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Metal-metal bearings have advantages for active patients because their wear resistance is much greater than that of polyethylene, they do not fracture and they perform better with larger diameter balls. It has been known for three decades that the serum and urine levels of cobalt and chromium (the main elements in a metal-metal hip bearing) are increased in patients with a metal-metal bearing. The health impact of such ion levels has not been established and continues to be debated. Trace element analysis requires very sensitive equipment since the levels are in parts per billion (ppb). Absent some environmental exposure, serum and urine levels of cobalt and chromium are effectively zero in people with no metallic implants. In patients with a well-functioning total joint with a metal-on- polyethylene bearing, the serum levels are generally less than 1 ppb. In patients with a well-functioning prosthesis with a metal-metal bearing, the ion levels are generally between 1 and 5 ppb. These levels are very low and are 1/10th to 100 times less than posted industrial toxic levels. If there is any increase in overall health risk to patients with a metal-metal bearing prosthesis, it is very low. The JRI continues to closely monitor patients with metal-metal bearing prostheses to better define the benefits and risks of this technology.

 

There is one potential problem with metal-on-metal bearings. Normal wear results in release of metal particles into the body. One problem with any artificial bearing surface is the wear regenerated by normal daily wear and tear. Large volumes of plastic generated by metal on plastic (traditional) replacements result in large amounts of bone destruction, (osteolysis) around implants and has been the major cause for failure in young patients. Both ceramic-on-ceramic and metal-on-metal devices generate about 99% less wear debris than traditional bearings and the debris generated seems to cause less irritation to the bone than plastic debris does. There has been some speculation of the potential for metal debris to cause cancer. However, with careful studies to date, no links have been demonstrated.

My opinion is that the advantage of metal on metal bearings strongly outweighs the potential risks. Now we can return patients to normal function with almost no restrictions and expect their implants to last more than 10 years at very high activity levels. Most patients will never require another operation on their hip. If ceramic-on-ceramic or metal-on-crosslinked polyethylene bearings are used, wear is also not a problem, but significant restrictions remain. No running or jumping can be allowed due to the risk of fracture of the implant: no crossing of the legs or extreme bending can be allowed due to the risk of dislocation. I am surprised that any person would choose anything other than a large metal-on-metal bearing!
 

Metal ion dispersal

The current generation of surface replacements are metal-on-metal bearings. That means both the ball and the socket are made entirely of metal. Although this cuts down dramatically on the wear and tear of the components, it has been shown to cause metal ions to be dispersed through the body. Cobalt and chromium ions are measurable in the blood stream, but have not been shown to cause cancer or any other disease in humans. Although the metal ions are measurable, no one knows what a safe level is. Generally, people with functioning kidneys are able to excrete the ions in their urine.

 

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