The Food and Drug Administration has told companies that make "metal on metal" artificial hips to take a closer look at how patients fare after their hip replacement surgery. The request involves about 20 manufacturers and comes in response to an increasing number of consumer complaints about the implants, along with last year's voluntary recall of an all-metal model made by DePuy Orthopedics, a division of Johnson & Johnson. Company officials say the company is paying for "reasonable and customary costs of monitoring" the recalled product, including the cost of surgery, if needed, to replace damaged artificial hips. There are many varieties of hip implants and numerous materials are used in their construction, including ceramics, plastics and metals. In the case of all metal implants, both the ball and socket of the artificial joint are made of metal. When those two parts rub together during normal wear and tear, minuscule particles of metal can shed and be released into surrounding tissue and even into the bloodstream. This causes severe damage to local tissue, including necrosis, infection and allergic reactions.