Disabling lower back pain from degenerated disks often improves on its own, eventually. It’s not clear why because the disk is still degenerated. But the pain diminishes or even goes away.

“By middle age almost everybody has disk degeneration and a lot of people have back pain, but science has not been able to link the two,” Dr. Mirza said. “That’s the problem with fusion surgery for disk degeneration.”

Tony Delitto, dean of the University of Pittsburgh’s School of Health and Rehabilitation Sciences, says he tries to warn patients seeking fusion surgery for a deteriorated disk not to expect too much.

“I would be very, very hesitant, and most surgeons would be very, very hesitant to tell patients that after fusion they would be pain-free,” he said.

Dr. Richard Deyo, an emeritus professor of medicine at Oregon Health and Science University, and his colleagues conducted a study in Oregon and found that about half of fusion patients who had the procedure on their lumbar, or lower, spine were using opioids before their operations. After their surgery, only 9 percent stopped using the drugs. And 13 percent who had not used opioids became long-term users after the surgery.

It’s hard to know what constitutes success, Dr. Deyo said. For example, one study, one of whose co-authors was Richard Guyer of the Texas Back Institute, who was widely reported to be Woods’s surgeon, reported a “clinical success rate” of 57 percent after two years. It defined clinical success as at least a 25 percent improvement in overall functioning, with no device failure, no major complications and no neurological deterioration.