Implants made of metal have a thin coating around them to prevent flecks of metal from going into the body and blood stream. This coating can breakdown, releasing potentially toxic chemicals into the body.
Several studies have been done that show an increased number of patients with metal hip implants developing cancer. Leukemia and lymphoma are the two types of cancer seen in patients with cobalt-alloy total joint replacements. The same has been reported for metal-on-metal hip replacement.
There’s no proof yet that the metals are the problem. More study is needed before we know for sure.