In the July 2012 issue of Orthopedics, researchers once again examined metal-on-metal (MoM) hip implants like the DePuy ASR, DePuy Pinnacle, Zimmer Durom Cup, and Biomet M2A Magnum. These implants, which are typically composed of a metal ball and stem that fits inside a metal cup, were originally designed to be more durable and long lasting than older ceramic and plastic designs. Post-marketing reports and studies, however, have found that they have higher failure rates and can increase the risk of tissue damage and death, bone loss, and metallosis.
In this study, researchers from California, Chicago, and New Jersey examined 80 patients who underwent revision surgery for a failed MoM implant to determine the most common reason for premature failure. The researchers found that when these implants fail, they typically do so within the first two to three years.
Study Finds Loosening of Acetabular Cup Most Common Problem
The revisions examined by the researchers occurred between 2003 and 2010. The participants ranged in age from 58 to 68 years old. The most common reason for hip implant failure was the loosening of the acetabular cup, which occurred in 45 of the 80 patients. The next most common reason was infection, followed by metal hypersensitivity, and fracture.
Of the 45 cups that loosened, 35 (78 percent) were Zimmer Durom Cups. The second most common was the DePuy ASR cup, which was withdrawn from the market in August 2010. The remainder were Biomet Magnum, DePuy Pinnacle, and Wright Medical Conserve.
Researchers Examine Metal Contamination
During normal wear and tear, MoM implants can sometimes shed tiny shards of cobalt and chromium into the surrounding joint area. This may cause inflammation and damage to tissues and bone, and the metal shards can also travel into the bloodstream to cause systemic side effects.
Metal sensitivity was cited as the reason for implant failure in five of the 80 patients. In these cases, soft tissue damage and death was discovered in revision surgery. The acetabular cup was also found to be pitched abnormally forward. The average time from implantation to revision for metal sensitivity was a little over two years.
Patients who experienced failure for other reasons also showed signs of metallosis. Metallosis results in tissue damage caused by metal debris. In some cases, the implant loosened because of these changes in tissue health. Some patients also suffered from pseudotumors, which are small pockets of swelling caused by inflammation. These were removed during revision surgery.
Positioning of the Acetabular Cup Particularly Important
Overall, the researchers found that about 78 percent of revisions were performed within two years of the initial implantation, and 92 percent within three years.
The researchers noted that a high percentage of the failures were caused by two cup designs “that have been found to have a high rate of early revisions.” Specifically, the researchers referred to the Zimmer Durom Cup and the DePuy ASR. They also noted that in failures caused by metal sensitivity, most occurred in women. Soft tissue complications caused by the metal material, however, could have serious implications on the function of the implant in all patients, and can lead to instability and premature loosening.
Finally, the researchers stated that the position of the acetabular cup was a major factor in whether the patient experienced metal contamination. They advised surgeons to be especially careful when implanting these types of implants to optimize positioning.
DePuy, Zimmer, Biomet, and other manufacturers of MoM implants are currently defending thousands of lawsuits in state and federal courts across the country.
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