The anterior arch of the atlas is the shorter anterior bar of the ring-shaped C1 vertebra, bearing the anterior tubercle on its external surface (attachment for longus colli muscle) and the articular facet (fovea dentis) on its internal surface, which articulates with the anterior surface of the dens of the axis. The transverse ligament of the atlas holds the dens against this facet.
The anterior arch-dens relationship defines atlanto-axial instability. The atlanto-dental interval (ADI), the gap between the anterior atlas arch and the dens on flexion lateral radiograph, should be 3 mm or less in adults (5 mm in children). An increased ADI indicates transverse ligament insufficiency from rheumatoid arthritis, Down syndrome, trauma, or congenital anomaly. Anterior C1 instrumentation in atlantoaxial fusion uses the anterior arch as the fixation point for anterior transarticular screws. Os odontoideum (incomplete dens ossification) leaves the anterior arch-dens complex unstable.
Rheumatoid arthritis pannus destroying the transverse ligament or traumatic transverse ligament rupture produces atlantoaxial instability with an ADI exceeding 3 mm on flexion radiograph; posterior C1-C2 arthrodesis with lateral mass screws and rod constructs stabilises the segment before spinal cord compression develops.
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