Metallosis is a condition where the body is adversely affected by metal ions from metal abrasion of implants.
Friction in metal implants
Metallosis is a syndrome associated with metal joint implants, where friction between two metal surfaces releases metal ions (electrically charged particles) into the joint, and these trigger an inflammatory response.
Metal-backed patellar components are no longer in frequent use but of course there are still many patients who had had this type of implant done in previous years. Although rare in the knee, metallosis is a diagnosis that must be thought of when investigating a patient whose knee is playing up after a knee replacement
Synovial reaction to metal debris
The term 'metal-induced synovitis' describes those parts of the synovial joint lining that bloat up and become rubbery and filled with a florid black staining due to the metal ions.
In the knee this condition is usually related to a previous knee replacement (usually a total knee replacement, but also sometimes in a partial/unicompartmental knee replacement) in which the polyethylene components have worn down and inadvertently allowed metal components to come into contact where they normally would not.
In particular it is often the polyethylene part of the metal-backed patella that is at fault, where the polyethylene breaks down and allows the back of the patellar component to make friction contact with the metal femoral component.
Symptoms of metallosis
The combination of all of these factors may lead to severe knee pain and quite marked knee swelling, with excessive fluid in the joint crepitus, and feelings of instability associated with loosening of the implant, pseudotumours, fractures around the implant, skin rashes, confusion, tinnitus, deafness, and even dementia.
Some patients may have a frank allergy to certain metals, eg nickel,cobalt and chrome while in other patients it may be more a matter of metal toxicity.
Diagnosing metallosis
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.
Revision surgery for metallosis
Despite this, it appears that a revision from a cobalt-chrome femoral component to a ceramic one, or replacement of a metal-backed patellar component with a polyethylene one might be contemplated when there appears to be toxicity problems.