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.