The posterior atlantooccipital membrane is a broad, thin fibrous sheet connecting the upper border of the posterior arch of the atlas to the posterior margin of the foramen magnum. On each side its lower lateral edge arches over the vertebral artery and first cervical nerve as they cross the groove on the atlas, and this edge sometimes ossifies to form a bony bridge (ponticulus posticus, or arcuate foramen). The membrane is in series with the ligamenta flava below.
| Origin | Upper border of the posterior arch of the atlas |
|---|---|
| Insertion | Posterior margin of the foramen magnum |
Reinforces the posterior craniocervical junction, limits flexion at the atlantooccipital joint, and protects the vertebral arteries and suboccipital nerves as they pass through its lateral arcuate openings.
The vertebral artery groove on the posterior arch of C1 leads to the opening at the lateral edge of this membrane, a key landmark in suboccipital dissection, and the artery must be identified before the membrane is opened laterally. A ponticulus posticus can be mistaken for a thick posterior arch and misdirect C1 lateral mass screws, and it has been linked to vertebral artery compression symptoms. The membrane is opened with the posterior arch of C1 in Chiari malformation decompression and in posterior occipitocervical fusion.
The vertebral artery passing beneath the lateral edge of the membrane is at risk when the membrane is opened during posterior craniovertebral surgery, so it is identified in its groove on C1 first.
Ossification of the lateral edge of the membrane bridging the vertebral artery groove, usually incidental but relevant to C1 screw placement and occasionally linked to vertebrobasilar symptoms.
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