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How safe are metal-on-metal hip implants?

BMJ 2012;344:e1410 doi: 10.1136/bmj.e1410 (Published 28 February 2012)

How safe are metal-on-metal hip implants?

Deborah Cohen examines the evidence of risk from metal-on-metal hips, the manufacturers’ inadequate response, and how the regulatory bodies failed to give doctors and patients the information they need to make informed decisions.

Deborah Cohen investigations editor
BMJ, London WC1H 9JR

Hundreds of thousands of patients around the world may have been exposed to toxic substances after being implanted with poorly regulated and potentially dangerous hip devices, a BMJ/ BBC Newsnight investigation reveals this week. Despite the fact that these risks have been known and well documented for decades, patients have been kept in the dark about their participation in what has effectively been a large uncontrolled experiment.

This isn’t the unlucky failure to spot the misdemeanours of one rogue company or the occasional unforeseen breakdown of a small number of devices. It is the inability to prevent a whole class of failing hip implant from being used in hundreds of thousands of people globally—a class of implant that the usually reticent National Joint Registry of England and Wales described recently as a “cause for concern.”1 2 The implants concerned are “metal on metal”—the head at the top and the lining of the cup it fits into are made of cobalt-chromium alloy rather than ceramic or polyethylene—and there are models for both total hip replacement and hip resurfacing.

From their arrival on the orthopaedic scene in 1997, they were marketed as the latest advance in hip replacement and were targeted at young active patients who needed a hip that would last a whole lifetime. And while there is evidence that hip resurfacing works well in young active men,3 the failure rates of resurfacing in women and of metal-on-metal total hip replacements in both sexes are higher than they should be. Average failure rates at seven years are 11.8% for resurfacing and 13.6% for metal-on-metal total hip replacement, although failure rates vary with the brand used. This compares with rates of 3.3%-4.9% for hip implants made of other materials.2

Metal-on-metal devices have been implanted into over 60 000 patients in England and Wales since 2003—when the National Joint Registry first began to record procedures. Before this date numbers are unreliable. In the US the figure is closer to a million and likely to increase. At the annual American Academy of Orthopedic Surgeons conference in February this year, a roll call of manufacturers was still promoting these products to the 40 000 attendees in the exhibitor hall.

Cobalt-chromium implants have been used successfully in orthopaedics for years—for example, in knee operations and fracture repair. They are known to release metal ions, but some metal-on-metal prostheses do so on a much greater scale than previously thought. These ions can seep into local tissue causing reactions that destroy muscle and bone and leaving some patients with long term disability.4 Local tissue reactions associated with ions from metal-on-metal hips were first described in detail as long ago as 1975.5 The ions can also leach into the bloodstream spreading to the lymph nodes, spleen, liver, and kidneys before being excreted in urine.6

Full article – How safe are metal-on-metal hip implants?

 

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