The internal vertebral venous plexus prevents compression of the dural sacduring atlanto-axial rotation

Deformation of the extradural space and the possibility of impression upon the dural sac during atlanto-axial rotation are investigated. Atlanto-axial rotation leads to a reduction in the cross-sectional area of the bony spin al canal of approximately 40%. Atlanto-axial rotation was recorded by endoc analar views from a video camera fixed inside the skull of six unembalmed c adavers. Axial thin-section T1-weighted MRI slice sets were acquired from t hree volunteers (midposition and maximal left and right rotation of the hea d and cervical spine). The axial cross-sectional areas of the bony spinal c anal, dural sac and spinal cord were measured. In two other persons post-ga dolinium contrast-enhanced T1-weighted MR1 volume scans with fat-suppressio n prepulse were acquired (midposition and rotation) to determine venous con tents of the extradural space. The 50:50 ratio between left and right extra dural halves in midposition changed to an ipsilateral:contralateral ratio o f 20:80 in maximum rotation at the level just above the lateral C1-C2 joint s. Directly below these joints the opposite occurred. The post-contrast stu dies showed an enhancing internal vertebral venous plexus (IVVP), which alm ost completely occupied the extradural space at the atlantoaxial level. Thi s could not be shown in the cadaver experiments, because of absence of bloo d and cerebrospinal fluid (CSF) pressure. During atlanto-axial rotation blo od displacement in the IVVP allows major deformations of the extradural spa ce. This prevents dural sac impression.