What it is
Spinal fusion is a surgical procedure used to correct problems with the small bones of the spine (vertebrae). It is essentially a "welding" process. The basic idea is to fuse together the painful vertebrae so that they heal into a single, solid bone.
Spine surgery is usually recommended only when your doctor can pinpoint the source of your pain. To do this, your doctor may use imaging tests, such as x-rays, computed tomography (CT), and magnetic resonance imaging (MRI) scans.
Spinal fusion may relieve symptoms of many back conditions, including:
Spinal fusion eliminates motion between vertebrae. It also prevents the stretching of nerves and surrounding ligaments and muscles. It is an option when motion is the source of pain, such as movement that occurs in a part of the spine that is arthritic. The theory is if the painful vertebrae do not move, they should not hurt.
If you have leg pain in addition to back pain, your surgeon may also perform a decompression (laminectomy). This procedure involves removing bone and diseased tissues that can put pressure on spinal nerves.
Fusion will take away some spinal flexibility, but most spinal fusions involve only small segments of the spine and do not limit motion very much.
Source from AAOS, Viewmedica, WebMD