How problematic, exactly, are metal on metal hip replacements? According to research conducted at the University of Bristol in the United Kingdom, they are up to four times as likely to fail as other types of hip replacements. US News & World Report reports that researchers at the school analyzed data on more than 400,000 hip replacements (including over 31,000 metal on metal implants) performed between 2003 and 2011. The scientists found that stemmed metal on metal implants had a five-year failure rate of 6.2 percent, which was much higher than implants made from ceramic and polythene. They also found that failure was related to head size. Each 1 millimeter increase in head size was associated with a 2 percent increased risk of failure. Curiously, the failure rate for ceramic-on-ceramic implants actually improved with larger head sizes. Read more from US News & World Report. Read the study in The Lancet.