Posterior Arch of the Atlas

arcus posterior atlantis

The posterior arch of the atlas (C1) is the posterior curved bar of the ring-shaped atlas, connecting the two lateral masses posteriorly. It bears the posterior tubercle (a rudimentary spinous process), and the vertebral artery groove (sulcus arteriae vertebralis) on its superior surface where the vertebral artery crosses from the transverse foramen to the foramen magnum. The suboccipital muscles attach to the posterior arch.

Region: Neck

Clinical Relevance

Clinical Notes

The posterior arch of C1 is removed in C1 laminectomy for posterior decompression of the upper cervical spinal cord and foramen magnum. The safe lateral limit of C1 posterior arch removal is approximately 1.5-2 cm from the midline, beyond which the vertebral artery groove is encountered. In Chiari malformation type I, the posterior arch of C1 may be removed as part of foramen magnum decompression. C1 posterior arch stress fractures occur from axial loading in gymnastics. The atlas ring fracture (Jefferson fracture) involves both anterior and posterior arches.

Pathology

Common Injuries & Conditions

Posterior Atlas Arch Fracture in Jefferson Injury

Axial loading of the skull fractures both the anterior and posterior arches of C1 (Jefferson fracture), exploding the ring outward; the transverse ligament integrity determines instability: if the overhang of lateral masses exceeds 6.9 mm on the open-mouth radiograph (Rule of Spence), ligament rupture is assumed and surgical stabilisation is required.