In April, the United Kingdom’s Medicines and Healthcare Products Regulatory Agency (MHRA) issued a Medical Device Alert on metal-on-metal (MOM) hip devices. The alert does not impose any restrictions on MOM device use, but notes reports of revisions of MOM total hip replacement or resurfacing procedures involving soft-tissue reactions or pseudotumors.

Although patients “are thought to be at a low risk of developing serious problems,” the alert states that progressive reactions to wear debris of MOM articulations, which can cause soft-tissue necrosis and adversely affect the results of revision surgery, may develop in a small number.

The MHRA recommends annual follow-up of all patients with MOM devices for the first 5 years and more frequent examinations for symptomatic patients. It recommends measuring cobalt and chromium ion levels in the blood of patients with pain, and cross-sectional imaging with MRI or ultrasonography.

Ion levels above 7 parts per billion merit another measurement after 3 months, the MHRA advises. If imaging reveals soft-tissue reactions, fluid collection, or tissue masses, revision surgery should be considered.

UK health authorities have been monitoring this issue for several years, particularly after a group of researchers at Oxford University reported “what they thought was a fairly unusual reaction,” says Joshua J. Jacobs, MD, whose research focus at Chicago’s Rush University Medical Center is metal biocompatibility. The adverse reactions “weren’t particularly common but when they occurred, some were of such severity that revision was difficult.”

No U.S. bulletin
The U.S. Food and Drug Administration has not taken any action on MOM bearing use, but is monitoring the issue.

“As more U.S. surgeons use MOM bearings—both for resurfacing and total hips—and as the length of follow-up increases, we’re starting to see more cases,” says Dr. Jacobs, “and getting experience in evaluating and managing them.”

According to Dr. Jacobs, metal ion analysis can have considerable variation and every step of the process, from collection to laboratory analysis, must be done with precision. “Seven parts per billion is a very small amount, and the specimens have to be collected quite carefully to make sure that you’re not getting contamination, which can lead to spurious results,” he says.

“Regardless of the type of bearing, patients should be followed on a periodic basis,” Dr. Jacobs advises. “A patient with a symptomatic MOM bearing should be followed more frequently and additional imaging should be considered, as should metal ion analysis and possibly metal allergy testing.”

Terry Stanton is senior science writer for AAOS Now. He can be reached at tstanton@aaos.org

References:

Pandit H, et al: Pseudotumours associated with metal-on-metal hip resurfacings. J Bone Joint Surg Br 2008;90:847-851

Medical Device Alert: DePuy ASR acetabular cups used in hip resurfacing arthroplasty and total hip replacement