The transverse ligament of the atlas is the thick horizontal band running between the medial tubercles of the C1 lateral masses, holding the dens of C2 against the anterior arch of the atlas. It is the primary stabiliser of the atlantoaxial joint, preventing anterior atlantoaxial subluxation and protecting the cervical spinal cord.
Primary restraint to anterior atlantoaxial subluxation; holds the odontoid process against the anterior C1 arch; maintains a safe space for the spinal cord in the C1 ring.
Transverse ligament rupture or attenuation produces atlantoaxial instability — a potentially catastrophic injury. It occurs in trauma (Jefferson C1 fracture, odontoid fracture), rheumatoid arthritis (synovitis erodes the ligament), Down syndrome (ligament laxity), and ankylosing spondylitis. The Steel rule of thirds divides the C1 ring into thirds: odontoid, ligament, and spinal cord — the ligament occupies the middle third. Atlantoaxial distance greater than 3 mm in adults or 5 mm in children on lateral flexion X-ray indicates instability requiring surgical fusion.
Anterior atlantoaxial subluxation from transverse ligament failure in trauma, rheumatoid arthritis, or Down syndrome, producing cervical myelopathy and risk of sudden death from cord compression, requiring posterior C1-C2 fusion.
C1 ring fracture with transverse ligament rupture from axial loading producing unstable atlantoaxial subluxation, identified by lateral mass spread greater than 7 mm on open-mouth odontoid view, requiring halo immobilisation or surgical fusion.
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