Investigation of coupled bending of the lumbar spine during dynamic axial rotation of the body

Little is known about the coupled motions of the spine during functional dynamic motion of the body. This study investigated the in vivo characteristic motion patterns of the human lumbar spine during a dynamic axial rotation of the body. Specifically, the contribution of each motion segment to the lumbar axial rotation and the coupled bending of the vertebrae during the dynamic axial rotation of the body were analyzed. Eight asymptomatic subjects (M/F, 7/1; age, 40-60 years) were recruited. The lumbar segment of each subject was MRI scanned for construction of 3D models of the vertebrae from L2 to S1. The lumbar spine was then imaged using a dual fluoroscopic system while the subject performed a dynamic axial rotation from maximal left to maximal right in a standing position. The 3D vertebral models and the fluoroscopic images were used to reproduce the in vivo vertebral motion. In this study, we analyzed the primary left-right axial rotation, the coupled left-right bending of each vertebral segment from L2 to S1 levels. The primary axial rotations of all segments (L2-S1) followed the direction of the body axial rotation. Contributions of each to the overall segment axial rotation were 6.7A degrees A A +/- A 3.0A degrees (27.9 %) for the L2-L3, 4.4A degrees A A +/- A 1.2A degrees (18.5 %) for the L3-L4, 6.4A degrees A A +/- A 2.2A degrees (26.7 %) for the L4-L5, and 6.4A degrees A A +/- A 2.6A degrees (27.0 %) for the L5-S1 vertebral motion segments. The upper segments of L2-L3 and L3-L4 demonstrated a coupled contralateral bending towards the opposite direction of the axial rotation, while the lower segments of L4-L5 and L5-S1 demonstrated a coupled ipsilateral bending motion towards the same direction of the axial rotation. Strong correlation between the primary axial rotation and the coupled bending was found at each vertebral level. We did not observe patterns of coupled flexion/extension rotation with the primary axial rotation. This study demonstrated that a dynamic lumbar axial rotation coupling with lateral bendings is segment-dependent and can create a coordinated dynamic coupling to maintain the global dynamic balance of the body. The results could improve our understanding of the normal physiologic lumbar axial rotation and to establish guidelines for diagnosing pathological lumbar motion.