The vertebral artery passes just posterior to the superior articular facets of the atlas and then through the foramen magnum to the medulla oblongata of the brainstem. At the superior end of the medulla the two vertebra arteries fuse to form the basilar artery. There is some controversy on whether the vertebral arteries are sufficiently stretched by the movements in the atlanto-occipital joint to compromise flow in the arteries. We can begin to answer that question by looking at how much distance lies between the posterior margin of the superior articular facet and the basilar artery when the head is taken through a full range of motion.
The foramen for the vertebral artery lies in the posterior atlanto-occipital membrane about 3/4 of a unit lateral to the center of the atlas and slightly above its horizontal meridian, say 0.3 units superior to the Q point. That will give it the location vector –
The distance that the vertebral artery spans is the difference between these two loci.
The first location travels with the atlas and the second travels with the occiput. If we perform the calculation of the alignment in neutral position, then the distance between the two loci is 2.73 units in neutral, as one can determine by vector subtraction. If the neck is flexed 20° then the distance between the two loci is 2.26 units and if it is extended 20° then it is 3.13. The relationship is plotted in the above chart. The increase in distance between the two points is a little less that 15%. At 15° extension the change is about 11.5%.