EFFECTS OF POSTURE AND STRUCTURE ON 3-DIMENSIONAL COUPLED ROTATIONS IN THE LUMBAR SPINE - A BIOMECHANICAL ANALYSIS

Study Design. A biomechanical lumbar spine model was constructed to si mulate three-dimensional spinal kinematics under the application of pu re moments. Parametric analysis of the model allowed for the estimatio n of how much of the coupled motions could be predicted by the lumbar lordosis and the intrinsic mechanical properties of the spine. Objecti ves. To evaluate the relative effects of lordosis and intrinsic mechan ical spine properties on the magnitude and direction of coupled rotati ons. Summary of Background Data. Clinical evidence Suggests that abnor mal 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 f rom nine fresh-frozen (L1-S1) cadaveric specimens, were used to establ ished the mechanical properties of the intervertebral joints. Two diff erent submodels were considered in simulating the three-dimensional in tervertebral rotations in response to the applied moments. In the firs t, 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. Interverte bral 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 t hat the lumbar lordosis and intrinsic mechanical properties of the spi ne 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 mec hanical properties of the spine were equally important in predicting t he magnitude and direction of the coupled rotations.