The atlas is the first cervical vertebra, a ring-shaped bone with no vertebral body that supports the skull via two concave superior articular facets and allows the nodding motion of the head at the atlantooccipital joint.
| Articulations |
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|---|---|
| Landmarks |
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| Ossification | Three primary ossification centres appearing by birth, fusing by age 7. |
The atlas is the most distinctive vertebra, lacking a body and consisting of two lateral masses connected by anterior and posterior arches. Jefferson fractures are burst fractures of the atlas from axial loading (e.g., diving into a shallow pool), producing the classic four-fragment pattern on open mouth odontoid radiograph. Transverse ligament integrity determines stability, if the combined overhang of the lateral masses exceeds 6.9 mm on open mouth view, the ligament is disrupted.
Axial loading of the head bursts the ring of the atlas at its anterior and posterior arches, spreading the lateral masses; stability depends on whether the transverse ligament is intact.
Laxity or rupture of the transverse ligament, seen after trauma, in rheumatoid arthritis, or in Down syndrome, allows the atlas to slide forward on the axis and threaten the spinal cord.
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