I had a lumbar fusion at L5S1 and ended up with problems in the sacroiliac joint on both sides. My low back pain is better but now I have SI pain. Is this a typical response after surgery?
Sacroiliac (SI) joint dysfunction is common in adults even without a spinal fusion. As many as 40 per cent of the "normal" adult population have SI joint changes observed on CT scans.

Seventy-five per cent of patients with a lumbar (L45) or lumbosacral (L5S1) fusion develop degenerative changes in the SI joint. The reason for this response is probably related to the location of the sacrum at the bottom of the spine. It is a wedge-shaped bone that sits between the two bones of the pelvis.

The sacrum distributes force transferred to it from the upper body. It is able to withstand six times the amount of shear (side-to-side) force applied to the lumbar spine. But the SI is not as resistant to rotational forces. When the lumbar spine is fused, the shear and rotational forces on the SI joint increase dramatically. This is especially true for anyone who has had a L5S1 fusion.