Three-dimensional translational movements of the upper cervical spine.

In 10 fresh cadaveric specimens of occiput (C0) to C3, six pure moments (maximum 1.5 Nm) were applied and the resulting physiological motions were studied. We determined the quantitative three-dimensional relative translations of two specific midsagittal points on C0 and C1: the anterior (point A) and posterior (point B) margins of the foramen magnum of C0, and the posteroinferior margin of anterior arch (point C) and the anteroinferior margin of posterior arch (point D) of C1. In flexion/extension, translational movements of these four points were mainly in the sagittal plane. In axial torque and lateral bending moment, both points on C0 shifted laterally, to the same side of axial torque and to the opposite side of lateral bending. The pattern of the translation of C1 in left axial torque was similar to that in the right lateral bending moment, although the magnitude was larger in the former. In these situations, point C moved to the left, and point D moved to the right and anterior. The vertical translation had two phases: superior translational phase, followed by inferior translational phase. We presented the quantitative translations but, because of large standard deviations between the specimens, it was difficult to define translational thresholds for instability.