Overview
The atlas is the most atypical vertebra: it has no body and no spinous process, and consists of two arches (dorsal and ventral) joined by two lateral masses that extend into the wings of the atlas (alae atlantis), wide and palpable just caudal to the skull.
Key features
- Cranial articular foveae: concave surfaces that receive the occipital condyles. The atlanto-occipital joint allows flexion and extension ("yes").
- Caudal articular foveae: articulate with the axis (C2). The atlantoaxial joint allows rotation ("no") around the dens of the axis.
- Fovea dentis: on the dorsal surface of the ventral arch, where the dens of the axis rests, held by the transverse ligament.
- Lateral vertebral foramen and alar notch: passage of the first cervical nerve root and the vertebral artery.
- Transverse foramen: at the base of the wing.
Clinical relevance
Atlantoaxial instability (from hypoplasia or aplasia of the dens, or rupture of the transverse ligament) causes cervical spinal cord compression in young toy breeds. The wings of the atlas are the landmark for cerebrospinal fluid collection from the cerebellomedullary cistern (between the occiput and the atlas). Atlas fractures are rare and usually traumatic.