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VPT1-00654-EN | B | Vitamin E Polyethylene

Steve Spiegelberg

Human

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Vitamin E Polyethylene

My name is Steve Spiegelberg. I'm going to be talking to you today about ECMO, which is a highly cross-linked oxidatively stable polyethylene that was jointly developed between the researchers at Massachusetts General Hospital and Cambridge Polymer Group back in the late 90s. We recognized that we could really improve the wear behavior of polyethylene by exposing them to ionizing radiation, effectively cross-linking the polyethylene. This is a very typical wear behavior plot, showing weight change of the polyethylene as a function of number of articulating cycles, and you can see the benefit of going from an cross-linked polyethylene all the way down to a CrossLink polyethylene. And we started to recognize that the wear resistance of polyethylene were related to their molecular weights. High density polyethylene, which is a low molecular weight polyethylene, has a much higher wear rate compared to ultra high molecular weight polyethylene, which has a molecular weight of about ten times higher. If we then further increase the molecular weight by cross-linking polyethylene, you can see we get a wear rate that continues to decrease as we increase the radiation dose. So the question is why does cross-linking help wear? Well, if you look at conventional polyethylene, which is typically sterilized with around 2.5MW of radiation, I show cross-links represented by this little yellow bead here. And a CrossLink is just a covalent bond between two adjacent polymer chains shown represented by the blue lines here. So when you're subjecting this to a wear cycle, such as one you might see in a hip and to a lesser degree in a, in a in a knee, the the surface of the polyethylene is subjected to a biaxial motion kind of stretching in two dimensions. And that biaxial motion wants to move the polymer chains around. If you only have a few of the chains connected through cross links, you can actually move the chains around quite a bit. And that can create wear debris. Now, if you increase the number of cross-links by increasing the radiation dose and subjected to the same kind of wear behavior, you get very little chain motion and subsequently very little wear in the material. So that was the motivation behind the first generation of highly CrossLink polyethylenes, which were introduced in the late 1990s. Several companies were taking polyethylene, irradiating it with either electron beam or gamma radiation. And then a critical step occurred after the radiation, where they either melted the material or they annealed it. The purpose of these steps was to try to reduce the number of free radicals, which are created when you irradiate the polyethylene, and then those go on to form either crosslinks, or they can react with oxygen to form oxidative species. If you melt the material, you'll see a slight loss of crystallinity, and that will result in a loss of some of the mechanical properties. But you see very little oxidation in the material over the seven year period or so, and thus no change in mechanical properties. If you anneal the material which is taking the material up to below the melting point, you can get rid of some of the free radicals, but not all of them. And that allows. But you don't melt the material so the crystallinity doesn't change. You can tend to keep some of the mechanical properties in the short term, but over the long term you get some oxidation. And that can lead to a large decline in the mechanical properties. I'm going to explain this a little bit more in the next couple of slides. But these are the first generation technologies that came around in the late 1990s. There were six major manufacturers or sorry, six different types of highly CrossLink polyethylenes that were introduced crossfire, prolonged Aerosol longevity, marathon, and Xlp. And most of these were remelted materials. In other words, they had taken above the melt, so they had very few residual free radicals left behind, as measured by electron spin resonance spectroscopy. Only one of them was annealed, and that was crossfire, which did contain some free radicals at the end of the day. Now, I talked about the loss of mechanical properties as a function of the either melting process or the oxidation process. And I show here an example of that where you show fatigue crack propagation resistance. If you look at conventional polyethylene, this is just gamma irradiated polyethylene that has a fairly high fatigue crack propagation or fatigue crack propagation resistance when you take it out of the box. If you accelerate age the material, though, it does have some free radicals left behind, which can form oxidative species shown by these white bands in the material, and that can result in a fairly large decline in the fatigue crack propagation resistance. So it's more brittle if you do the same thing to a annealed, highly CrossLink material like crossfire, it'll have a loss of mechanical properties out of the box because of the cross-linking that's understood, but because it has all those free radicals left behind, it will really oxidize and lose a lot of its mechanical properties. The remelted materials I mentioned, they lose more mechanical properties because of that remelting process compared to the annealed, but because they don't oxidize over the long term, they do have a hold on to their mechanical properties after you accelerate, age them. So what we learned from from the first generation CrossLink Polyethylenes is that the melt treated materials and this encompasses DermACELL longevity, and xlpe do very well in terms of wear behavior, but they do have some reduced mechanical strength due to this resistance or reduction in fatigue crack propagation resistance. And that will have an effect on what you can do with locking mechanism design, liner thickness, and its use in posterior stabilized knees. The annealed materials and this is namely a crossfire, does have problems with oxidation over the long term, and that does lead to embrittlement which can lead to fracture of the implants long term. But something that we've noticed over time is that both of these types of devices will oxidize in vivo if there's lipid absorption in these materials as well as the result of fatigue induced free radical production. And that's something that wasn't real appreciated in the simulator studies that were done in the development of these materials. So this was a goal for the second generation of these. The highly CrossLink polyethylenes was to keep the same wear properties as we had in the first generation, which are very good, but try to improve the oxidation resistance both short term and long term. In other words, in the body. At the same time, we wanted to improve the mechanical properties over the first generation materials to try to get back to something close to conventional polyethylene. So this was the motivation for the second generation polyethylene that came along in the late 1990s. And companies used two approaches to making this. The first approach was the E1 technology that was developed by by Biomet. In this material they take virgin polyethylene, they consolidate it, they radiation CrossLink it, and then they diffuse in an antioxidant, which is vitamin E. We're going to talk more about vitamin E in a bit. And then they do a terminal sterilization step. The other approach is what's called a blending process. And this is what Zimmer uses to make their vivacity material. They take the virgin polyethylene powder. They blend in an antioxidant again vitamin E, then they mold it and then they irradiate, cross the radiation, CrossLink it, and then do a terminal sterilization step. And what this does is give you, in both cases, a high CrossLink density, which is correlated to a good wear resistance. You get an improved oxidation resistance both in the short term, because you're not doing any remelting or annealing of the material, but you also get good long term resistance in the body because you have this antioxidant present to take care of any lipid attack that might occur. And of course, you get good mechanical properties because you're not doing any remelting of these materials. So this is the process used to to make eshima. You take a Polyethylene powder and blend in vitamin E, and the vitamin E gives you a good in vivo oxidation resistance. So no lipid attack, no fatigue will affect it. You then mold it and irradiate the material, and the radiation gives you your good wear resistance. You're forming those high number of crosslinks in the material. We then perform a mechanical annealing step, and that mechanical annealing helps get rid of residual free radicals present in the material, and that provides even more oxidation resistance beyond what the vitamin E can supply. A thermal annealing step is applied to allow it to recover after the mechanical annealing step without any melting, and that helps you retain your good mechanical properties of the material. And then it goes through a final machining step followed by packaging and sterilization. So this is what the wear behavior looks like of the material shown here and here in just two different directions to show it's uniform compared to conventional polyethylene. This is gamma sterilized polyethylene, which has much higher wear rates and also compared to remelted first generation materials. Duracell longevity. So you can see we're equal or slightly better than those first generation materials. How about the free radical content in these materials, which allows you to get some indication of oxidation resistance? Well, ECMA has about half the free radical content compared to a diffuse vitamin E like E1, or even a blended material like vivacity. How about the oxidation resistance? Actually measured under a challenge conditions. So this is materials that have been exposed to a lipid like environment like in the body. This is a remelted material shown here which does show some oxidation over time on the surface of the material. The second one here is a 0.1 weight percent vitamin E containing material, which does have a little bit of oxidation on the surface. But because ECMA has even fewer free radicals, we saw no free radicals, no oxidation of these materials over time. So it's a very oxidatively stable material, which means it's going to last in the body a good long time. And we talked a lot about the mechanical properties that we wanted to improve on over the first generation. This is shown here. The ultimate tensile strength the yield strength and the energy it break. In other words, just doing a stretching experiment on the part versus a remelted material like longevity or or prolong. And you can see we have much better mechanical properties, at least in terms of ultimate tensile strength and energy at break. So this is a summary of the mechanical properties and the wear rate of Sima versus the second generation doping material, such as E1 and the first generation Remelted materials compared to conventional. You can see in all these camps here we do better in terms of mechanical properties and wear rate in the Sima. So it's definitely an improved material over these first and second generation systems. So your take home messages here Eshima provides improved wear behavior over conventional polyethylene. And this is due to the cross-linking behavior. This is going to give you much less potential for bone loss and implant loosening over time. Eshima has improved mechanical properties, and that's because we're not melting it to make the material. That gives you more resistance to creep and gives you more design options. And most importantly, it gives you very good long term oxidation resistance. That's due to both the mechanical annealing steps and to the vitamin E that's present. And it's also grafted to the polyethylene. So that means lower free radical content. You're not going to lose your vitamin E in the body. And that's going to give you really good resistance to fatigue and lipid attack over the long term. So that's eshima and why this is we think it's a really good material.

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