Effect of changes in lordosis on mechanics of the lumbar spine-lumbar curvature in lifting.

Using a realistic nonlinear three-dimensional finite element model, biomechanics of the entire lumbar spine L1-S1, risk of tissue injury, and required local lumbar muscle exertion in extended and flexed postures are investigated under moderate to relatively large compression loads as great as 2800 N as the lumbar lordosis is altered from the undeformed value of -46 degrees by + 15 degrees in extension or by as much as 38 degrees in flexion. To prevent the instability of the passive structure in compression, the changes in segmental rotations are prescribed and the required sagittal/lateral moments at each level calculated. The effect of load distribution is considered by applying the whole compression on the L1 vertebra alone or among all vertebral levels with 90% or 80% of the compression on the L1 and the remaining evenly shared by the rest. The results are markedly affected by the postural changes and load distributions. The primary global displacement responses are stiffened in the presence of combined loads. The axial compression load substantially increases the intradiscal pressure, facet loads, and disc fiber strains. The large facet loads at the caudal L5-S1 level causes large differential sagittal rotations at vertebral posterior and anterior bony structures, resulting in large stresses in the pedicles and pars interarticularis. The contribution of the passive structures in carrying the load is influenced by the lumbar lordosis and compression load magnitude. Slight flattening of the lumbar spine under large compression reduces the maximum disc fiber strains and required equilibrating moments without adversely affecting the disc pressure and ligament forces. During lifting tasks, the passive spinal structures are protected by slight to moderate flattening in the lumbar curvature, whereas larger flexion angles impose significantly higher risk by increasing the disc pressure, disc anulus fiber strains, ligamentous forces, and facet forces. Changes in lordosis also markedly affect the stabilizing sagittal moments; the required moments diminish in small flexion angles, thus requiring smaller forces in local lumbar muscles. Thus, the lumbar posture during heavy lifting could be adjusted to minimize the required moments generated by lumbar muscle exertions and the risk of tissue injury.