Biomechanical study of proximal adjacent segment degeneration after posterior lumbar interbody fusion and fixation: a finite element analysis

A healthy male volunteer was recruited. He had no previous spinal disease or lower back pain. A healthy L3–5 lumbar nonlinear finite element model was established using scanned computed tomography images of the lumbar region. The vertebral body consisted of cancellous bone, cortical bone, endplates, and posterior bone, and the thickness of the cortical bone and endplate was 1 mm. Intervertebral discs were consisted of a nucleus pulposus, an annulus fibrosus matrix, and annulus fibrosus fibers. The fluid traits of the annulus fibrosus matrix and nucleus pulposus were modeled using Mooney-Rivlin hyperelasticity [16]. The contact between the facet joints of the vertebral body was similar to that of facial contact without friction. The anterior longitudinal ligament, posterior longitudinal ligament, ligamentum flavum, interspinous ligament, supraspinous ligament, intertransverse ligament, and annulus fibers were all simulated with uncompressed truss elements [16, 17]. Three models of L3–4 intervertebral discs were established with different degrees of degeneration (normal, mild degeneration, and severe degeneration). The L3–4 intervertebral disc degeneration models were established by changing the height of the intervertebral disc, where the mildly degenerated and severely degenerated disc heights were reduced by 16.5% and 33%, respectively [7]. Nuclear parameters associated with mild and severe degeneration were obtained from the work of Guo et al. [16]. The L4–5 posterior interbody fusion model was simulated using pedicle screws, interbody fusion cages, and grafts placed inside the cage (Fig. 1). The material properties used in the models were obtained from the literature [16‐18] (Table 1). The mesh refinement was locally controlled at the highly stress-concentrated sites and the articulating surfaces. Using aspect ratio and the Jacobian check, the quality of all elements was monitored to avoid sharp discontinuities and unrealistically high-stress concentrations. Mesh refinement was performed for modeling accuracy until excellent monotonic convergence behavior with less than 3% difference in the total strain energy was achieved. Healthy disc with no fusion lumbar model has a total of 137,700 elements. Healthy disc with the fusion model has a total of 359,016 elements. Mild degenerated disc and fusion model have a total of 353,471 elements. Moderate degenerated disc fusion model has a total of 368,912 elements.

We limited the movement of all models in all directions of the lower endplate of the L5 vertebral bodies. We applied 400 N of vertical downward pressure to the upper endplate of the L3 vertebral bodies to simulate the body’s own weight [17]. On the L3 upper endplates, bending moments of 10 N O were applied to simulate the movement of the human body in six directions [17] (flexion-extension, lateral bending, and torsion).

We measured the range of motion (ROM) of the L3–4 and L4–5 vertebrae under a 400 N load and 10 N l bending moment using the L3–5 model. The model was verified by comparison with previous experimental data obtained from corpses.

The ROM, nucleus pressure, and annulus pressure of L3–4 were measured. All finite element analyses were performed using the ABAQUS software, version 6.13 (Dassault Systtult Simulia Corp. Providence, Rhode Island, USA).

The healthy L3–5 lumbar finite element model was compared with the previous cadaveric experimental data. Compared with the previous cadaveric experimental data, the L3–4 and L4–5 ROMs derived from the lumbar finite element model were similar to those in a previous cadaver experiment (Table 2), thereby validating the model.

We found that after L4–5 PLIF, the L3–4 ROM increased by 34.2%, 4.6%, and 7.8% in flexion-extension, lateral bending, and rotation, respectively. However, in the degeneration models, the L3–4 ROM was significantly reduced compared with that in the model without degeneration. In mild degeneration,the ROM decreased by 24.5%, 30.5%, and 9.7%. In severe degeneration, the ROM decreased by 54.0%, 38.0%, and 16.9% (Fig. 2).

The pressure on the L3–4 nucleus pulposus after PLIF was increased by 2.2%, 32.7%, 2.7%, and 5.1% in flexion, extension, lateral bending, and rotation, respectively. The pressure on the nucleus pulposus increased further with the degeneration of adjacent segments. In mild degeneration, the nucleus pulposus pressures increased by 6.4%, 16.5%, 7.6%, and 25.4%. In severe degeneration, the nucleus pulposus pressure increased by 23.5%, 38.1%, 32.3%, and 47.8% (Fig. 3).

The pressures increased on the L3–4 annulus fibrous after PLIF were increased by 1.9%, 176.0%, 2.8%, and 8.0% in flexion, extension, lateral bending, and rotation, respectively. The pressures on the annulus fibrosus increased further with the degeneration of adjacent segments. In mild degeneration, the annulus fibrosus pressure increased by 10.3%, 2.8%, 7.4%, and 1.9%. In severe degeneration, the annulus fibrosus pressure increased by 22.5%, 17.8%, 17.6%, and 22.6% (Fig. 4).