Axis (C2)

At a glance

English

Axis (C2)

Latin (TA2)

Axis (C2)

System

Bone

Region

Neck

Side

Single, on the midline

Series

Cervical vertebrae

Muscles on it

Levator scapulae

About the axis (c2)

The axis is the second cervical vertebra, directly below the atlas. Its name means axle, because it provides the pivot around which the atlas and head turn. It is easy to tell apart from every other vertebra by the dens, known too as the odontoid process or peg, a strong tooth-like column that rises straight up from its body into the ring of the atlas.

The dens is slightly waisted where it joins the body. Its front carries an oval facet that meets a matching facet behind the front arch of C1, and its back is grooved by the transverse ligament that holds it in place. The pointed tip anchors the thin apical ligament, which is a remnant of the notochord, and on each side below the tip a rough mark receives an alar ligament; these tie the dens to the margin of the foramen magnum and check excessive turning of the head. Developmentally the dens represents the body of the atlas that has become fused to the axis.

The body itself is deeper at the front, where it hangs down over the top of the third vertebra, and has a midline ridge between hollows for the longus colli. The pedicles and laminae are thick and strong. The superior articular facets are large and round and sit on the body and pedicles, while the inferior facets match those of the vertebrae below. The transverse processes are small, each with a single tubercle and a foramen for the vertebral artery. The spinous process is large and heavy, forked at its tip, and easily felt high in the back of the neck.

Fractures of the dens are graded into three types: through the tip, usually stable; through the base, the most common and prone to poor healing; and extending into the body. A separate injury, the hangman's fracture, breaks the bony bridge between the front and back of the axis. A dens that never united with the body, os odontoideum, can leave the upper neck unstable, so contact sports are usually advised against.

Function and supply

Function

Forms the pivot of the atlanto-axial joint, letting the head rotate from side to side.