The third condyle, also known as condylus tertius or the median occipital condyle, is a rare anatomical variant of the occipital bone that may mimic an occipital condyle fracture. It is part of the spectrum of occipital vertebrae.
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Epidemiology
The third condyle is a rare variant, found in approximately 0.5-2.5% of individuals 1,2. It has an association with os odontoideum 3.
Clinical presentation
A third condyle is usually an incidental finding. If it articulates with the atlas (C1) or axis (C2) it may disturb the occipitocervical articulation by forming a three-column mechanism bearing the skull. This can lead to pain, reduced neck range of motion or bony torticollis that may necessitate surgery 1,4. A prominent third condyle projecting posteroinferiorly may cause neural compression 5.
Radiographic features
When present, the third condyle appears as a midline bony protuberance on the inferior surface of the clivus, at the anterior rim of the foramen magnum 6. It is located between the tips of the occipital condyles and may exist as an isolated accessory ossicle, or be fused to varying degrees to the occipital bone 1.
A third condyle may range in size from barely perceptible to prominently hyperplastic 5. It frequently articulates with the apex of the dens and/or the anterior arch of the atlas 3-5.
Development
The third condyle is formed due to incomplete regression of the midline part of the hypochordal arch of the embryonic proatlas, with normal regression of the lateral parts 1,3.
See the article on occipital vertebrae for a detailed discussion of the embryonic origin of this spectrum of variants.
Differential diagnosis
fracture fragment: unexpected location with broken cortical margin
os odontoideum: will be accompanied by hypoplasia of the dens 1,4
os terminale: well-corticated ossicle located at the tip of the dens, more posterior than a third condyle
calcification of the apical ligament of the dens: a more ill-defined peaked bony spur at the tip of the dens, more posterior than a third condyle 1
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