Posterior Atlantoaxial Membrane

membrana atlantoaxialis posterior

The posterior atlantoaxial membrane is a broad, thin fibrous sheet connecting the lower margin of the posterior arch of the atlas to the upper borders of the laminae of the axis. It represents the ligamentum flavum at the C1-C2 level. The second cervical nerve pierces its lateral part. The vertebral arteries do not cross it but run above the posterior arch of C1 in the suboccipital triangle.

Anatomical Data

Attachments

Origin	Lower border of the posterior arch of the atlas
Insertion	Upper borders of the laminae of the axis

Biomechanics

Function & Actions

Limits flexion at the atlantoaxial joint, reinforces the posterior C1-C2 joint capsule, and provides the posterior ligamentous boundary of the atlantoaxial space.

Clinical Relevance

Clinical Notes

The posterior atlantoaxial membrane is encountered and divided during posterior surgical approaches to the atlantoaxial junction, such as transarticular screw fixation or C1 lateral mass with C2 pedicle screw fixation. The C2 nerve root and ganglion pass through its lateral portion, a key relationship during C1-C2 screw placement, and the vertebral artery running above the posterior arch of C1 must be avoided. In patients with atlantoaxial instability from rheumatoid arthritis or trauma, the posterior membrane may be the only remaining posterior soft tissue restraint.

Pathology

Common Injuries & Conditions

C1-C2 Facet Cyst

Synovial cysts arising from the C1-C2 facet joints project through the posterior atlantoaxial membrane into the spinal canal, producing upper cervical myelopathy or radiculopathy managed by cyst aspiration at C2 ganglion level or posterior C1-C2 decompression and fusion.

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