Now federal regulators and medical researchers are scrambling to determine how many implant recipients have been injured by the devices, which can shed dangerous metallic debris through wear.

In a highly unusual move, the Food and Drug Administration last month ordered manufacturers of all metal hips to undertake emergency studies of patients. And lawmakers and others are now calling for a tightening of how the F.D.A. scrutinizes new implants — both before and after they are sold.

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