SI Degeneration After Lumbar Spine Fusion

Spinal fusion is known to cause degeneration of the adjacent vertebral segment (next to the fusion). Many studies have been done to understand and possibly prevent this from happening. This is the first study to look at the effect of lumbar fusion or lumbosacral fusion on the sacroiliac joint (SIJ).

The SIJ is formed by the sacrum and pelvic bones on each side of the sacrum. It is the lowest segment of the spine before the coccyx (tail bone). Force from the upper body is transferred down the spine to the SIJ. Small movement at the SIJ occurs and affects movement of the lumbrosacral spine (where the last lumbar vertebra attaches to the top of the sacrum).

All patients in this study had a posterolateral (from the back and side) fusion of the lumbar spine or lumbrosacral spine. Bone graft was harvested from the patient's own iliac crest (top of the pelvic bone). The graft material was packed inside titanium-threaded cages and placed between the bone segments being fused.

Three groups of patients were compared. The control group was made up of healthy adults without spine problems. The fusion group was divided into two subgroups. The first subgroup had a fusion to L5. This was called the floating fusion group. The second fusion subgroup was the fixed fusion group. Their fusion was to S1.

CT scans were used to measure degeneration at the SIJ following fusion. Imaging was done before surgery, two weeks after surgery, then again, one year and five years after surgery. SIJ degeneration was seen on CT as areas of erosion, sclerosis (hardening of the soft tissues), or bone spur formation. Any sign of bone fragments, joint space narrowing, or cysts was also defined as SIJ degeneration.

As might be expected, SIJ was much more common in the fusion group (both subgroups) compared to the control group. The group with a fixed fusion had the greatest amount of SIJ (on both sides). The amount of SIJ degeneration did not appear to be linked to the number of fused segments. The area of fusion seemed to have more of a cause-effect (more SIJ degeneration with S1 fusion than with L5 fusion).

The authors suggest the force delivered to the pelvis is higher with a sacral fusion thus causing a greater effect on the SIJ. The authors also report that removing bone from the iliac crest seems to have a negative effect on the SIJ. SIJ degeneration is more likely after graft harvest. The reason for this is unknown since no damage is done to the SIJ during the graft removal.