The cruciform ligament of the atlas is the cross-shaped ligament complex at C1-C2 formed by the transverse ligament of the atlas (the horizontal beam, the primary stabiliser) and the superior and inferior vertical bands connecting the occiput above to the C2 body below. Together these form the cruciate pattern on the posterior surface of the dens. The transverse ligament is the critical component whose failure produces atlantoaxial instability.
| Origin | Medial tubercles of the lateral masses of the atlas (transverse band); body of the axis (inferior band) |
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| Insertion | Transverse band passes behind the dens to the opposite lateral mass; superior band ascends to the anterior margin of the foramen magnum |
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Holds the dens in place against the atlas and limits flexion at the craniovertebral junction.
The cruciform ligament complex is assessed on MRI in high-energy cervical trauma and in rheumatoid arthritis, where the transverse and alar ligaments are destroyed by pannus. Its transverse band is the critical component: an anterior atlantodental interval greater than about 3 mm in adults on flexion radiographs indicates insufficiency, and a posterior atlantodental interval under about 14 mm signals risk to the cord. Down syndrome is associated with laxity and atlantoaxial instability in a minority of patients. C1-C2 posterior fusion (transarticular or C1 lateral mass with C2 pedicle screws) treats symptomatic instability.
Cruciform ligament failure producing excessive C1-C2 translation with cord compression risk from rheumatoid pannus or trauma, requiring C1-C2 posterior fusion.
Ligamentous laxity allowing increased C1-C2 movement, usually asymptomatic but occasionally causing myelopathy and requiring fusion.
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