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A study of metal-on-metal hip implants found that the metal ions released when the parts of the cobalt-chromium-molybdenum devices rub against each other were able to penetrate into bone marrow, where they impeded the formation of bone-growing cells. 
Metal-on-metal hip implants came under intense scrutiny following the high-profile August 2010 recall of DePuy Orthopaedics’ ASR XL acetabular and ASR hip resurfacing systems. Johnson & Johnson pulled the devices off the market after receiving reports that a higher-than-normal number of patients required surgeries to correct or remove defective implants. The devices, many of which have been recalled or otherwise pulled from the market, have been found to deliver failure rates as high as 43% after 9 years.
Some reports warned that hundreds of thousands of patients may have also been exposed to toxic compounds from metal-on-metal implants, putting them at risk of developing cancer, cardiomyopathy, muscle and bone destruction and changes to their DNA. Since 2010 the controversy has ensnared other device makers, with personal injury lawsuits piling up even against metal-on-metal implants that haven’t undergone a recall.
The new study, published in the August issue of the journal Biomaterials, found that cobalt and chromium release contributes to bone loss. A Berlin-based team of researchers tested adjacent tissues, joint fluids and bone marrow to discover that both metal particles and dissolved metals play a role.