Composite motion in cervical disc segments.

This study of composite motion in cervical motion segments used biplanar radiography and the principles of rigid body mechanics to determine the relationship of the axes of motion to the planes of the discs and zygapophyseal joints. Each cervical disc has an uniinate process posterolaterally on each side and the zygapophyseal joints are orientated at approximately 45° to the long axis of the spine. When considering the full range of composite motion from left to right the axis of motion lies in the mid-sagittal plane, but when unilateral movements to the left or right are studied the axes are eccentrically situated. When viewed in the lateral projection the line of the axis passes obliquely upwards and backwards through the front of the disc, then through the posterior part of the moving vertebral body, to cross the plane of the zygapophyseal joints approximately at right-angles. The orientation varies depending on whether the moment is applied in axial rotation or lateral flexion. The range of variation of the axis is small in the upper two disc segments where the superior articular facets have a slightly medial inclination, and larger in the lower cervical motion segments where the facets are inclined slightly laterally. The position and inclination of the axes of composite motion suggest that the anterior part of each cervical disc acts like a pivot, while the posterior part of the disc with the uncinate process on each side acts like a socket within which the upper vertebral body rolls.