Articulation of the Atlas with the Epistropheus or Axis

Anatomy > Gray's Anatomy of the Human Body > III. Syndesmology > 5b. Articulation of the Atlas with the Epistropheus or Axis

Henry Gray (1821–1865). Anatomy of the Human Body. 1918.

Articulation of the Atlas with the Axis

(Articulatio atlantoepistrophica)

The articulation between the atlas (C1) and the axis (C2), also called the atlantoaxial joint, is a complex anatomical structure consisting of multiple articulations and ligaments. These joints allow a significant degree of rotation of the head and neck and provide important structural support for the craniovertebral junction.

Joints Involved

This articulation comprises four distinct synovial joints:

• Two lateral zygapophyseal joints (gliding or arthrodial joints) between the inferior articular facets of the atlas and the superior articular facets of the axis.
• One median pivot joint between the odontoid process (dens) of the axis and the anterior arch of the atlas.
• A second median joint between the odontoid process and the transverse ligament of the atlas.

These articulations are supported and stabilized by multiple ligaments.

Ligaments

The ligaments connecting the atlas and axis include:

• Two articular capsules – one for each lateral atlantoaxial joint.
• Anterior atlantoaxial ligament
• Posterior atlantoaxial ligament
• Transverse ligament of the atlas
• Accessory ligament (reinforcing the articular capsule)
• Cruciate ligament of the atlas (comprising superior and inferior crus)

Articular Capsules

The articular capsules are thin and loose, surrounding the lateral joints. Each is reinforced posteriorly and medially by the accessory ligament, which extends from the base of the dens to the lateral mass of the atlas near the transverse ligament.

Anterior Atlantoaxial Ligament

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Anterior atlanto-occipital and atlantoaxial ligament (Gray's Anatomy, 1918)

A strong membrane that connects the anterior arch of the atlas above to the anterior surface of the body of the axis below. In the midline, it is reinforced by a rounded cord continuous with the anterior longitudinal ligament.

Posterior Atlantoaxial Ligament

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Posterior atlantoaxial ligament (Gray's Anatomy, 1918)

A broad, thin membrane that connects the posterior arch of the atlas above to the laminae of the axis below. It is the analog of the ligamenta flava found in the thoracic and lumbar spine. Posteriorly, it lies deep to the Obliquus capitis inferior muscles.

Transverse Ligament of the Atlas

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Articulation of the atlas and axis showing transverse ligament (Gray's Anatomy, 1918)

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Membrana tectoria, transverse and alar ligaments (Gray's Anatomy, 1918)

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Sagittal section of atlantoaxial and atlanto-occipital region (Spalteholz)

A thick, horizontal band extending between the medial surfaces of the lateral masses of the atlas. It arches across the ring of the atlas, holding the dens against the anterior arch. The ligament is convex posteriorly and concave anteriorly.

The superficial fibers give rise to:

• Superior crus – Ascends to the basilar part of the occipital bone
• Inferior crus – Descends to the posterior surface of the body of the axis

These components, together with the transverse ligament, form the cruciform ligament of the atlas.

This ligament divides the vertebral foramen of the atlas into anterior and posterior compartments:

• The anterior compartment holds the dens
• The posterior compartment transmits the spinal cord and meninges

Synovial Membranes

Each articulation has its own synovial membrane, although the median atlantoaxial joint's cavity is sometimes continuous with the atlanto-occipital joint's synovial cavity.

Movements

The atlantoaxial joint permits a significant degree of rotation of the head and neck. Rotation of the atlas (and therefore the skull) around the dens of the axis is the primary motion allowed at this joint.

Range and Limitation of Motion

• Rotation is limited by the alar ligaments.
• The transverse ligament holds the dens securely in place, acting as a pivotal restraint.
• Both joint surfaces are convex in the sagittal plane, allowing gliding movements during rotation.

Muscles Involved

Primary rotators include:

• Sternocleidomastoid (contralateral rotation)
• Semispinalis capitis (contralateral rotation)
• Splenius capitis and Longissimus capitis (ipsilateral rotation)
• Rectus capitis posterior major
• Obliquus capitis superior and Obliquus capitis inferior

Physiological Considerations

According to Corner (Journal of Anatomy and Physiology, Vol. XLI), rotation occurs in two phases:

1. Asymmetric rotation: One atlantoaxial joint is fixed by neck musculature while the opposite side glides anteriorly and inferiorly.
2. Symmetric rotation: The dens becomes the axis of rotation for the atlas and skull.

Clinical Relevance

• Injury to the transverse ligament can result in instability of the cervical spine, especially in conditions such as rheumatoid arthritis or trauma.
• Atlantoaxial subluxation is a potential complication in patients with congenital ligamentous laxity (e.g., Down syndrome).
• Surgical stabilization may be required in cases of atlantoaxial instability.

See Also

• Atlanto-occipital joint
• Cruciate ligament of the atlas
• Odontoid process
• Spinal cord
• Cervical vertebrae

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Adapted from the Classic Grays Anatomy of the Human Body 1918 edition (public domain)