Metal-on-metal total hip replacements have a much higher failure rate than other options and "should not be implanted", say researchers.They said data from the world's largest register of hip implants provided "unequivocal evidence" of the failure rate.The risk was greatest for women, the young and for large implants.The UK health regulator has already advised annual checks for people with large head metal-on-metal implants.Hip replacements are one of the great successes of modern medicine, restoring movement and cutting pain. Yet, all replacements have a risk of failure, such as the new joint coming loose, dislocating or through wear and tear. This study, published in The Lancet, showed some fail far more than others.Hip replacements come in different varieties such as all metal, all ceramic or metal-on-plastic.Researchers at the University of Bristol analysed information from 402,051 hip replacements recorded in the National Joint Registry of England and Wales.It showed that, overall, 6.2% of metal-on-metal hips had failed within five years. At the same point only 1.7% of metal-on-plastic - and 2.3% of ceramic-on-ceramic - had failed.The risks were greater in women. The report said: "Revision rates for stemmed metal-on-metal implants in women were up to four-times higher." Men were three-times more likely to need a replacement.Larger implants were also linked to an increased risk of problems with metal-on-metal with "each 1mm increase in head size being associated with a 2% increase" in the risk of revision. Ceramic implants performed better as the head size increased.Risks were also higher for younger patients.DangersConcerns about metal-on-metal implants were also raised two weeks ago when the UK's Medicines and Healthcare products Regulatory Agency (MHRA) said some patients would need annual blood tests.It is thought tiny pieces of metal break off the implants and leak into the blood. This might cause muscle and bone damage as well as neurological issues.It said 49,000 patients with large-head hip implants were affected.After the latest data, the clinical director of the MHRA, Dr Susanne Ludgate, said: "We recognise that there is emerging evidence of increased revision rates associated with large head metal on metal hip replacements. But the clinical evidence is mixed and this does not support their removal from the market."We will take quick action if we need to and, if patients have any questions, they should speak to their orthopaedic surgeon or doctor."The number of patients being fitted with metal-on-metal implants has fallen rapidly in recent years in the UK. There were 8,072 fitted in 2008, in 2011 that number fitted was 673.In the US, however, about a third of implants are metal-on-metal.Dr Art Sedrakyan, from the Weill Cornell Medical College, said: "When failures take a long time to develop, many faulty products can enter the market."We are left with more than 500,000 patients with metal-on-metal prostheses in the USA and more than 40,000 in the UK who are elevate risk of device failure, which will inevitably result in the burden of further surgical treatment."