HELICAL AXES OF MOTION OF THE SUB-AXIAL CERVICAL SPINE

Abstract

Information regarding the axes of motion or centers of rotation of the normal cervical spine are necessary to evaluate the similarity of the motion allowed by cervical total disc replacement designs to the natural cervical spine. However, little data has been presented previously regarding the three-dimensional axes of motion of the cervical spine for the three primary motions of flexion/extension, lateral bending and axial rotation. The objective of this study was to measure the three-dimensional axes of motion (Helical axis of Motion) in the natural sub-axial cervical spine using ex-vivo human cadaveric cervical spines.

To measure the Helical Axes of Motion (HAM) for the sub-axial cervical spine under flexion/extension, lateral bending and axial torsion moments and evaluate the effect of a physiologic axial preload on the axes locations and orientations.

This study demonstrated the feasibility of calculating the HAM in the cervical spine using an ex-vivo experimental protocol.

The HAM is a three-dimensional analogue to the two-dimensional center of rotation. The data presented here can be used to evaluate the similarity of the motion allowed by total disc replacement designs to the natural cervical spine. They can also be applied for the characterization of spinal trauma, pathology, instability or surgical devices.

The orientation and locations of the HAMs for axial torsion loading are presented in Figure 1. In flexion/extension the HAM penetrated the sagittal plane near the posterior aspect of the vertebral body and near the cranial endplate. The lateral bending results were similar to the axial torsion results. The addition of axial preload had little effect on the position and orientation of the HAM.

Sub-axial (level C2-C7) cadaveric cervical spine functional spinal units (n=7) were subjected to pure moments of 1 Nm. Specimens were tested with and without axial preloads of 200 N. Vertebral kinematics were measured using an optoelectronic motion analysis system. These data are particularly applicable to the evaluation and design of “motion-retaining” devices such as total disc replacements, facet joint replacement systems or flexible stabilization systems.

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