Atlanto-occipital articulation

The atlanto-occipital articulation (also known as the C0-C1 joint/articulation) is comprised of a pair of condyloid synovial joints that connect the occipital bone (C0) to the first cervical vertebra (atlas/C1).

Gross anatomy

Articulations

Each joint is comprised of two concave articular surfaces on the superior aspect of the lateral mass of atlas which articulate with a convex surface on the occipital condyle. The joint is reinforced by fibrous capsules which support each joint. The atlanta facets are tilted medially.

Attachments

• anterior atlanto-occipital membrane: broad, dense fibrous structure which joins the upper border of the anterior arch of the atlas (C1) to the anterior inferior margin of the foramen magnum; it is a continuation of the anterior longitudinal ligament and and prevents excessive neck extension. Laterally it blends with the joint capsule and medially is strengthened by a strong, rounded cord which connects the basilar occipital bone to the anterior atlantal tubercle.

• posterior atlanto-occipital membrane: a broad but thin fibrous membrane which attaches the upper border of the anterior surface of the posterior arch of the atlas (C1) to the posterior margin of the foramen magnum. It is continuous with the posterior atlantoaxial membrane inferiorly (part of the ligamentum flavum) and ligamentum nuchae posteriorly posteriorly and lies immediately posterior to the spinal dura. Posteriorly lie the suboccipital muscles. The atlantic (V3) portion of the vertebral artery runs anteriorly to pierce the membrane and dura and become the dural (V4) portion.

• tectorial membrane: continuation of the posterior longitudinal ligament from the dens to the anterior aspect of the foramen magnum.

Innervation

The joint capsule is innervated by the C1 nerve.

Arterial supply

Anastomosis between the deep cervical, occipital and vertebral arteries.

Function

Allows for 25^o of of flexion/extension and around 5^o of of axial rotation.

Relations

• posterior: vertebral artery

• anterior: rectus capitis anterior

• posteromedial: dorsal ramus of the first cervical nerve and rectus capitis posterior major

Variant anatomy

• variable degree of communication between the atlanto-occipital joint and the cavity between the dens (odontoid peg) and transverse ligament of the cruciform ligament

• third occipital condyle (condylus tertius) which may articulate with the anterior arch of the atlas, or the dens

• paracondylar process

• occipital condyle hypoplasia

• division of the superior articular facet of the atlas

• atlanto-occipital assimilation

• arcuate foramen: foramen visualised by the atlantic portion of the vertebral artery as it courses through a calcified posterior atlanto-occipital membrane

Related pathology

• atlanto-occipital dissociation (AOD) represents represents the most common cervical spine injury following a motor vehicle accident and is usually highly unstable with significant neurological morbidity