Mechanical stress on the rod in posterior spinal fusion with and without rod fracture: a finite element analysis.

BACKGROUND: Rod fracture (RF) is a common complication of posterior spinal fusion surgery, with a high reoperation rate. This study evaluated the mechanical stress on the rods and intervertebral discs under various loading conditions in a simplified short-segment lumbar fusion model (L3-S1) with and without a unilateral rod fracture (URF) via 3D-computed tomography finite element analysis (CT/FEA), to clarify the spinal motion directions most associated with the mechanical stress on the rods. METHODS: Two 3D nonlinear finite element models of L3-S1 posterior spinal fusion were created using the preoperative CT images of 10 patients: (1) a no rod fracture (NRF) model and (2) a URF model with a simulated left RF at L4/5. A 10-Nm load was applied in six directions (flexion, extension, right and left lateral bending, and right and left axial rotation), and the maximum von Mises stress on the right rod at L4/5 and the minimum principal stress on the L2/3 adjacent and L4/5 intervertebral discs were measured and compared between both models. Furthermore, the maximum von Mises stress on the right rod was also compared among six loading directions. RESULTS: Mechanical stress on the L4/5 intervertebral disc was significantly higher in the URF model under all loading directions, but there was no significant difference in the mechanical stress on the adjacent L2/3 intervertebral disc between both models. In the NRF model, mechanical stress on the right rod was significantly higher under left and right axial rotational loadings versus the other loading directions. In the URF model, mechanical stress on the right rod was significantly higher under left lateral bending and axial rotational loading versus flexion loading. CONCLUSIONS: After posterior spinal fusion surgery, restricting spinal axial rotation and lateral bending toward the fractured side may help reduce mechanical stress on the rods.