Effect of sacral slope on the biomechanical behavior of the low lumbar spine

The present study investigated the influence of sacral slope (SS) on the biomechanical responses of the lumbar spine under specific physiological conditions. Firstly, based on computed tomography scan images of a 30-year-old healthy male volunteer (SS, 55°), a three-dimensional finite element (FE) model including the L4-S1 segment was established. Flexion, extension, lateral bending and torsion motions were simulated and compared with cadaveric test data in the literature to validate the lumbar spine FE model. The model was then modified with different SS values (40 and 25°) for the same simulations to describe the process of structural compensation. Numerical results showed that with the reduction of SS, the range of motions (ROMs) reduced for flexion and lateral bending, but increased for extension and torsion. For displacement, the maximum magnitudes of L4/5 annulus fibrosus (AF) reduced by 10-25% in flexion, lateral bending and torsion, but less effect was observed for extension with only a 4% drop. Nearly the same displacement distribution appeared on the L5/S1 AF with small changes in the four motions. For the stress field of L4/5 AF, in contrast to flexion, the magnitudes for extension and lateral bending varied markedly, and under torsion the value increased by ~10%. For L5/S1 AF, the stresses changed little under flexion, extension and lateral bending, but strongly declined for torsion by ~71.8%. In conclusion, the present study indicates that the change in SS due to structural compensation affects the biomechanical behavior of the spine structure, and attention should be paid to SS when conducting surgical procedures or selecting intervertebral fusion implants.