Here's a paragraph I found in a CR article (subscription probably needed):
Spinal Fusion for Chronic Back Pain
(from Consumer Reports, "4 Medical Treatments That Are Sometimes Unnecessary"
By Hallie Levine, August 30, 2022)
What it is: This surgery permanently connects two or more vertebrae in the spine—using metal plates, rods, or screws—to eliminate the motion between them that can cause pain.
Spinal fusion is appropriate in situations such as a spinal fracture from a car accident, a severe bone infection, or a tumor that causes part of your spine to collapse, says Steven Atlas, MD, MPH, director of The Primary Care Practice-Based Research & Quality Improvement Network at Massachusetts General Hospital in Boston. But generally, “that’s not when it’s being done,” he says.
When it’s questionable: Surgeons may advise spinal fusion for chronic lower back pain from severe arthritis or age-related wear from spinal disks, Atlas says. But studies suggest that in these circumstances, the procedure is no more effective than nonsurgical approaches such as physical therapy, he notes.
(This section of the article also adds the following, which of course on a TMS forum we can't comment on, but I provide it for context and completeness:
If you have severe chronic back pain and have used measures like PT for six to 12 months without much improvement, however, consider asking your doctor about laminectomy, Atlas says. In that procedure, part or all of the vertebral bone is removed to enlarge the spinal canal and ease pressure on your spinal cord, disks, and nerves.)