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Metal-on-metal hip replacement systems, which are designed to provide the patient with excellent durability, range of motion and joint stability, are made of cobalt-chrome alloy. The ball and socket can be manufactured to closely match the size of the patient’s own joint, providing a more natural functioning. These advantages of the metal-on-metal design make this an artificial hip system preferred for younger, more active patients. Disadvantages and risks include metallic debris that might lodge in tissues surrounding the joint, metallic ions that travel through the body in the bloodstream and possible dislocation of the joint. Severe complications are sometimes addressed using a revision surgery to replace the artificial hip.
One advantage of metal-on-metal hip replacements when compared to metal-on-plastic replacements is the durability of the all-metal implants. Frictional wear is significantly less in the all-metal devices, increasing the length of time that the prosthetic joint functions optimally. This is one of the reasons that metal-on-metal hip replacement systems are used in younger patients who have an active lifestyle.
Additional benefits of metal-on-metal systems include an enhanced range of motion of the joint when compared with other types of hip replacements and the increased stability provided by the larger size of the metal implants. Increased stability helps prevent dislocation of the hip replacement, which is a major risk with smaller implants. Both of these benefits of metal-on-metal hip replacement systems make them suitable for active patients.
The increased durability, greater range of motion and stability of metal devices greatly improve the quality of life for patients after hip replacement surgery. Patients should have little to no pain within a few months of the surgery, and with some precautions, they can return to their normal activities. Jogging and contact sports are discouraged, however.
Two of the risks of metal-on-metal hip replacements are the release of metallic debris into surrounding tissues and metallic ions into the bloodstream. Varying levels of metallic ions have been found in the blood of patients who have received certain metal-on-metal implants. Research has shown that the levels of metal ions in the bloodstream increase over time. Cases of cobalt toxicity have been reported in patients after metal hip replacement surgery. The risk that metallic ions might adversely affect an unborn child has raised the issue of whether metal-on-metal systems are the best choice for women of childbearing age.
Frictional wear of the metal parts of the metal hip replacement might cause the release of metallic debris along with metal ions. Risks associated with this include inflammation and swelling of soft tissue around the joint, damage to connective tissue and bone and the occurrence of noncancerous tumors. Research has found that younger women are at the greatest risk for tumor formation and soft tissue damage around the metal hip replacement.
The remedial action for early metal device failures and other serious side effects, such as severe reactions to metal debris and ions and permanent dislocation of the artificial joint, include revision surgery to correct the problem. Damage to the bone and soft tissues resulting from the first metal hip prosthesis might cause lasting problems even after revision surgery has implanted a new hip replacement. Some metal-on-metal hip replacement devices have been discontinued because of their higher-than-expected failure rates.