The influence of fixed sagittal plane centers of rotation on motion segment mechanics and range of motion in the cervical spine.

The center of rotation (CoR) has become an increasingly used metric for biomechanical evaluation of spinal joints however traditional methods of determination remain prone to high degrees of uncertainty. The objective was to use a novel robotic testing protocol to investigate the effects of placement of fixed CoRs in the cervical spine. Human cadaveric C4-C5 (n=3) and C6-C7 (n=5) motion segment units (MSU) were rotated in flexion-extension to limits of 2.5 N m bending or 225 N resultant force about three points in a disc plane (A1, C1, P1) located at 25%, 50% and 75% along the length of the midline of the intervertebral disc respectively in the sagittal view, and three points (A2, C2, P2) in a sub-adjacent plane 5mm below the disc plane. Significant differences in range of rotation occurred between CoRs within the same plane but not between the same points in different planes (e.g. A1-A2). In flexion and extension axial forces at posterior points of rotation (P1, P2) were significantly different from those at anterior and central points. Shear forces were significantly different between points within the same plane except for the disc plane in extension, and between the same points in different planes in flexion and extension. The results indicate that the native cervical MSU is highly sensitive to the CoR location in terms of mechanics and range of motion, and that the CoR location likely varies between flexion and extension. The methodology developed has potential for application towards investigation of optimal CoR locations and in-vitro evaluations of the effects of implantable instrumentation.