The cruciform ligament of the atlas is formed by the transverse atlantal ligament (horizontal band) crossing with vertical longitudinal bands running from the dens to the occiput above and C2 below. Together they form a cross-shaped structure that is the primary stabiliser of the atlantoaxial joint.
Constrains the dens within the anterior ring of the atlas, preventing posterior dens displacement into the spinal cord. The transverse band is the critical component, resisting anterior atlantoaxial subluxation.
Cruciform ligament disruption (transverse ligament rupture) is the primary injury in atlantoaxial instability. It occurs in high-energy trauma, rheumatoid arthritis, Down syndrome, and os odontoideum. The atlantodental interval (ADI) greater than 3mm on lateral X-ray indicates transverse ligament failure. Surgical fusion (C1-C2) is required for symptomatic instability.
Disruption of the transverse atlantal ligament producing C1-C2 instability with ADI >3mm on flexion X-ray, requiring posterior C1-C2 arthrodesis.
Ligamentous laxity of the cruciform ligament producing asymptomatic or symptomatic atlantoaxial instability in 10-20% of Down syndrome patients, requiring screening X-rays before contact sports.
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