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The assessment of sacrum angular motions and stress across sacroiliac joint (SIJ) articular surfaces using finite element lumbar spine-pelvis model and simulated posterior fusion surgical procedures.To quantify the increase in sacrum angular motions and stress across SIJ as a function of fused lumbar spine using finite element lumbar spine-pelvis model.A review of the literature suggests that for 20% to 30% of spine surgery patients, failed back surgery syndrome as a possible complication. The SIJ might be a contributing factor in failed back surgery syndrome in 29% to 40% of cases. The exact pathomechanism which leads to SIJ pain generation is not well understood. We hypothesized that lumbar spine fusion leads to increased motion or stresses at the SIJ; this alone could be a trigger of the pain syndrome.A finite element model of the lumbar spine-pelvis was used to simulate the posterior fusion at L4–L5, L4–S1, and L5–S1 levels. The magnitude of the sacrum angular motion and average of stresses across SIJ articular surfaces were compared with intact model in flexion, extension, lateral bending, and axial rotation motions.The computed sacrum angular motions in intact spine, after L4–L5, L5–S1, and L4–S1 fusion gradually increased with maximum value in L4–S1 fusion model. Also, the average stress on SIJ articular surfaces progressively increased from minimum in L4–L5 to maximum in L4–S1 fusion models.The fusion at the lumbar spine level increased motion and stresses at the SIJ. This could be a probable reason for low back pain in patients after lumbar spine fusion procedures.