Motion characteristic of the normal lumbar spine in young adults: instantaneous axis of rotation and vertebral center motion analyses.

To determine the motion characteristic of the normal lumbar spine, 61 of 612 optimal radiographs of healthy young Japanese adults in the neutral erect, active maximum flexion and maximum extension positions were carefully selected. The coordinates of the instantaneous axis of rotation (IAR) and vertebral center from L1 to L5 vertebra were analyzed. The directional shifts in IAR in the x-axis and the y-axis closely reflected translational and rotational movements of the vertebra. The IAR showed qualitative values with very large, 95%, probability ellipses in the normal lumbar spine. Analysis of coordinates of the vertebral center provided quantitative data of the extension-to-flexion motion of each vertebra. In extension-to-flexion motion, the L4 vertebra showed a translation-predominant motion characteristic. The L5 vertebra had a rotation-predominant motion characteristic. The patterns of motion of the L1-L4 vertebrae were correlated to the intervertebral disc height and the inclination angle of the facet joint. The L5 vertebra alone showed some specific patterns suggestive of the involvement of a separate ligamentous stabilizer, the iliolumbar ligaments, which reduces translation.