Ipsilateral arcuate foramen and high-riding vertebral artery: implication on C1-C2 instrumentation

Several methods for C1-C2 stabilization have been described in the literature. These include C1-C2 transarticular and C1 lateral mass screws. In patients with aberrant anatomy such as a high-riding vertebral artery (VA) or the presence of an arcuate foramen at C1, there exists a higher risk of VA injury. This may lead to excessive bleeding, stroke, and possibly death. There have been several studies determining the incidence of arcuate foramen and high-riding VA occurring individually in the general population, but none have determined their occurrence simultaneously.

To determine the prevalence of ponticulus posticus and high-riding VA occurring simultaneously in the general population.

Radiological study.

One hundred consecutive computed tomography (CT) scans of the cervical spine were reviewed. Scans that contained an arcuate foramen were identified, and it was indicated whether the foramen was right sided, left sided, or bilateral. In the same group, the thickness of the isthmus and the internal height of the lateral mass of C2 were measured. The VA was considered high riding if the isthmus thickness was less than 5 mm or the isthmus internal height was less than 2 mm.

Fourteen out of one hundred (14%) patients had a fully formed arcuate foramen. Of these, six were left sided, three were right sided, and five were bilateral. In addition, there were 24 (24%) patients with partially formed ponticulus posticus. Thirty-two (32%) patients were identified to have a high-riding VA. Of these, 13 were left sided, 9 were right sided, and 10 were bilateral. Five (5%) had an ipsilateral arcuate foramen and high-riding VA.

The arcuate foramen and high-riding VA are common anomalies that are often not recognized. Although ipsilateral high-riding VA and arcuate foramen rarely occur in the general population, proper identification of these anomalies on preoperative CT scan facilitates planning the safest technique for C1-C2 instrumentation.