Coupled rotation patterns in cervical spine axial rotation can change when the head is kept level

The biomechanical literature describes axial rotation occurring coupled with lateral bending and flexion in the cervical spine. Since the head is kept level during some activities of daily living, we set out to investigate the changes in total and segmental motion that occur when a level gaze constraint is applied to cadaveric cervical spine specimens during axial rotation. 1.5Nm of left and right axial rotation moment was applied to sixteen C2-T1 cadaveric specimens with C2 unconstrained and C2 constrained to simulate level gaze. Overall and segmental motions were determined using optoelectronic motion measurement and specimen-specific kinematic modeling. Without a kinematic constraint on C2, motions were as described in the literature; namely, flexion and lateral bending to the same side as axial rotation. Keeping C2 level reduced that total axial rotation range of motion of the specimens. Changes were also produced in segmental coupled rotation in all specimens. The observed changes included completely opposite coupled motion than in the uncoupled specimens, and traditional coupled behavior at one load extreme and the opposite at the other extreme. Constraining C2 during axial rotation to simulate level gaze can produce coupled motion that differs from the classically described flexion and lateral bending to the same side as axial rotation. Statement of Clinical Significance: Activities of daily living that require the head to be kept level during axial rotation of the cervical spine may produce segmental motions that are quite different from the classically described motions with implications for biomechanical experiments and implant designers.