The craniovertebral junction (CVJ) is a highly complex region, which includes the occipital bone and condyles, the atlas, and the axis, is unique from the remainder of the cervical spine with regard to its bony anatomy, joint shape and orientation, and relationship to the vertebral artery complex.
It is a transition between the cranium and the mobile cervical spine. The bony confines of the CVJ encompass the transition of the brainstem to the cervical spinal cord. This is the most mobile region of the cervical spine, and it functions as a “funnel” that is uniquely adapted to stability and motion 1).
Transverse and alar ligaments appear to be the main stabilizers of the craniocervical junction. The vertical structures attached to the clivus and OA joint capsules function as secondary stabilizers. Craniocervical dislocations seem to affect FE and lateral bending the most, whereas increased translation seems to occur primarily in the AP and CC directions 2).
OC-C1-C2 joint is an osseoligamentous complex and extremely mobile; a component of the cervical spine. OC-C1 joint is primarily a “yes-yes” joint (flexion-extension of 23-24.5 degrees) and C1-C2 joint is primarily a “no-no” joint (axial-rotation of 23.3-38.9 degrees to each side) with atlas acting as a washer between occiput and the rest of the cervical spine. It is largely believed that flexion at OC-C1 is limited by impingement of odontoid tip and extension by the tectorial membrane. There occurs a small flexion-extension movement at C1-C2 joint (of 10.1-22.4 degrees) with transverse ligament limiting flexion and tectorial membrane along with C1-C2 articular joint limiting extension 3).
Of the skeletal CVJ anomalies, AAD is most common in India followed by occipitalization of atlas, and basilar invagination, whereas basilar invagination is most common in the western countries.
The CVJ anomalies are usually congenital and occur since birth, but the mean age of manifestation is around 25 years. Males are most commonly affected.
Predisposing factor in the form of trivial neck trauma may be identified in nearly 50% of the cases 4).
The craniovertebral junction is anatomically complicated. Representative vertebral artery (VA) variations include the persistent first intersegmental artery (FIA), fenestration of the vertebral artery above and below C1 (FEN), posterior inferior cerebellar artery (PICA) from C1/2, and high riding vertebral artery (HRVA). The ponticulus posticus (PP) is a well-known osseous anomaly at C1.
Among 480 subjects with a mean age of 63.1 years, one hundred and eighteen subjects were female, and 269 were male. HRVA was observed in 10.1 % of patients (39 out of 387 cases), FIA in 1.8 % (7 cases), FEN in 1.3 % (5 cases), and PICA in 1.3 % (5 cases). PP was observed in 6.2 % of patients (24 cases).
According to past reports, many VA anomalies could be attributed to congenital or acquired conditions (e.g., rheumatoid arthritis). However, VA anomalies appear to exist even in patients without any such cervical diseases 5).