People who suffer from a spinal problem that can cause back and leg pain have an alternative to difficult surgery with a newly approved device that requires a much less invasive procedure to implant.

The Food and Drug Administration authorized use of the "X-stop" — a thumb of titanium on a mount that fits to a vertebra in the lower back — to reduces pain from lumbar spinal stenosis. The FDA's approval was announced Tuesday by the St. Mary's Spine Center in San Francisco, which developed the device.

The condition is the most common cause of back surgery in people over 50. It occurs when the tube for nerves in the spine becomes constricted as a person ages. Pain, numbness and weakness usually manifest by standing up; it is often relieved when sitting down.

Previously, the condition could be treated with physical therapy, anti-inflammatory drugs and injections, or by a laminectomy, a difficult surgery that involves full anesthesia and the removal of parts of bone and tissue to open up the canal in the spine.

The X-stop achieves a similar effect by pressing against parts of either side of a vertebra, pushing open the tube without removing any of it.

"By wedging those bones apart, the tube is indirectly opened up," said Dr. James Zucherman, who invented the device. He is the medical director of the St. Mary's Spine Center. "The bones don't collapse on the nerves like they did before. The patient doesn't have to bend over to protect the nerves."

Because it is implanted so close to the surface of the skin, it usually requires only local anesthesia to insert. People may have up to two of the devices implanted on vertebrae in the lower spine.

Zucherman, who has overseen the implantation of more than 100 of the devices during trials, said he will recommend it to most people with the condition who don't benefit from physical therapy and drug treatments. He will only recommend a laminectomy if the device doesn't help.

However, FDA documents released last year before a public hearing on the product raise concerns that the X-stop's effectiveness decreases over time, saying studies found that about 15 percent of patients who had the implant reported symptoms returning.

"It appears that the X-stop is effective to about a year and then begins to decline," the FDA said.

Dr. Zucherman disputed those figures, saying the percentage of implant patients reporting a dropoff was about half that — and that people who received only therapy and drug treatments reported a much greater rate of symptoms returning.

The documents also say roughly 50 percent of people who received the implant said they had a significant relief of all symptoms related to stenosis, as opposed to 5 percent who reported relief from nonsurgical treatment.

Zucherman pointed to another figure provided to the FDA — that 73 percent of patients who received the implant reported being satisfied with it after two years. People who underwent laminectomies report similar success rates, he said.

The treatment is also less expensive than the more invasive alternative. Implanting a single X-stop costs about $17,000; implanting two costs $24,000, he said. A laminectomy is about $50,000.

About 4,000 X-stop devices have been implanted around the world, he said. Versions of the device are already available in Europe and Japan. About 400,000 Americans are affected annually by lumbar spinal stenosis.

Zucherman said he invented the device in 1997 after watching a patient who received a successful laminectomy become mentally disabled — he couldn't remember where he was moment to moment — because of the full anesthesia used.