Effects of posture and structure on three-dimensional coupled rotations in the lumbar spine - A biomechanical analysis

Study Design. A biomechanical lumbar spine model was constructed to simulate three-dimensional spinal kinematics under the application of pure moments. Parametric analysis of the model allowed for the estimation of how much of the coupled motions could be predicted by the lumbar lordosis and the intrinsic mechanical properties of the spine. Objectives. To evaluate the relative effects of lordosis and intrinsic mechanical spine properties on the magnitude and direction of coupled rotations. Summary of Background Data. Clinical evidence Suggests that abnormal coupled motion in the lumbar spine may be an indicator of low back disorders. Methods. The biomechanical lumbar spine model consisted of five vertebrae separated by intervertebral joints that provided three rotational degrees of freedom. In vitro experimental data, obtained from nine fresh-frozen (L1-S1) cadaveric specimens, were used to established the mechanical properties of the intervertebral joints. Two different submodels were considered in simulating the three-dimensional intervertebral rotations in response to the applied moments. In the first, it was assumed that the coupled motions were generated solely as a result of the vertebral orientation caused by lordosis. In the second, additional intrinsic motion coupling was assumed. Results. Intervertebral coupling was partially predicted by lumbar lordosis; however, the inclusion of intrinsic mechanical coupling dramatically improved the simulation of the intervertebral rotations (root mean square error <1 degrees). Comparison of the results from the two models demonstrated that the lumbar lordosis and intrinsic mechanical properties of the spine had about an equal effect in predicting the coupling between axial rotation and lateral bending. In contrast, coupled flexion, associated with lateral bending, was almost fully accounted for by the presence of lumbar lordosis. Conclusions. The lumbar lordosis and intrinsic mechanical properties of the spine were equally important in predicting the magnitude and direction of the coupled rotations.