crosslinking makes UHMWPE extremely wear resistant, with as much as 90% to 99% reduction of wear in hip simulators compared to conventional UHMWPE.
Jackson: Dr. Callaghan, as surgeons interacting with purchasing policies in our hospitals we often need to justify the additional costs of new technology. Do highly crosslinked UHMWPE knee and hip replacements merit the increased cost in the low-demand elderly patient?
Callaghan: Implant matching, including which UHMWPE should be utilized, is a philosophical debate. What is a low-demand elderly patient? The life expectancy for the elderly is continuing to increase. Many elderly patients also remain active, especially following total hip or total knee replacement. Since the wear resistant properties of conventional polyethylene (gamma irradiated with 2.5 to 4 megarads in an oxygen-free environment) are much better than traditional polyethylene (gamma irradiated in air), one can easily make the argument that conventional polyethylene will most likely perform well throughout the elderly patients’ lives, especially when one considers the fact that we have no long-term data comparing the newer highly crosslinked polyethylene to conventional polyethylene.
Jackson: What are your clinical indications for its use in the hip and knee at this time?
Callaghan: I philosophically disagree with implant matching so I have used highly crosslinked polyethylene in the hip in all patients since it initially became available. The material is currently only available for a few knee replacement designs. There has been less interest in use of the highly crosslinked material in the knee because of the decreased strength properties of the material (initially) compared to conventional polyethylene.
Since femoral component-polyethylene surface wear damage in the knee (delamination and pitting) is thought to be related to fatigue of polyethylene, there is concern that the highly crosslinked material may not be a definite improvement over conventional polyethylene. However, with more concern for backside wear with modular tibial trays and with the use of mobile-bearing designs, the adhesive wear that occurs at the hip is also a concern with these knee designs. Hence the properties of the highly crosslinked material may be advantageous to decreasing wear in these situations, but no clinical follow-up has been performed.
Jackson: How can clinicians decide on which processing is best for the highly crosslinked UHMWPE? Do we have enough results to say? What should we be watching for in scientific studies?
Callaghan: Only time will tell whether one process to highly crosslinked polyethylene is better than another. Fortunately prospective randomized studies comparing the new materials to conventional materials are underway in the hip, including RSA studies. I know of no study comparing various highly crosslinked materials to each other clinically. As these studies become available, clinicians should evaluate the data in terms of how close the wear reduction in situ mimics that reported by wear simulators. In addition, surgeons should report any fractures of polyethylene that they observe with use of this material.
Finally, the clinician should recognize that the bar has been set high for highly crosslinked polyethylenes. Polyethylene has allowed millions of patients with disabling hip and knee arthritis to live active, functional lives (most without ever having to be revised in their lifetime) even with traditional sterilizing methods, ie, gamma irradiated in air or ethylene oxide sterilization without crosslinking.
The conventional polyethylenes used today (gamma irradiated in an inert environment) with moderate crosslinking should provide improved results over a highly successfully used material. Highly crosslinked polyethylene should be the icing on the cake and provide potential increased longevity and durability of total joint replacments especially in the patients with high risk for UHMWPE wear (young patients, males, highly active patients).