The role of lumbar spinal elements in flexion.

The forces induced in the disc and ligaments of a lumbar motion segment in resisting a quasi-static external load, using a semi-experimental approach, are presented. The lines of action of ligaments (direction cosines) and disc center for the initial position of the specimen were determined using a morphometer. The changes in these lines of action for a known external load were computed by using the three-dimensional load-deformation characteristics of an intact motion segment. The load-deformation behavior were obtained by applying a known load to the motion segment's superior vertebra and recording the motion produced. A seven dial gauge motion measuring system was used for this purpose. The six equations of equilibrium yielded a statically indeterminate model. A linear optimization technique in conjunction with a cost function enabled the computation of forces in the ligaments as well as forces and moments in the disc. This approach made it possible to determine the component forces without a priori knowledge of the structural properties of ligaments. Typically for an external flexion moment of 6.9 Nm the supraspinous ligament experienced the most force (60 N), followed by capsular ligaments (25 N), and transverse ligaments (15 N). A compressive force of 100 N within the disc was predicted. The load-deformation curve, obtained from this study, for the supraspinous ligament was nonlinear and is in agreement with published experimental results.