Role of posture in mechanics of the lumbar spine in compression

Biomechanics of the entire lumbar spine L1-S1 in erect postures was investigated under physiological axial compression loads of up to 800 N with and without right axial torques of up to 10 Nm using a nonlinear finite-element model. The sagittal curvature is varied from an initial value of 46 degrees by +15 degrees to model more lordotic posture and by -7.5 degrees as well as -15 degrees to model flattened lumbar spines. The primary/coupled displacements, disc pressures, ligament forces, facet forces, disc fiber strains, along with the required stabilizing sagittal/lateral moments, are computed at different levels, loadings, and lordotic postures. The role of facetectomy and partial nucleotomy at some levels is also studied for a few cases. The primary/coupled motions, disc pressures, facet loads, and disc fiber strains are all affected by postural changes and the addition of 10-Nm axial torque. The primary responses are stiffened when axial compression and axial torque are applied together rather than separately. Flexion and left lateral moments are required to stabilize the lumbar spine in compression. The changes in lordosis markedly affect the stabilizing sagittal moments. The lateral moments, however, are significantly influenced only in the presence of axial torque. Axial compression generates small disc fibers that decrease from the innermost layer to outer ones at all disc levels. The contact forces are largest at the L5-S1 facets. These forces cause large differences in rotation between L5 anterior and posterior bony structures, suggesting the high stresses on corresponding pedicles and pars interarticularis. Finally, the stabilizing flexion moments are generated primarily by the off-centeredness of the gravity load. Relatively small muscle forces are required to balance the remaining portion.