Anatomic and radiologic analysis of the atlantal part of the vertebral artery.

The horizontal third segment (V3h) of the vertebral artery (VA) in 7 cadavers (14 sides) was dissected and the anatomical measurements recorded. Measurements from 24 healthy individuals (48 sides) were taken for comparison using multislice CT scanning. The distance between the medial tip of the VA V3h and the line passing through the mid point of the posterior tuberculum of the atlas was marked as length A. The distance between the medial tip of the VA V3h and the point penetrating the dura mater was classified as length B. The angle between these lines was the alpha (alpha) angle. Measurements were taken when the head was in a neutral position, as well as in maximum right and left rotation, extension and flexion. In cadavers, the mean alpha angle (+/-S.D.) was 82.42+/-10.34 degrees and 83.21+/-10.81 degrees on the right and left side, respectively. On multislice CT scanning, the mean alpha angle was 81.64+/-10.15 degrees on the right and 83.77+/-10.65 degrees on the left. These angles varied with the position of the head.