Resurfacing was originally targeted to active younger adults because of better wear resistance and because the preserved bone may make it easier to perform total hip replacement later on.

But according to Art Sedrakyan, MD, an associate professor at Weill Cornell Medical College in New York and an authority on comparative effectiveness research, hip resurfacing wasn't limited to this relatively small population. "Thousands of people on Medicare received these implants," he says. "Any technology adopted in the world can be adopted in very unexpected ways. That's why it's important to have clear criteria about who is going to benefit."

Dr. Sedrakyan wrote a commentary accompanying the Lancet report but was not involved in the study. In the commentary, he brings up the issue of the metal-on-metal bearings in resurfacing implants.

A growing number of serious complications including bone damage and cardiovascular and neurological problems have been linked to THR with a metal ball and socket (called metal-on-metal, or MoM) due to the microscopic metal particles that are released into the surrounding tissues and bloodstream as the ball and socket components rub together.

The current study did not look at the safety of resurfacing implants, but researchers say they could present the same risks as metal-on-metal THRs.

Dr. Sedrakyan notes – as he has in the past when speaking about the potential hazards of metal on metal hips – that there are no good studies on the long-term effects of chromium and cobalt ions produced when these metal bearings rub together.

"People get resurfacing implants at a relatively young age, and we don't know what happens after 20 or 30 years of metal ion exposure," he says. "If hip resurfacing devices are found to be unsafe, then the implications are grave."

Regulatory agencies in the U.K. advise annual checks and imaging tests for people with metal-on-metal hip systems, but no such guidelines exist in the U.S. In June, a Food and Drug Administration advisory panel took the first step by recommending that patients experiencing symptoms from all-metal hips have annual X-rays and metal ion testing.

Dr. Blom tells resurfacing patients not to worry too much, but suggests similar precautions: "If you have pain, I would advise an annual review by surgeons, have your metal levels checked and a MARS MRI of the hip," he says. A MARS (Metal Artifact Reduction Sequence) MRI cuts down on distortions to the image caused by the metal implant.

As to whether female patients should continue to undergo hip resurfacing, Thomas Vail, MD, chair of the department of orthopaedic surgery at the University of California, San Francisco, agrees with the researchers. "I would concur with the findings of the authors that indications for metal-on-metal implants should be stringently applied,” he says. “Based upon the emerging information, it would seem that the indications for metal-on-metal hip resurfacing in women are increasingly limited to surgeons with technical expertise in resurfacing and female patients with special cases where total hip replacement might be less desirable."