Arcuate foramen of atlas: incidence, phylogenetic and clinical significance.

Arcuate foramen is less known trait of the human atlas vertebra formed by a delicate bony spiculum, which arches backward from the posterior end of the superior articular process. Examination of 1044 human atlas vertebra revealed that the trait was present in 13.8% of the samples. The mean length of the arcuate foramen form was 7.16 mm on the left side and 9.99 mm on the right side in bilateral positive samples while it was 8.14 mm and 9.26 mm respectively in unilateral positive samples. The mean vertical height of this foramen was 6.57 mm on the left side and 6.52 mm on the right side in bilateral positive samples while it was 4.91 mm and 5.38 mm respectively in unilateral positive samples. The sides did not show any statistical significant differences. The importance of the arcuate foramen lies in the external pressure it may cause on the vertebral artery as it passes from the foramen transversarium of the first cervical vertebra to the foramen magnum of the skull.