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Axis (C2)

Axis

The second cervical vertebra, the axis provides the pivot around which the head rotates. Its defining feature is the dens, or odontoid process, a tooth-like projection that extends superiorly from the body to articulate with the anterior arch of the atlas above it. This arrangement accounts for roughly half of the rotation available in the cervical spine. Odontoid fractures are common cervical injuries, particularly in elderly patients, and a hangman's fracture involves bilateral fractures of the pars interarticularis of C2. Atlantoaxial instability, in which the joint between the atlas and axis becomes abnormally mobile, is associated with rheumatoid arthritis and Down syndrome.

L'axis se caractérise par la dent (processus odontoïde), une projection en forme de dent qui s'étend vers le haut depuis le corps pour s'articuler avec l'arc antérieur de l'atlas. Cette disposition unique permet environ 50% de la rotation cervicale.

On this page
Type Bone
Système corporel Squelettique
Région corporelle Colonne vertébrale
Nom latin Axis
FMA ID 12520

Fonction

La deuxième vertèbre cervicale, qui fournit un point de pivot pour la rotation de la tête. Le processus odontoïde (dent de l'axis) se projette vers le haut et agit comme l'axe autour duquel tourne l'atlas.

Signification clinique

Les fractures du processus odontoïde sont des lésions fréquentes de la colonne cervicale, en particulier chez les patients âgés. La fracture du pendu implique des fractures bilatérales de la pars interarticularis de C2. L'instabilité atlanto-axiale se produit dans la polyarthrite rhumatoïde et le syndrome de Down.

Frequently Asked Questions

What is special about the axis vertebra?

The axis (C2) is the second cervical vertebra and is unique for its prominent superior projection called the dens (odontoid process). The dens projects upward through the ring of the atlas (C1) and acts as a pivot around which the atlas and skull can rotate. This arrangement allows approximately 50% of total cervical rotation, enabling the head to turn left and right.

How does the axis connect to the atlas?

The axis articulates with the atlas (C1) primarily through the atlantoaxial joint complex. The dens of the axis fits into the anterior arch of the atlas, held in place by the transverse ligament of the atlas. This forms a pivot joint. Additional articulations occur at two lateral facet joints between C1 and C2. The transverse ligament is critical; its rupture can cause life-threatening cord compression.

What injuries affect the axis vertebra?

Axis fractures include odontoid (dens) fractures, which are classified into three types by the Anderson-D'Alonzo system. Type II fractures at the base of the dens are most common and often require surgical stabilization. Traumatic spondylolisthesis of the axis (Hangman's fracture) involves bilateral pedicle fractures of C2, typically from hyperextension injuries such as motor vehicle accidents.

References

  1. Foundational Model of Anatomy — FMA:12520 Reference anatomy ontology (University of Washington)

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Educational Disclaimer

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Data sources: Terminologia Anatomica, Foundational Model of Anatomy, Wikidata.