Spontaneous fusion isn't an everyday thing, but it's not uncommon. It usually occurs withv aging, arthritis, or disc degeneration. Bone spurs form around a joint or on either side of the spine and stop motion. Bridges of bone can also form to fuse the spine.
Sometimes the fusion site isn't stable and an operation is needed. Bone graft and/or plates, screws, or wires can be used to gain a solid fusion. Your doctor will be able to see the fusion on an X-ray. The presence of any movement may signal the formation of a false joint. Increased pain will alert you to any problems.