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

BACKGROUND CONTEXT Axial rotation of the cervical spine with the cranium held level occurs during certain daily activities such as looking over one's shoulder when walking or driving. These activities require rotation from the suboccipital and subaxial cervical spine. The literature on cervical spine axial rotation and coupled rotations has been obtained from experiments where the segmental axis of rotation is aligned with the global axis of rotation. In looking over one's shoulder, the segmental rotation axes may not be aligned with the global and primarily vertical rotation axis due to curvature of the spine. Furthermore, fusion may limit the amount of rotation that can be obtained from the subaxial spine and reduce the facility with which the daily activities are performed. PURPOSE (1) To define the rotation of the intact subaxial spine during horizontal axial rotation of the cranium, and (2) To measure the effect of C5-C7 fusion on the rotation of the subaxial spine during horizontal axial rotation of the cranium. STUDY DESIGN/SETTING Biomechanical experiment. PATIENT SAMPLE Five C0-T1 cadaveric spine specimens (38-54 years old, 3 female and 2 male). OUTCOME MEASURES Rotation of the subaxial cervical spine in response to applied axial rotation moment. METHODS A left and right axial rotation moment of 1Nm was applied to specimens mounted in an apparatus that held the cranium in an orientation consistent with horizontal gaze (horizontal Frankfort line). The axial rotation axis passed through the midpoint between the two external auditory meatuses. Rotations of the individual vertebrae before and after C5-C7 fusion using lateral mass screws and posterior rods were measured using an optoelectronic motion measurement system. RESULTS C5-C7 fusion reduced the C3-T1 axial rotation range of motion from a mean of 31±5° to 22±5°. In the nonfused levels, the axial rotation range of motion changed less than half a degree. Holding the cranium level (horizontal gaze) changes the segmental and overall coupled rotation behavior of the cervical spine during axial rotation. In 3 specimens, left axial rotation induced both right lateral bending in the lower segments and left lateral bending in upper segments, and vice versa with right axial rotation. In the three specimens in which both left and right lateral bending was induced by axial rotation, there was a 0.1° to 3.7° reduction in the coupled C3-C4 lateral bending range of motion. In the other two specimen, C3-C4 lateral bending range of motion increased by 0.8° to 1.8°. C5-C7 fusion changed the amount but not the direction of coupled lateral bending of unfused segments. CONCLUSIONS Without the horizontal gaze constraint, right axial rotation has been shown to induce right lateral bending in the cervical spine. There are various activities of daily living, such as looking over one's shoulder, that involve holding the gaze horizontal while axially rotating the cervical spine. In this biomechanical experiment, a constrained horizontal axial rotation resulted in specimen-specific coupled rotation of the subaxial cervical spine. In three of the specimen, axial rotation induced left and right segmental lateral bending regardless of the axial rotation direction.This specimen-specific difference in the coupled lateral bending and in the ability to perform constrained axial rotation after C5-C7 fusion leads us to wonder whether there may also be a patient-specific difference. This difference would have clinical implications for patients who are candidates for C5-C7 fusion. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.