Lumbar fusion leads to increases in angular motion and stress across sacroiliac joint: a finite element study.

STUDY DESIGN: 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.

OBJECTIVE: 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.

SUMMARY OF BACKGROUND DATA: 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.

METHODS: 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.

RESULTS: 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.

CONCLUSION: 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.