Analysis of symmetry of vertebral body loading consequent to lateral spinal curvature.

Abstract

A biomechanical model was used to calculate muscle and intervertebral forces in a spine with and without a lumbar scoliosis. To quantify the loading of the motion segments in a lumbar scoliosis. Scoliosis is thought to cause asymmetric loading of vertebral physes, causing asymmetric growth according to the Hueter-Volkmann principle. The magnitude of vertebral loading asymmetry as a function of scoliosis magnitude is unknown, however, as is the sensitivity of growth to asymmetric loading. The analysis included five lumbar vertebrae, the thorax, and the sacrum/pelvis and 90 pairs of multijoint muscles. Five spinal geometries were analyzed: the mean spinal shape of 15 patients with left lumbar scoliosis (38 degrees Cobb angle, apex at L1-L2, the reference or "100%" geometry), and the geometry scaled to 0%, 33%, 67%, and 132% of the asymmetry of the reference shape. The muscle and intervertebral forces for maximum efforts opposing moments applied to the T12 vertebra in each of the three principal directions were calculated. The loading at each intervertebral level was expressed as the resultant force (P), the axial torque, the lateral and anteroposterior offset of P from the disc center, and the angle of P from the axial direction. With increasing scoliosis, there was a weak trend of increasing lateral offset of P, but not consistently to either the convex or concave direction. There was a much stronger trend of increasing angle between the force P and the motion segment longitudinal axis with increasing Cobb angle. Typically, this angle was 10-30 degrees for the largest scoliosis (51 degrees Cobb) and in a direction tending to increase the scoliosis. This angulation of the force results from shear loading of the disc. Axial torques tending to increase the transverse plane deformity increased with scoliosis for extension efforts. These analyses indicate that lumbar scoliosis produces asymmetric spinal loading characterized by shear forces tending to increase the scoliosis, but with little increase in the asymmetric compression of motion segments. If scoliosis progression results from asymmetric loading, it appears that the shear force component is responsible.