Coupled motions in human and porcine thoracic and lumbar spines.
Coupled motions, i.e., motions along axes other than the loaded axis, have been reported to occur in the human spine, and are likely to be influenced by inclined local axes due to the sagittal plane spine curvature. Furthermore, the role of facet joints in such motions is as yet unclear. Therefore, this study aimed at assessing coupled motions in multiple spine sections in vitro, before and after removal of posterior elements. Six elderly human and 6 young porcine spines were sectioned in four segments (high thoracic, mid thoracic, low thoracic and lumbar), each consisting of four vertebrae and three intervertebral discs. Segments were loaded along each of the three axes, and three-dimensional rotations of the middle segment were quantified. Subsequently, posterior elements were removed and the protocol was repeated. To avoid mixed loading between Axial Rotation (AR) and Lateral Bending (LB), in contrast to other studies, local axes at the vertebrae were defined as aligned with the loading device prior to each load application. Expressed as a percentage of motion in the loaded direction, coupled motions were on average larger in human (22.7%, SD = 2.2%) than in porcine (11.9%, SD = 1.2%) spines (p < .001). Largest coupled motions were obtained in AR loading of the lumbar spine segments, with mean magnitudes averaged over coupling axes for human L2-L3 joints of 48.9% (SD = 13.2%), including somewhat more LB (56.4%, SD = 18.6) than FE (41.4%, SD = 14.1%) coupling. For porcine L3-L4 joints average coupling in AR loading was 29.3% (SD = 8.2%). In human segments removal of posterior elements only had substantial effects in the lumbar spine segments, where posterior element removal decreased coupled motion during AR loading, averaged over LB and FE coupling, from 48.9% (SD = 13.2%) to 27.7% (SD = 6.1%), mainly through increased motion in the loaded direction. The present results indicate that coupled motions were largest in the lumbar spine. In human spines, posterior elements only contributed to coupled motions in lumbar axial rotation loading.