The prebasioccipital arch is a rare anatomical variant of the occipital bone that is part of the spectrum of occipital vertebrae 1.
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Epidemiology
The prebasioccipital arch is a very rare variant; one study reported a prevalence of 0.025% 2.
Clinical presentation
The prebasioccipital arch is a rare incidental finding. If it articulates with the atlas (C1) or axis (C2) it may theoretically disturb the occipitocervical articulation and causing neck pain and dysfunction, however symptomatic cases are not well described so its true clinical significance remains uncertain 3. The prebasioccipital arch is not seen to encroach posteriorly on the foramen magnum, so resulting neural compression is considered unlikely 4.
Radiographic features
The prebasioccipital arch is a U-shaped bony arch arising from the inferior surface of the clivus, draped across the anterior rim of the foramen magnum between the tips of the occipital condyles 1,4. It may exist as an isolated accessory ossicle, or be fused to varying degrees to the occipital bone 1,3.
Various bony articulations of the prebasioccipital arch may occur, including to the occipital bone, anterior arch of the atlas and tip of the odontoid process of the axis. One case has been described of an arch articulating with all three of these structures, with accompanying osteoarthritic changes seen on CT 1.
Development
Abnormal persistence and subsequent ossification of the entire hypochordal arch of the embryonic proatlas results in the prebasioccipital arch 1,4,5. Functionally, this structure represents the combination of bilateral basilar processes with a third condyle 3.
See the article on occipital vertebrae for a detailed discussion of the embryonic origin of this spectrum of variants.
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