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) . 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 .

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, projects up from the body, smooth posterior surface
• body
• lateral mass bears the weight of skull and transfers through to C3 vertebral body 
• transverse process with foramina transversarium, L-shaped, directed up and out to allow lateral bend in vertebral artery 
• superior articular facets, slopes down from body like shoulders, extends over pedicles and lateral masses 
• inferior articular facets, face anteroinferiorly like typical cervical vertebra 

Posterior elements of the axis are composed of:

• pedicle
• lamina, thick and rounded 
• spinous process, large and bifid 

Articulations

• atlantoaxial joint 
  • median:
    • synovial with joint capsule and bursa between posterior dens and cruciform ligament 
    • articulation: anterior surface of dens with the posterior surface of anterior arch of C1, surfaces covered in hyaline cartilage 
    • movement: head rotation, flexion/extension
  • lateral
    • synovial with joint capsule
    • articulation: inferior articular facet of C1 with superior articular facet of C2, surfaces covered in hyaline cartilage  
    • movement: head rotation, flexion/extension
• C2/C3 uncovertebral joint 
• inferior articular facets of C2 with superior articular facets of C3 (facet joints)
• intervertebral joint with C3 via the C2/C3 intervertebral disc

Attachments:

• musculotendinous
  • attached to the anterior surface of the vertebral body 
    • longus colli
  • attached to transverse processes 
    • levator scapulae
    • scalenus medius
    • splenius cervicis
  • attached to spinous processes 
    • semispinalis cervicis
    • rectus capitis posterior major
    • obliquus capitis inferior 
  • attached to posterior surface of lamina
    • multifidus and longissimus
• tectorial membrane 
  • extends upward in continuity with posterior longitudinal ligament
  • attached to back of body of C2 and anterior margin of foramen magnum 
• ligaments 
  • apical ligament, embryological remnant of the notochord
    • apex of dens to anterior margin of foramen magnum
  • paired alar ligaments 
    • sloping upper margin of dens to margin of foramen magnum
  • cruciform ligament with synovial bursa and fibrous capsule
    • longitudinal part - attached from the body of the axis to the basiocciput
    • transverse part - laterally attached to the medial aspect of the lateral masses of C1, provides the most stability retaining the dens in contact with the atlas (C1)​
    • found between the tectorial membrane and apical ligament 
  • accessory atlantoaxial ligament: from posterior body of C2 to lateral mass of C1 
  • anterior atlantoaxial membrane: from the body of the C2 to the anterior arch of C1
  • posterior atlantoaxial membrane (upper part of the ligamentun flavum between C1 and C2) 
  • ligamentum nuchae
  • anterior longitudinal ligament: passes anterior to the anterior atlantoaxial membrane before attaching to the anterior tubercle of C1

Relations

Anteriorly

• anterior atlantoaxial membrane
• anterior longitudinal ligament

Posteriorly

• posterior atlantoaxial membrane 
• ligamentum nuchae
• posterior to dens
  • important ligamentous structures are attached to the dens and play an important role in C1/C2 stability
    • cruciform ligament
    • apical ligament: embryological remnant of the notochord
    • alar ligament
    • accessory ligament
    • tectorial membrane

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 (common)
• os odontoideum
• bicornuate odontoid
• basilar invagination - upward displacement of the tip of odontoid process above foramen magnum
• persistent ossiculum terminale 

See also: vertebral anomalies.

Radiographic features

Plain radiograph

• 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 centers in the axis (C2).

Primary ossification centers:

• vertebral arch
  • two primary centers
  • appear at the 7 or 8 week of life
• vertebral centrum/body
  • one primary center
  • appears at the 4 or 5 month of life
• dens (odontoid process)
  • bilateral centers
  • appears at the 6 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 12 year
• dens separated from the vertebral body by a cartilaginous disc
  • circumference ossifies

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

Siehe auch:

• Dens axis
• Densfraktur
• kongenitaler Blockwirbel HWK 2-3
• Atlas
• hangman fracture
• Knochenkerne Dens axis
• Fraktur Axis
• atlanto-axial separation

und weiter:

• cervical spine: open mouth annotated view