Effect of lordosis angle change after lumbar/lumbosacral fusion on sacrum angular displacement: a finite element study.

Objective: To assess and characterize the sacrum angular displacements in response to lumbar lordosis after lumbar/lumbosacral fusion.

Methods: A finite element model of the lower lumbar spine-pelvis was established and used to simulate the posterior fusion at L3-L5 and L4-S1. The lordosis angle in the fusion segments was set to five different conditions with respect to the intact model: 10° less than intact, 5° less than intact, same as intact, 5° more than intact, and 10° more than intact. Variations of the sacrum angular displacements with lordosis changes were analyzed under loading setting of axial compression, flexion, extension, lateral bending, and axial rotation.

Results: Compared with the intact lordosis, both increased and decreased lumbar lordosis angles caused the sacrum angular displacements to be increased. The lordosis angle increased by 10° induced the most substantial increase in sacrum angular displacements. In addition, the sacrum angular displacements of the L4-S1 fusion model at different lordosis angles were higher than those of the L3-L5 fusion model.

Conclusions: The sacrum angular displacements occur as a result of the fusion surgery (L4-S1) and the changes in lumbar lordosis.