Problems with Knee Replacement? New Plastic Really Pisses Off Body

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Are there serious problems with knee replacement? The past 5 years, the concept of nasty wear particles in knee and hip replacement devices has come to the forefront. The worse offenders seem to be metal on metal devices, the ones commonly used for “minimally invasive” replacement or “resurfacing” of the joint. To get away from that and some of the problems seen with traditional plastic joints, device manufacturers began using cross linked polyetheylene. Now a new study shows that this new material may be a bigger problem than what it replaced.

The issue with wear particles and knee replacement devices is huge. As the device wears (which happens faster in younger and more active patients) it sheds small particles. These particles burst themselves into the collective consciousness as some of the first metal knee or hip replacement devices began to be removed and replaced. The surgeons often reported that the joint cavity was black due to these oxidized metal shavings. Additional research soon demonstrated that some of these hip replacement particles found their way into the blood stream, sometimes leading to metal toxicity in the blood. Other studies showed that a surprising number of patients were allergic to the metals found in knee and hip replacement devices, leading to many allergic reactions. Still other research showed that these local tissue reactions were a major cause of early failure of the joint replacement device. 

Polyethylene is one material that’s been used to replace metal in knees and hips. However, one down side is that it wears more quickly. Cross-linked polyethylene is better because it wears more slowly, allowing a longer survival of the prosthesis. However, when it does wear and produce particles, what happens?

The new study authors injected both polyethylene and it’s longer lasting cross linked cousin into the knees of an animal model. They then measured markers of inflammation and cartilage breakdown. The cross linked polyethylene caused the knee tissues to produce a much nastier stew of cartilage inflammation and breakdown chemicals.

The upshot? So much for cross-linked polyethylene! These findings are pretty disturbing as I’ve talked to many patients who believe that this type of plastic knee is the one that will keep them out of trouble. So is there a perfect material with which to craft a knee device? Maybe, but not one that I can see at this point.

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