The axis is the second cervical vertebra commonly called c2 it is an atypical cervical vertebra with unique features and important relations that make it easily recognisable its most prominent feature is the odontoid process also know as the dens or peg which is embryologically the body of the atlas c1 1 2 it plays an important role in rotation of the head.

Os odontoideum plural ossa odontoidea is an anatomic variant of the odontoid process of c2 and needs to be differentiated from persistent ossiculum terminale and from a type 2 odontoid fracture it can be associated with atlantoaxial instability although it was originally thought to be a congenital lesion due to a failure of the center of ossification of the dens to.