June 4, 2012 — Adding to the increasing evidence about the dangers of metal-on-metal hip implants, a new study has found that pseudotumors may be 10 times more likely to occur in people who have metal-on-metal hip replacements. The study was published in the U.K. version of the Journal of Bone and Joint Surgery. The researchers are very concerned about the slow, insidious growth of pseudotumors, and they are warning doctors to closely monitor patients who have these devices.
A pseudotumor is the abnormal growth of tissue, greater than 2-cm in diameter. It is not a cancerous malignancy, scar tissue, or a bursa. The growth of a pseudotumor can increase the risk of hip replacement failure, but it may not cause symptoms until it is very large. These symptoms may include pain, swelling, clicking, misalignment, or dislocation of the hip.
The study was based on an analysis of 119 patients who were implanted with a metal-on-metal hip replacement with a large femoral head. In total, 108 patients were available for CT scans, ultrasounds, and testing for the presence of a pseudotumor. The patients were tested an average of 3.6 years after being implanted with the new hip. Of these patients, there were 42 (39%) who had a pseudotumor.
The researchers also found that the best predictor for whether a patient had a pseudotumor was elevated levels of cobalt — patients with high amounts of cobalt in their bloodstream were four times more likely to have a pseudotumor. This indicates that high amounts of cobalt in the bloodstream could be a risk factor for developing a pseudotumor. Cobalt poisoning is a known side effect of metal-on-metal hip replacements.
Metal-on-metal hip replacements are known to shed cobalt and chromium into the body, due to the grinding effect of the metal ball-and-socket design. Over time, particles of metal can accumulate in the surrounding bone, tissues, muscles, and bloodstream. This can cause a side effect called metallosis, or metal poisoning.
There were 13 patients who had severe problems with their hip replacement and needed to have an additional surgery to replace the defective device. These surgeries revealed the presence of pseudotumors, extensive bone loss, tissue damage, metallosis (toxic metal poisoning), swelling, and other serious problems. All of the 13 patients were given a metal-on-plastic hip replacement instead of a metal-on-metal device.
The researchers warned that the pseudotumors tended to grow without causing symptoms, until the pseudotumor was very large and was also causing extensive tissue damage. They advised doctors and patients to monitor closely for signs of emerging problems with the device.
There are three FDA-approved metal-on-metal hip replacements currently available. Though the use of these products is on the decline internationally, they remain popular in the United States:
- Birmingham Hip Resurfacing (BHR) System
- Conserve Plus Total Resurfacing Hip System
- Cormet Hip Resurfacing System
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