The government’s system for regulating many medical devices should be abandoned and replaced because it fails to examine their safety and effectiveness before sale, according to one of the nation’s top scientific groups. In a report, released Friday, a panel of the Institute of Medicine found that the existing rules were never intended to provide safeguards for screening out dangerous or ineffective products. The panel urged the Food and Drug Administration to devise a new regulatory system for the so-called moderate risk devices — a category that now includes artificial hips, external heart defibrillators and hospital pumps — because the current system was not fixable.