Atlas is the first cervical vertebra. It articulates with the occipital bone above and the axis vertebra below. It plays an important role in movement of the skull and the neck. A rare variation of the atlas vertebra is found where the posterior arch of the atlas had one accessory foramen just behind each lateral mass. In the atlas vertebra, this retroarticular canal or the lateral bridge are examples of bony outgrowths which may cause external pressure on the vertebral artery as it passes from the foramen transversarium of the vertebra to the foramen magnum of the skull. The knowledge of this variation may be of importance to orthopedic surgeons, neurosurgeons, radiologists and anthropologists. Of the 80 atlas vertebrae observed 17.5 % showed presence of retro transverse groove or canal.
Arcuate foramen, Atlas vertebra, Ponticulus posticus, Vertebral artery