The posterior atlanto-occipital membrane bridges from the posterior arch of the atlas to the posterior margin of the foramen magnum (opisthion). The vertebral artery passes through an opening in the lateral part of this membrane on each side, accompanied by the first cervical nerve. The posterior membrane is the upward continuation of the ligamentum flavum at the craniovertebral junction.
Restrains flexion at the atlanto-occipital joint, reinforces the posterior joint capsule, and bridges the foramen magnum posteriorly while accommodating vertebral artery passage through its lateral arcuate defects.
The posterior atlanto-occipital membrane is accessed during suboccipital craniotomy and posterior approaches to the craniovertebral junction. The vertebral artery groove on the posterior arch of C1 leads to the opening in this membrane, a key landmark in suboccipital dissection. The membrane must be opened carefully in posterior decompression of the craniovertebral junction to avoid vertebral artery injury. In Chiari malformation decompression, the membrane is opened along with the posterior arch of C1 to release the constrictive element at the foramen magnum.
During suboccipital dissection for craniovertebral junction exposure, the vertebral artery entering through the posterior atlanto-occipital membrane after emerging from the C1 transverse foramen is at risk when the membrane is opened; the artery must be identified in its bony groove on the posterior arch of C1 before any membrane incision.
