Gross morphology of the bridges over the vertebral artery groove on the atlas.

The bony bridges of the atlas over the "groove of the vertebral artery" are commonly seen in plain radiographs of the cervical spine, and it is a subject of controversy whether they cause compression of the underneath lying vertebral artery. To clarify this we examined a total of 176 dried and complete atlas vertebrae and found the presence of a "canal for the vertebral artery" (CVA) in 10.23% and an incomplete "canal for the vertebral artery" in 24.43%. The CVA and incomplete CVA is more common in males (11.11% and 24.9%) than in females (9.3% and 24.42%). We found a higher incidence of CVA in laborers (37.5%) than in nonlaborers (4.16%). The incomplete CVA appeared to be more characteristic in the age group of 5-44 years. In the age group of 45-90 years the CVA was characteristic, which probably means that an incomplete CVA is the precursor of a CVA. The superoinferior diameter of the CVA canal ranged from 5.1 to 6.1 mm at the right side and from 4.6 to 5.8 mm at the left side, while the anteroposterior diameter was 5.6-6.9 mm at the right side and 6.1-7.2 mm at the left side. We also found a high incidence of coexistence of CVA and the "retrotransverse foramen" (72.22%) which means that because of possible compression of the vertebral veins the blood flow is directed into the small vein of the retrotransverse foramen. Finally, in 93.5% of unilateral CVA a deeply excavated contralateral "groove of the vertebral artery" was found.