Axis

Original Editor - Rachael Lowe

Top Contributors - Mandeepa Kumawat, Admin, Kim Jackson, George Prudden and WikiSysop  

Introduction

The Axis (C2 vertebra) also known as epistropheus forms the pivot upon which the first cervical vertebra (the Atlas), which carries the head, rotates. The axis is composed of a vertebral body, heavy pedicles, laminae, and transverse processes, which serve as attachment points for muscles. The axis articulates with the atlas via its superior articular facets, which are convex and face upward and outward.

Structure

Lateral view

Posterosuperior view

Odontoid process

The most distinctive characteristic of this bone is the strong Odontoid Process(also known as the "dens") which rises perpendicularly from the upper surface of the body.It is a prominent rounded superior projection from the body.This tooth-like elevation serves a stable point around which the atlas bone rotates. Posteriorly the Odontoid process is flattened where it articulates with the transverse ligament.

Vertebral body

The body is deeper in front than behind, and prolonged downward anteriorly so as to overlap the upper and front part of the third vertebra. It presents in front a median longitudinal ridge, separating two lateral depressions for the attachment of the Longus colli muscles. Its under surface is concave from before backward and convex from side to side.

Other Structures

The pedicles are broad and strong, especially in front, where they coalesce with the sides of the body and the root of the odontoid process. They are covered above by the superior articular surfaces.

The laminae are thick and strong, and the vertebral foramen large, but smaller than that of the atlas.

The transverse processes are very small, and each ends in a single tubercle; each is perforated by the transverse foramen, which is directed obliquely upward and laterally.

The superior articular facets are round, slightly convex, directed upward and laterally,It articulates with the inferior articular facet on the altas bone above (C1) to form the atlantoaxial joint.

The inferior articular facets lies posterior to the transverse process and is directed downwards and forwards to articulate with the third cervical vertebra.

The superior vertebral notches are very shallow, and lie behind the articular processes; the inferior lie in front of the articular processes, as in the other cervical vertebrae.

The spinous process is large, very strong, deeply channelled on its under surface, and bifid.

Attachments

• The odontoid process provides attachment at its apex to the apical ligament and on each side below the apex to the alar ligaments.
• The transverse processes serve as the attachment sites of muscles that assist in rotating the head.They are levator scapulae , the sclenus medius anteriorly and the splenius cervicis posteriorly.
• The foramina (the holes) give passage to the vertebral artery and vertebral vein.
• The spinous process serves as the attachment site for many muscles of the spine, particularly those close to the skull, as well as the nuchal ligament. The muscles are semisplinalis servicis ,the rectus capitis posterior major,the inferior oblique, the spinalis cervicis , the interspinalis and the multifidus.
• The lamina provides attachment to the ligamentum flava.

Clinical significance

• Odontoid fractures

• The condition, where the dens is separated from the body of the axis, is called os odontoideum, and may cause nerve and circulation compression syndrome.