Overview

The axis is the strongest cervical vertebra. From its body the dens projects superiorly; its anterior facet articulates with the anterior arch of the atlas and its posterior groove with the transverse ligament of the atlas, together forming the median atlanto-axial (pivot) joint. The apical ligament joins the tip of the dens to the anterior margin of the foramen magnum and the strong alar ligaments pass from the sides of the dens to the medial aspects of the occipital condyles, limiting rotation.

Large, flat superior articular facets on either side of the dens form the lateral atlanto-axial joints. The axis has a large, bifid spinous process — the first palpable midline bony point below the external occipital protuberance — and small transverse processes with transverse foramina. Its inferior aspect resembles a typical cervical vertebra, with the first intervertebral disc (C2–C3) below.

Location: Second vertebra of the neck, below the atlas.

Parts & subdivisions

Dens (odontoid process)
Pivot for the atlas; embryologically the body of the atlas.
Superior articular facets
Lateral atlanto-axial joints (plane).
Bifid spinous process
Large; attachment of rectus capitis posterior major and obliquus capitis inferior.
Pars interarticularis (isthmus)
Between superior and inferior articular processes; fractured in hangman fracture.

Anatomy

Articulations: Atlas (median and lateral atlanto-axial joints), C3 (intervertebral disc and facet joints).
Ligaments: Transverse ligament of atlas, alar ligaments, apical ligament, tectorial membrane (upward continuation of the posterior longitudinal ligament), cruciform ligament.
Ossification: Five primary centres — body, two halves of the arch and two for the dens — plus a secondary centre for the dens tip (os terminale). The dens fuses to the body ≈3–6 years (subdental synchondrosis may mimic a fracture in children).

Function

Provides the pivot for rotation of the head ("no" movement)
Transmits the weight of the head to the lower cervical spine

Clinical significance

Odontoid (dens) fractures

Anderson–D’Alonzo classification: type I (tip — avulsion of alar ligament), type II (base of the dens — commonest, poor blood supply, high non-union rate), type III (extends into the body — usually heals). Common in elderly falls.

Hangman fracture

Bilateral fracture of the pars interarticularis of C2 (traumatic spondylolisthesis of the axis) from hyperextension with distraction — judicial hanging or a chin striking a dashboard.

Atlanto-axial instability

Laxity or erosion of the transverse ligament in rheumatoid arthritis and Down syndrome allows the dens to compress the cord; assessed by the atlanto-dental interval (> 3 mm abnormal in adults).

Compiled from standard anatomy and physiology references — see the Skeletal System sources. For education only; not medical advice.