A Novel Methodology for Improving Understanding of the Multiplanar Kinematics of the Human Cervical Spine

Abstract The biomechanics of the spine are naturally multiplanar, but their experimental characterization remains primarily conducted in pure moment bending in anatomical planes: Flexion-Extension (FE) in the sagittal plane, Lateral Bending (LB) in the coronal plane, and Axial Rotation (AR) in the transverse plane. This leaves the biomechanical behavior between anatomical planes under-characterized. Computational tools for evaluating spinal implants and surgical treatments, like finite element models, are validated by comparison to experimental spinal loading. Thus, they are only able to represent spine behavior that is characterized through testing. A novel testing protocol was developed using a six-axis industrial robot to apply multiplanar experimental loading trajectories to characterize the spine's multiplanar behavior. One postmortem cervical spinal specimen was loaded in combined FE and LB bending about the craniocaudal axis, capturing its multidimensional stiffness behavior at several hundred unique joint kinematic “poses” throughout the spine's physiologic range of motion. The multiplanar trajectories are designed to enable parameterization of spinal stiffness behavior at each pose to the joint kinematic pathway taken to achieve the pose. Visualizing the multiplanar behavior of the spine also reveals spinal movement patterns that are not visible in planar bending alone. This method has elucidated that spinal stiffness under multiplanar loading cannot be inferred exclusively from behavior in planar loading, and that directionality of spinal loading has an impact on stiffness behavior. This information can be incorporated into finite element models and other tools for more robust predictions for spinal health.