There seems to be two types of problems related to the situation we are discussing:
- On the one hand the polyethylene wear leaves real particulate debris of polyethylene, which is taken up by the lymph cells and transported to the lymph glands. As the body tries to rid itself of these particles a process called 'osteolysis' may occur, where bone is broken down and weakened in areas around the implant. This can be seen on X-ray or CT imaging.
- On the other hand, the breakdown of the polyethylene may distort the mechanics of the implant and allow metal to come into contact with metal. The resultant friction may release charged metal ions, which are not the same as the particulate matter of the polyethylene, although sometimes, but rarely, there may be actual metal particles. The presence of metal ions can be inferred from X-ray changes .
Skin patch testing may differentiate between the metals, although it may be necessary to do blood tests and histology on the lymph nodes. Potentially the most harmful components are cobalt from cobalt-chromium alloy, nickel from stainless steel, and vanadium from titanium alloy. X-rays may show excessive fluid within the joint and osteolysis around the implant. The joint lining and suprapatellar pouch may show a fine linear radio-opacity called a 'metal-line. Other radiographic signs called 'clouds' and 'bubbles' may be indicative of the metal within the synovium as well as the florid synovial outgrowth, seen during surgery as areas of joint lining (synovium) heaped up into what look like soft black lumps.
It would seem logical to simply monitor knee replacement patients for raised metal levels, but this is not as simple as it seems. The biggest culprit for toxicity seems to be cobalt-chrome but there is no generally accepted threshold beyond which serum or blood concentrations of cobalt and chromium are known to be toxic.
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