Knee and Hip Replacements and Wear Particles: More Data

Published on Mar 13, 2011

One of the big swirling controversies in knee and replacement surgeries is “wear particles”. These particles are generated as the artificial knee or hip joint undergoes normal wear. These tiny particles can leach toxins that can cause problems in the body. All types of prostheses used today release these wear particles-polyethylene (plastic), metal, and ceramic.

As an example, recent research has shown that the particles released from metal and plastic artificial joints can cause inflammation, changes in the surrounding tissue, and even genetic instability (meaning damage to the genes of the cells that come in contact with the particles and the toxins they release). I have recently blogged on metal on metal prostheses (MOM) side effects, as these surgeries have become very popular for resurfacing in the hip as well as partial knee replacements.

The concern surrounding MOM hips made national news due to research presented at the 2011 American Academy of Orthopedic Surgeons conference that demonstrated that the wear particles can cause side effects and complications in patients. These included local swelling, breakdown of the surrounding bone, and nerve dysfunction. Based on all the attention MOM prostheses used in hip resurfacing have been receiving, one might think that only MOM hips were the problem. However other implant types have generated concerns as well: for example, longer lasting ceramic knee and hip replacements have the same issue. These hip replacement joints last longer as ceramic on ceramic (both sides or surfaces made of ceramic) joints have much less wear than metal on metal.

Since ceramic knees and hips have less wear over time, it also makes sense that less of these “wear particles” would enter the patient’s blood steam and lymphatic system than metal knees. However, a new study questions this concept-finding that ceramic knees produce as many of these wear particles as do metal knees. Another popular new prostheses for hip replacement is highly cross-linked polyethylene (plastic) used with either metal or ceramic. The idea behind these newer combination joints is that by combining a special plastic that is manufactured to reduce wear (cross-linked polyethylene) with the traditional metal or ceramic components, the patient will get the comfort of a softer plastic joint with the durability of a metal or ceramic joint.

Again, recent research shows that both of these plastic/metal and ceramic/metal designs still produce wear particles. Another study out this month shows that even after an offending prostheses is removed, amazingly the wear particles already captured by the lymphatic system can continue to release cobalt ions that can lead to side effects. This is really concerning, as it may mean that removal of a prostheses years from now and replacement with a future joint shown not to produce toxic wear particles, may not help the patient escape exposure to the chemicals leaching from the particles already circulating in the lymph system.

The upshot? No matter what materials are used (plastic, metal, ceramic or a combination of these), the knee or hip replacement wear particles side effects the patient will still be exposed to include potentially toxic wear particles. These particles will increase likely based on weight of the patient (heavier patients experiencing more wear and particles) or activity (more active patients wearing off particles quicker than more sedentary patients).

Will these particles cause immediate problems in everyone? No, many patients don’t notice some of these side effects and it’s a minority of patients who are very sensitive to these toxins. Over the long run will these wear particles cause side effects in most patients? This is an unknown at this time-with data being collected and proponents on both sides of the argument. Detecting things like small increases in the cancer rate due to wear particles from artificial hips or knees usually requires careful registry tracking (this is performed only in Europe at this time) and tracking thousands to tens of thousands of patients for decades. I’ve blogged already on one of these studies, but as mentioned the study had some problems with it’s methods. In addition, for this type of association (cancer rates are increased due to these knee and hip replacement wear particles), most physicians would want to see multiple studies all pointing in the same direction.

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