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 (or dens), 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 centers which are discussed below in more detail 2-4.

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
  • superior articular facet with inferior articular facet of C1
  • 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
  • intervertebral joint with C3 via the C2/3 intervertebral disc
  • 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
  • vertebral arteries and veins
    • traverse foramina transversarium moving laterally
  • spinal cord traverses vertebral foramen
  • basivertebral veins

See also: vertebral anomalies

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

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

Primary ossification centers:

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

Secondary ossification centers:

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

rID: 29878
Section: Anatomy
Synonyms or Alternate Spellings:
  • C2
  • Axis
  • Second cervical vertebrae
  • Second cervical vertebra

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Cases and figures

  • Figure 1: superior view
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  • Figure 2: anterior view
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  • Figure 3: dens articulating with C1
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  • Figure 4: alar and cruciform ligament anatomy
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  • Figure 5: median atlanto-occipital and atlantoaxial joints (Gray's illustration)
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