Absence of foramen transversarium in the human atlas vertebra: a case report.

Anatomical texts allude to many variations in the foramen transversarium of the seventh cervical vertebra, including its absence, but absent foramen transversarium of the atlas vertebra has not been mentioned. This is the case report of an atlas vertebra, the transverse process of which did not exhibit this foramen on the left side. However, the groove for the vertebral artery was present bilaterally on the posterior arch. The possible factors responsible for this interesting variant are discussed.