Published On September 1, 2011
IN 1998 PHYSICIANS IMPLANTED THE FIRST HIP JOINT made of highly cross-linked polyethylene—so called because of the way the polymer’s molecules bond when dosed with radiation. Cross-linking makes joints more durable than those formed of traditional polyethylene, which eventually disintegrates, shedding particles that ignite an immune reaction that destroys bone around the replacement joint. Ten years and more than a million implants later, highly cross-linked polyethylene had become the market standard.
Yet despite that success, highly cross-linked polyethylene joints are far from perfect. The electron beam irradiation used to make them also generates free radicals, which over time can cause joints to oxidize and break down. Orhun Muratoglu of the Harris Orthopaedic Laboratory at MGH describes how he and his colleagues came up with a better alternative to the original cross-linked polyethylene.
Q: How did you deal with the problem of free radicals?
A: You can melt cross-linked polyethylene to quench the free radicals, but this sacrifices some of its strength. Vitamin E is a well-known antioxidant, and we discovered that adding it to cross-linked polyethylene stabilizes the free radicals without weakening the joint.
Q: So you just mixed in vitamin E?
A: We tried to mix it into the polyethylene, but vitamin E inhibited the cross-linking because the vitamin scavenges the free radicals needed for the cross-linking. I was about to give up when I realized: We have to put the vitamin E in after we cross-link the polyethylene, not before. So now we take vitamin E, which is an oil with the consistency of maple syrup at room temperature, and heat it in an oven so that it’s more like water. We then soak cross-linked polyethylene in this hot oil, almost like deep frying. That helps the vitamin E diffuse into the cross-linked polyethylene. It can take 6 to 12 hours, or sometimes even days, to form a uniform concentration across the entire thickness of the joint. But once that happens, you have a material that’s resistant to oxidation, very durable and stronger than the original cross-linked polyethylene.
Q: The new material was approved by the Food and Drug Administration in 2007. What’s next for your work?
A: Now we’re trying to find materials we can use to help heal cartilage before the joint degenerates—so we can actually eliminate the need to replace the replacements.
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