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Occipital vertebrae are rare anatomical variants that result from incomplete or aberrant fusion of occipital bone ossification centers. There is a broad spectrum of occipital vertebrae, the more common variants include:
Occipital vertebrae are usually identified incidentally. If they fuse or articulate with the atlas (C1) or axis (C2) they can disrupt the occipitocervical articulation, causing neck pain and dysfunction that may necessitate surgery 1,2.
These variants are best detected on CT, where their characteristic location and appearance aids in differentiating them from fractures 1,3.
The hypochordal arch (or bow) is a part of the embryonic proatlas, derived from the first cervical sclerotome and the caudal part of the fourth occipital sclerotome 4. It lies on the ventral surface of the the clivus, extending across the anterior rim of the foramen magnum between the anteromedial borders of the occipital condyles 1,4. The hypochordal arch gives rise to the anterior arch of the atlas and subsequently undergoes a process of controlled assimilation into the occiput, with the midline part disappearing before the lateral parts 1.
Incomplete regression of the hypochordal arch leads to a spectrum of anatomical variants:
third condyle (condylus tertius): midline part persists
basilar process: lateral part persists (can be unilateral or bilateral)
prebasioccipital arch: entire hypochordal arch persists
The paracondylar process develops due to a phenomenon known as caudal shifting, whereby the component sclerotomes of the embryonic proatlas fail to separate normally. The broad array of paracondylar process sub-variants is said to be a result of variable degrees of separation between the sclerotomes 5. In recent literature, the prevailing mechanism is the one described above. Some authors would advocate for an alternate acquired or post-traumatic mechanism secondary to tension on the rectus capitis lateralis muscle or lateral atlanto-occipital ligament, as both attach near the jugular process from which the paracondylar process arises 5,6.
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- 2. Prescher A, Schuster D, Brors D. GMS. Some Rare Osseous Variations of the Craniocervical Junction. 7th Congress of the European Skull Base Society Held in Association with the 13th Congress of the German Society of Skull Base Surgery. 2009. doi:10.3205/05esbs02
- 3. Narayanan R, Shankar B, Paruthikunnan S, Kulkarni C. Paracondylar Process of the Occipital Bone of the Skull: A Rare Congenital Anatomical Variant. Case Reports. 2014;2014(oct15 1):bcr2014205315. doi:10.1136/bcr-2014-205315 - Pubmed
- 4. Offiah C & Day E. The Craniocervical Junction: Embryology, Anatomy, Biomechanics and Imaging in Blunt Trauma. Insights Imaging. 2016;8(1):29-47. doi:10.1007/s13244-016-0530-5 - Pubmed
- 5. Schumacher M, Yilmaz E, Iwanaga J, Oskouian R, Tubbs R. Paramastoid Process: Literature Review of Its Anatomy and Clinical Implications. World Neurosurg. 2018;117:261-3. doi:10.1016/j.wneu.2018.06.056 - Pubmed
- 6. Nolet PS, Friedman L, Brubaker D. Paracondylar process: a rare cause of craniovertebral fusion—a case report. Journal of the Canadian Chiropractic Association. 1999;43(4):229-235. Pubmed