Normal coupling behavior between axial rotation and lateral bending in the lumbar spine - biomed 2009.

Lumbar spine kinematics, a three-dimensional phenomenon, involves motion about a primary axis coupled with motions about secondary axes induced by posture and intervertebral joint structure. Due to the role of coupling in normal spinal mechanics, abnormal coupling magnitude and quality may indicate clinical instability. Although lumbar coupling was previously investigated, a definitive coupling relationship between axial rotation and lateral bending was not established. This study characterized normal coupling in younger specimens (33.8+/-11.1 yr) between axial rotation and lateral bending. Ten specimens (T12-S1) were loaded to 6 Nm in pure lateral bending and pure axial rotation. Stereophotogrammetry recorded three-dimensional kinematics and motion analysis computed level-by-level angulations. Motions were defined as primary lateral bending and coupled axial rotation during applied lateral bending moments, and primary axial rotation and coupled lateral bending during applied axial rotation moments. Primary lateral bending increased from 3.0 degrees at T12-L1 to 4.9 degrees at L4-L5, and reduced to 3.4 degrees at L5-S1. Coupled axial rotations decreased from 1.5 degrees at T12-L1 to 0.2 degrees at L5-S1, rotating contralateral to primary lateral bending for all segments, except L5-S1. Primary axial rotation increased in caudal segments, with a maximum of 1.4 degrees at L5-S1. Minimal coupled lateral bending occurred at cranial segments, with mean ipsilateral bending of 2.3 degrees at L4-S1. Level-dependent variations in coupled motion quality and quantity exist between axial rotation and lateral bending in normal lumbar spines. Cranial lumbar segments dominated coupled axial rotation while caudal segments demonstrated primarily coupled lateral bending. Variation from normal coupling patterns may indicate lumbar spine instability.