Biomechanical Effect of L4-L5 Intervertebral Disc Degeneration on the Lower Lumbar Spine: A Finite Element Study

Objective To ascertain the biomechanical effects of a degenerated L-4-L-5 segment on the lower lumbar spine through a comprehensive simulation of disc degeneration. Methods A three-dimensional nonlinear finite element model of a normal L-3-S-1 lumbar spine was constructed and validated. This normal model was then modified such that three degenerated models with different degrees of degeneration (mild, moderate, or severe) at the L-4-L-5 level were constructed. While experiencing a follower compressive load (500 N), hybrid moment loads were applied to all models to determine range of motion (ROM), intradiscal pressure (IDP), maximum von Mises stress in the annulus, maximum shear stress in the annulus, and facet joint force. Results As the degree of disc degeneration increased, the ROM of the L-4-L-5 degenerated segment declined dramatically in all postures (flexion: 5.79 degrees-1.91 degrees; extension: 5.53 degrees-2.62 degrees; right lateral bending: 4.47 degrees-1.46 degrees; left lateral bending: 4.86 degrees-1.61 degrees; right axial rotation: 2.69 degrees-0.74 degrees; left axial rotation: 2.69 degrees-0.74 degrees), while the ROM in adjacent segments increased (1.88 degrees-8.19 degrees). The largest percent decrease in motion of the L-4-L-5 segment due to disc degeneration was in right axial rotation (75%), left axial rotation (69%), flexion (67%), right lateral bending (67%), left lateral bending right (67%), and extension (53%). The change in the trend of the IDP was the same as that of the ROM. Specifically, the IDP decreased (flexion: 0.592-0.09 MPa; extension: 0.678-0.334 MPa; right lateral bending: 0.498-0.205 MPa; left lateral bending: 0.523-0.272 MPa; right axial rotation: 0.535-0.246 MPa; left axial rotation: 0.53-0.266 MPa) in the L-4-L-5 segment, while the IDP in adjacent segments increased (0.511-0.789 MPa). The maximum von Mises stress and maximum shear stress of the annulus in whole lumbar spine segments increased (L-4-L-5 segment: 0.413-2.626 MPa and 0.412-2.783 MPa, respectively; adjacent segment of L-4-L-5: 0.356-1.493 MPa and 0.359-1.718 MPa, respectively) as degeneration of the disc progressively increased. There was no apparent regularity in facet joint force in the degenerated segment as the degree of disc degeneration increased. Nevertheless, facet joint forces in adjacent healthy segments increased as the degree of disc degeneration increased (extension: 49.7-295.3 N; lateral bending: 3.5-171.2 N; axial rotation: 140.2-258.8 N). Conclusion Degenerated discs caused changes in the motion and loading pattern of the degenerated segments and adjacent normal segments. The abnormal load and motion in the degenerated models risked accelerating degeneration in the adjacent normal segments. In addition, accurate simulation of degenerated facet joints is essential for predicting changes in facet joint loads following disc degeneration.