Can lumbar fusion or sacroiliac-joint surgery shift mechanical stress to the SI joints during weight-bearing?

Lumbar and sacroiliac-joint (SIJ) procedures can change the mechanical pathway by which forces move from the spine through the pelvis to the lower limbs. The strongest evidence is biomechanical, with clinical postoperative SIJ findings consistent with—though not individually diagnostic of—this mechanism.

Biomechanical and weight-bearing evidence

• Finite-element studies show that lumbar fusion, particularly constructs extending to S1, increases SIJ motion, contact pressure, articular stress, and ligament strain. In a validated 2023 model, SIJ contact pressure rose 171%–676% and stress 130%–424% across loading directions after fusion.
• These effects are relevant to daily loading: walking models find SIJ stress is gait-phase dependent and greatest during single-leg stance, when pelvic loading is asymmetric.
• SIJ fusion can also modify pelvic mechanics. In a small prospective study of unilateral SIJ fusion, standing-load symmetry and several gait measures improved, but pelvic obliquity and rotation remained elevated—indicating redistribution rather than simple normalization of load transfer.

Clinical correlates

• Systematic-review and meta-analytic evidence identifies new-onset SIJ pain after spinal surgery, with higher pooled estimates when fusion extends to the sacrum.
• Imaging-based SIJ degeneration is more prevalent after lumbosacral fusion than lumbar-only fusion. These findings support clinical relevance, but pain and imaging changes cannot establish the mechanical cause in a particular patient.

Mechanistic interpretation

• The SIJs normally transmit spinal load to the legs with small but important motion. Restricting lumbar motion, especially at the lumbosacral junction, can redirect movement and force toward SIJ cartilage, ligaments, and periarticular tissues.
• Direct postoperative human measurements of SIJ pressure, ligament strain, or tissue loading during walking and standing are not yet available.

Bottom line

• Lumbar fusion and SIJ surgery can alter spinopelvic load transfer and shift stress to SIJ and surrounding tissues, most clearly with fusion to the sacrum; the magnitude and clinical consequences are procedure- and patient-specific.