Axis (C2)

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, which is embryologically the body of the atlas (C1)^1,2. It plays an important role in rotation of the head with the majority of movement occurring around the dens and at the atlanto-axial joint. There are five primary and two secondary ossification centres which are discussed below in more detail ^{2, 3, 4}.

Gross anatomy

The axis is formed by a body with the attached dens, two lateral masses, a posterior neural arch (formed by the pedicle and a thick lamina), and a large spinous process, which is commonly bifid.

Anterior components of the axis are composed of: 

• dens - conical in shape
• body
• lateral mass
• transverse process with foramina transversarium
• superior articular facets
• inferior articular facets

Posterior elements of the axis are composed of: 

• pedicle
• lamina
• spinous process
  • has several muscle attachments
    • semispinalis cervicis
    • rectus capitis posterior major
    • inferior oblique

Articulations

• superior articular facet with inferior articular face of C2
• dens articulates with the posterior aspect of the anterior arch of C1
• inferior articular facet with superior articular facet of C3
• uncovertebral articulation with C3

Relations

Anteriorly

• anterior atlanto-axial membrane
• anterior longitudinal ligament

Posteriorly

• posterior atlanto-axial membrane
• ligamentum nuchae
• posterior to dens
  • important ligamentous structures are attached to the dens and play an important role in C1/C2 stability
    • cruciate ligament
      • Transverse ligament provides the most stability retaining the dens in contact with the atlas (C1)
    • apical ligament
      • embryological remnant of the notochord 
    • alar ligament
    • accessory ligament

Laterally

• vertebral arteries and veins
  • traverse foramina transversarium moving laterally

Supero-posteriorly

• sub-occipital triangle

Centrally

• spinal cord traverses vertebral foramen
• basivertebral veins

Variant anatomy            

• hypoplasia
• aplasia (rare)
• bifid spinous process
• os odontoideum
• persistent ossiculum terminale ⁴

See also: vertebral anomalies. 

Radiographic features

Plain film

• the odontoid process and atlanto-axial joint are best appreciated in an AP open mouth view 
• soft tissue contours are visible on lateral views

Development

There are five primary and two secondary ossifications centres in the axis (C2).

Primary ossification centres:

• vertebral arch
  • two primary centres
  • appear at the 7^th or 8^th week of life
• ​vertebralvertebral centrum/body
  • one primary centre
  • appears at the 4^th or 5^th month of life
• dens (odontoid process)
  • bilateral centres
  • appears at the 6^th month of life
  • fuse before birth forming conical mass

Secondary ossification centres:

• apex of dens
  • from cuneiform cartilage
  • variable appearance and fusion
    • most commonly appear from 5 to 8 years of age
    • fuse at around the 12^th year
• dens separated from the vertebral body by a cartilaginous disc
  • circumference ossifies ^3, 5

Related pathology 

• fractures
  • hangman fracture
  • odontoid fracture
  • lateral mass fracture
  • flexion teardrop fracture
  • extension teardrop fracture
• atlanto-axial subluxation
• rheumatoid arthritis
• osteoarthritis
• erosion of the odontoid process