Axis

The axis or second cervical vertebrae is the strongest of the cervical group. It shows similarity to typical cervical vertebrae with some obvious exceptions:

• odontoid peg or dens:
  • sited anteriorly in the midline
  • results from fusion with body of atlas
  • facet on its anterosuperior surface projects superiorly onto facet of anterior arch of atlas
  • pointed at apex where apical ligament attaches
  • bevelled sides are origins of alar ligaments
  • posteriorly flat surface lying anterior to transverse ligament
• vertebral canal: oval and broader in coronal plane
• body: connects to odontoid process medially and superiorly; superior articular processes and transverse processes laterally; and pedicles posterolaterally
• pedicles: relatively short and sited at junction of lamina, transverse process and body
• superior articular facet: the key weight-transmitting structure articulating with inferior facets of atlas
  • slightly convex and face superolaterally
  • permit rotation of atlas on axis
• inferior articular facet:
  • at junction of pedicles and laminae
  • slightly concave
  • surface slopes anteriorly and inferiorly
• transverse processes:
  • relatively small
  • contain foramen transversarium
  • no anterior tubercle
  • bounded by costotransverse lamella
• laminae: broad and project posteriorly to spinous process in median plane
• spinous process:
  • bifid
  • relatively large allowing use as a palpation landmark