The vertebra with a tooth-like peg, projecting upward, around which the head and the vertebra above it turn
Axis — the dens (odontoid process) is the axis of rotation.
Anatomy · Bones
Axis (C2)
Second cervical vertebra
Spine · cervical
The axis is the second cervical vertebra. It is recognised at once by the dens (odontoid process), a peg of bone that projects upward from its body and fits into the ring of the atlas. The head and atlas rotate about this peg, which is why the vertebra is called the axis.
How to spot them
- ▸A vertebra with a tooth-like peg, the dens, standing up from the front of its body
- ▸The dens articulates with the anterior arch of the atlas to form a pivot joint
- ▸A strong, often bifid spinous process, and large superior facets that meet the atlas
- ▸Its narrow bar between the upper and lower facets, the pars interarticularis, is the site of a famous fracture
What to know
Its name comes from the fact that it serves as the pivot when you turn your head from side to side
Axis — rotation at the medial atlanto-axial joint.
A fracture through the bony column between its upper and lower joint facets, typically from a high-velocity hyperextension such as judicial hanging, is often fatal
Axis — hangman's fracture of the pars interarticularis.
Fractures of its peg account for roughly 40% of the breaks of this vertebra and risk avascular necrosis because the tip loses its blood supply
Axis — odontoid (dens) fractures, about 40% of C2 fractures.
The odontoid process of this vertebra fits against the anterior arch of the topmost vertebra, held by the transverse ligament
Axis — the atlas transverse ligament holds the dens in place.
Key ideas
- The transverse ligament of the atlas holds the dens against the atlas, keeping the pivot stable while the head rotates.
Quick-fire facts
- •The dens articulates with the atlas to form the medial atlanto-axial joint, a pivot joint
- •The superior articular facets of the axis meet the inferior facets of the atlas at the two lateral atlanto-axial joints
- •Odontoid fractures represent about 40% of C2 fractures and are often unstable
- •Hangman's fracture is a fracture of the pars interarticularis of the axis
- •An old name for the axis is the epistropheus
- •The dens is developmentally the body of the atlas
- •The nuchal ligament attaches to the spinous processes of C1 to C7
- •Dislocation or subluxation of C2 on C3 can involve the spinal cord and may lead to quadriplegia or death
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Sources
- TeachMeAnatomy — The Cervical Spine
- OpenStax — Anatomy and Physiology 2e, 7.3 The Vertebral Column
- Kenhub — Anatomical structure of the axis (C2)