The transverse ligament of the atlas is the primary stabiliser of the atlantoaxial joint, holding the odontoid process firmly against the posterior surface of the anterior arch of the atlas and preventing forward displacement of C1 on C2. Its failure, from trauma or rheumatoid arthritis destruction, allows the odontoid process to migrate posteriorly into the spinal cord, a potentially fatal complication.
| Origin | Medial surface of the lateral mass of C1 (atlas) |
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| Insertion | Medial surface of the opposite lateral mass of C1 |
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Holds the dens against the anterior arch of the atlas, preventing posterior displacement of the dens toward the spinal cord.
Transverse ligament rupture from high-energy trauma produces atlantoaxial instability that may compress the spinal cord. An atlantodental interval greater than about 3 mm in adults (5 mm in children) on lateral or flexion radiographs indicates ligament insufficiency, and Steel's rule of thirds (one third dens, one third cord, one third free space) describes the reserve space at C1. In a Jefferson (C1 burst) fracture, combined lateral mass overhang greater than about 7 mm on the open-mouth view suggests transverse ligament rupture. Rheumatoid arthritis erodes the ligament through synovitis, and Down syndrome is associated with ligamentous laxity, both causing atlantoaxial instability that may require C1-C2 fusion.
Disruption from trauma or rheumatoid arthritis producing atlantoaxial instability with risk of odontoid compression of the spinal cord, requiring posterior C1-C2 fusion when instability is demonstrated on dynamic radiographs.
C1 ring burst fracture from axial loading with transverse ligament rupture, suggested by lateral mass spread over about 7 mm, treated with halo immobilisation or fusion.
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