The Lancet has published ‘unequivocal evidence’ from the world’s largest database on hip replacements, which confirms that stemmed metal-on-metal (MOM) implants, especially those with a larger head size and those implanted in women, whose failure rates are up to four times higher, have a substantially higher fail rate than hip implants made from other materials. The researchers are calling for a ban of using stemmed MOM hip implants.

The Lancet published the study 10 days following the Medicines and Healthcare products Regulatory Agency’s (MHRA) announcement of MOM implants requiring annual check-ups.

Whilst the use of stemmed MOM hip implants has declined in England and Wales, the USA still uses these implants extensively according to recent data. In 2009, 35% of hip implants in the U.S. were stemmed MOM implants. There was an increasing trend of using large-head stemmed MOM implants due to the belief that these implants would reduce the risk of dislocation and that they were highly resistant to wear.

In a study led by Ashley Blom from the University of Bristol in the UK, researchers evaluated data on over 400,000 hip replacements, including 31,171 MOM replacements from data obtained from the National Joint Registry of England and Wales. The data included all hip replacements between 2003 and 2011 and was tracked for up to 7 years post-surgery.

The researchers categorized the hip implants by materials before estimating the stemmed MOM implant failure rate by different head sizes and comparing them with ceramic and polythene implants.

They observed that stemmed MOM implants failed substantially faster, with a 5-year revision rate of 6.2% compared with other types of implants. The results showed that failure was related to the implant’s head size, with larger heads failing earlier, translating to a 2% higher risk of failure for each 1mm increase in head size. Unlike MOM implants, ceramic-on-ceramic implants performed better with larger head sizes.

Failure rates for stemmed MOM implants in women were higher than hose in men regardless of the implant’s head size. In addition they were also nearly four-times higher compared to implants with other bearing surfaces.

In a concluding statement, the researchers say: “Metal-on-metal stemmed articulations give poor implant survival compared with other options and should not be implanted. All patients with these bearings should be carefully monitored, particularly young women implanted with large diameter heads.”

Art Sedrakyan, from Weill Cornell Medical College at Cornell University in New York warns about the current situation in the USA in a linked comment, writing:

“The National Institutes of Health is interested in new discoveries and, until recently, not in infrastructure for comparative safety and effectiveness…There is also substantial pressure from Congress not to stifle innovation and to undertake faster reviews…[These practices] fail to recognize that only a large national, or even worldwide, registry can address the needs when more than 10 000 products are on the market for the same purpose.”

He concludes:

“Policy makers need to appreciate that registry data alone are not a substitute for good pre-marketing studies, which should include testing of implants. When failures take a long time to develop, many faulty products can enter the market. In the case of the ASR and metal-on-metal implants it took 4-5 years before evidence was accumulated and reported. We are left with more than 500 000 patients with metal-on-metal prostheses in USA and more than 40 000 in the UK who are at elevated risk of device failure, which will inevitably result in the burden of further surgical treatment as well as billions of dollars in costs to taxpayers.”

Written By Petra Rattue