Polyethylene has been around since the early 1960’s. The way it differs now is the way you process it makes a huge difference in how it wears. They used to radiate them in air. They found out when they did that it generated a lot of free radicals, and that lowered its wear resistance. What they do now is they radiate it in an inert atmosphere. The higher dosage of radiation increases the cross-linking in the polyethylene. If you take this polyethylene and do this 20 million times [moves it], that is about 20 years of use for somebody who is pretty active, and then you try to measure the wear. The wear is about 1/20th compared to the polyethylene we were using more than a couple of years ago. If you take the cup out after 20 million cycles and weigh it, you can’t even measure the amount of wear. Sometimes you still see the machining marks after 20 million cycles, and that is with body weight load.
Bone grows into the stem, just as it does into the cup. This is the titanium stem, this is the rough coating, and that is where bone grows into that. You wedge it into the bone. The femur is hollow in the middle. The flutes [on the side of the stem] help to stabilize it for that 6-week period while bone is growing into it. In six weeks, if you try to take this thing out, you would have to chisel the bone off to get it out. So bone really adheres onto the surface.
When you radiate it, the wear properties are much better, but the fracture toughness is lowered by about 40 percent. The manufacturers don’t know if that is going to make a difference. Nobody has fractured these yet. Engineers, however, are a little worried that maybe in 10 years they may crack or fracture. If that happens, it may be necessary to go in and change the insert.
Zimmer Cross Linked Metal-on-polyethylene Total Hip Replacement Device