Metal on metal total hip replacements have been in use for approximately 50 years. Because the combination of metal on polyethylene has been so effective and so popular, far fewer metal on metal total hip replacements have been performed.
The two opposing sides of a total hip replacement are called the bearing surfaces. The goal of a total hip arthroplasty is to create bearing surfaces which have low friction, good lubrication, will not break down, and will last a long time. The bearing surfaces must be compatible and must not give off particles when they wear that would cause problems within the joint or other parts of the body.
A metal femoral head articulating with ultra high molecular weight polyethylene has become the modern standard for total hip arthroplasty. As the polyethylene wears, however, it gives off small particles called particulate debris. The debris remains in the area around the joint. It causes thinning of the bone or osteolysis.
When osteolysis occurs in either the femur or the acetabulum, it can cause loosening of the components and ultimately result in the need for revision surgery. Alternative combinations of bearing surfaces are being studied in an effort to eliminate wear debris and the thinning of the bone that it causes.
A metal on metal prosthesis made of cobalt-chromium alloy was first developed by McKee and Farrar in the early 1960s. Since that time, newer designs have been put forward. It is felt that the combination of metal on metal will result in less wear debris.
A metal on metal hip joint replacement has the advantage of using larger sizes for the femoral head. Larger sizes mean that the hip joint is more stable and less likely to come out of the socket or dislocate. A second benefit is that the edge of the socket is less likely to impinge on the head or neck of the femur as the joint moves into extreme positions.
Large series of cases with long-term follow up are not yet available, but in some studies it appears that second generation metal on metal replacements will last as long as metal on polyethylene.
Metal surfaces, like polyethylene, can give off small particles of debris. The tiny metal particles are called ions. While they do not cause a reaction in the joint such as osteolysis or loosening, some particles may appear in other parts of the body.
Metallic ions have been found in blood serum, in red blood cells and in urine. It is not known if their presence will have toxicity or effect on any organs. Beyond this, it is not known if metal particles accumulating in the body would have a cancer causing or carcinogenic effect.
In a few rare cases, the metal may cause a local hyper-sensitivity reaction.
Metal on metal bearings have recently become more popular because they are used in hip resurfacing procedures. As noted above, the long-term effects of metal on metal in the human body are not known.
More information is needed on the tribology or the friction, lubrication, and wear of metal on metal surfaces.