The basilar process, also known as the mammillar or papillar process, 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 basilar process is a rare variant, with a reported prevalence of 4% 2.
Clinical presentation
A basilar process is usually an incidental finding. It may cause neck pain and dysfunction if it disturbs the occipitocervical articulation by articulating with the atlas (C1), but symptomatic cases are not well described so its true clinical significance remains uncertain 3.
Radiographic features
The basilar process is a small, rounded bony protuberance on the inferior surface of the clivus, at the anterior rim of the foramen magnum. It is found between the tips of the occipital condyles in a slightly paramedian location compared to the third condyle, which is a midline variant 1,4. The basilar process may articulate with the anterior arch of the atlas. It may also exist as an isolated accessory ossicle 4.
Basilar processes may be unilateral or bilateral. Large bilateral basilar processes may fuse in the midline, forming a structure known as a false third condyle. A false third condyle will have a remnant of its fused origin in the form of a slender intrabasilar canal passing through it in the anteroposterior direction. A true third condyle will lack this canal 3.
Bilateral bulging basilar processes may give a lipped appearance that has been described as the anterior labia of the foramen magnum 3,5.
Development
The basilar process is formed due to incomplete regression of one or both of the lateral parts of the hypochordal arch of the embryonic proatlas, with normal regression of the midline part 1,6.
See the article on occipital vertebrae for a detailed discussion of the embryonic origin of this spectrum of variants.
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