At least half the thousands of operations on Americans with ruptured discs in their lower back probably were unnecessary, says a study showing 90 percent of such patients can be healed without surgery. The three-year study indicates that surgeons should change how they decide whether to operate on patients with lumbar disc problems, one of the study's authors, Dr. Jeffrey Saal of the San Francisco Spine Institute, said Monday. Neurosurgeons often operate if a patient fails to improve after bed rest and suffers severe pain, leg weakness and if diagnostic exams show the disc's nucleus has broken through the disc wall and is pressing against a nerve root. Saal said none of those criteria should be used as overwhelming evidence that surgery is needed. He said the decision should be based on the patients' level of function and whether that can be improved by an aggressive rehabilitation program.