Can lumbar fusion increase sacroiliac joint stress and pain?

Lumbar and lumbosacral fusion can shift mechanical demand to the sacroiliac joint (SIJ), and SIJ pain is a clinically recognized contributor to persistent or new pain after surgery. The biomechanical explanation is well supported, whereas direct proof that altered SIJ mechanics causes an individual patient’s pain remains indirect.

Biomechanical evidence

• Finite-element models predict increased compensatory sacral/SIJ motion, articular stress, and contact pressure after fusion versus intact biomechanics. In one model, peak SIJ contact pressure increased 171% in flexion and 676% in extension, while SIJ stress increased 130% and 424%, respectively.
• Effects appear construct-dependent: longer and more caudal constructs—particularly those including L5–S1 or L4–S1—produce larger changes. Cadaveric work also identifies altered, sometimes increased, SIJ flexion-extension motion after L5–S1 fixation.
• Fusion increases strain in the iliolumbar and iliosacral ligaments, offering an additional pathway for periarticular mechanical irritation. Pelvic fixation, SIJ stabilization, loading direction, and existing SIJ injury can substantially modify these effects.

Clinical relevance

• Postoperative SIJ pain is not rare. A systematic review estimated an incidence of 15.8% overall and 32.9% when fusion extended to the sacrum; a later meta-analysis estimated 9.4% new-onset SIJ pain. Cohorts have reported approximately 12.0% after lumbar fusion.
• Higher rates with multilevel and sacrum-including fusion are consistent with the loading-transfer hypothesis, but do not establish that increased SIJ stress or motion is the direct cause of pain.

Practical implications

• In an 83-year-old with pain after lumbar surgery, prior fusion should heighten suspicion for SIJ involvement, particularly with a long construct or sacral extension, but should not replace diagnostic confirmation.
• Provocation-test clusters can guide assessment; image-guided, contrast-confirmed intra-articular anesthetic injection is the key confirmatory test.

Bottom line

• Lumbar fusion can increase SIJ stress, motion, and ligament strain, and SIJ pain is a recognized postoperative pain generator; the mechanical pathway is compelling but not proven as the causal explanation for pain in every individual.