By virtue of the relative slenderness of the neck and the weight of the head, the cervical spine can be subjected to considerable loads. Knowledge about the basic biomechanics. and load-sharing of the cervical spine may help to elucidate injury mechanisms or even suggest mechanisms leading to cervical joint degeneration. The aim of this study was to characterise the load transmission paths through cervical functional spinal units (FSUs). Compression and flexion, extension, lateral bending and torsional bending moments and anterior, posterior and lateral shear forces were applied. Under compression loading, the influence of superimposed flexion and extension postures was established. Under all other-loading modes the effect of a constant axial preload was evaluated. The preload represented an active neck musculature and the weight of the head. An optoelectronic motion analysis system was used to measure specimen kinematics. Load- sharing mechanisms were studied using a miniature pressure sensor to measure intervertebral disc pressure and tri-axial strain gauges mounted beneath each of the facet joints and on the anterior surface of the vertebral body.

Our results suggest that flexion and compression-flexion loading modes result in compressive force in the anterior column and tensile (or small compressive) forces in the posterior column. Extension moments resulted in compressive force at the posterior column and tension in the anterior column. Extension-compression caused compressive forces in both the anterior and posterior columns. Lateral bending moments were associated with higher loads in the facet toward which the segment rotated. Torsion moments caused equal loading at each of the facets. Under anterior shear the vertebra pivoted about the compressed annulus anteriorly and ramped up the facet joints posteriorly resulting in a net distraction of the intervertebral disc and decrease in disc pressure. Posterior shear resulted in a pure posterior translation tension at the anterior annulus and distraction of the facet joints. Lateral shear caused an impingement of the facet joint on the underlying lamina. These results identify which specific anatomic structures may be highly loaded under each of the loads applied. Clinically, they can be used to identify parts of the anatomy that should be examined for injury.