Posterior Atlantoaxial Membrane

membrana atlantoaxialis posterior

The posterior atlantoaxial membrane is a broad fibrous sheet connecting the inferior margin of the posterior arch of the atlas to the superior edge of the lamina of the axis. It represents the superior continuation of the ligamentum flavum at the C1-C2 level and has a central opening for the passage of the C2 dorsal root ganglion and posterior ramus. The vertebral arteries lie lateral to the membrane as they pass through the suboccipital triangle.

Region: Neck

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 during posterior surgical approaches to the atlantoaxial junction and must be carefully incised to expose the C1-C2 joint for fusion or cyst drainage. The C2 nerve root and ganglion pass through an opening in its lateral portion, and this relationship is critical during C1-C2 screw placement. 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.