Vertebral Artery Anatomy

The vertebral arteries are branches of the first portion of the subclavian arteries. These paired arteries are generally unequal in size, with the left the larger and dominant of the two. The typical course of the vertebral artery allows for its classic division into four segments: V1 to V4. The first segment (V1) starts with branching of the vertebral artery from the subclavian artery and follows as it courses anterior to the transverse foramen of C7 and into the transverse foramen of C6. The second segment (V2) includes the section of the artery as it passes through the successive vertebral foramina from C6 to C1. V3 comprises the portion from the superior aspect of the arch of the atlas to the foramen magnum, whereas V4 extends from the foramen magnum to the confluence with the contralateral vertebral artery; together, they form the basilar artery ( Figs. 50-1 and 50-2 ).

Schematic representation of normal vertebral artery anatomy with passage through the transverse foramina of C1 to C6. Note the midline orientation of the longus colli musculature. The inset depicts relationships of the artery within a transverse foramen, most notably with the exiting nerve root and associated uncovertebral joint.

Coronal reconstruction of a computed tomography angiogram demonstrating normal passage and filling of bilateral vertebral arteries.

Various anatomic relationships throughout the course of the vertebral artery are important to the spine surgeon. In the V2 region, the artery normally remains 1.5 mm or more lateral to the uncovertebral joint. Further, the bony architecture within the region of the V2 segment dictates a mildly convergent course of the arteries through this section; the mean interforaminal distance at C6 is approximately 29 mm compared with 26 mm at C3. Similarly, the mean distance from the medial edge of the longus colli to the medial edge of the vertebral artery decreases from 11.5 mm at C6 to 9 mm at C3. Whereas the transverse foramina of the subaxial spine are ring shaped, the transverse foramen of C2 is an angulated canal bordered by the pedicle and lateral mass. Its inferior and lateral openings allow the artery to deviate 45 degrees laterally before continuing its ascent to enter the transverse foramen of C1.

The V3 segment becomes important to the spine surgeon mostly during the posterior approach to the atlantoaxial joint. As the artery exits the foramen of C1, it travels posteriorly and medially inside the vertebral artery groove on the superior aspect of the atlas. At a distance ranging from 8 to 18 mm from the midline, the artery abruptly changes course, traveling anteriorly and superiorly toward the foramen magnum.

Anomalous Vertebral Artery Anatomy

Anatomic anomalies within the V2 segment are quite rare. However, their presence can be extremely important, particularly in patients undergoing anterior cervical spine operations. These anomalies can be divided into three major categories: intraforaminal, extraforaminal, and arterial.

Intraforaminal anomalies, or vertebral artery tortuosity, can be defined as a vertebral artery that is located medial to, or less than 1.5 mm lateral to, the uncovertebral joint. In general terms, this refers to the midline migration of the vertebral artery that causes erosion into the vertebral body. Several hypotheses have been proposed to explain this phenomenon, including degenerative changes and posttraumatic changes, as well as less common causes such as infection, tumor, systemic disease, or prior surgical nonunion. Cadaveric studies have shown the incidence of this condition to be 2.7%, with C3 and C4 the most commonly affected levels. More recent magnetic resonance imaging (MRI)–based studies showed a higher incidence, 7.6%, and found that patients with a tortuous vertebral artery tended to be older than patients without this finding ( Fig. 50-3 ).

Axial T2-weighted magnetic resonance image demonstrating a normal right vertebral artery at the level of C4 with a medialized left vertebral artery encroaching into the C4 body. This artery would potentially be at risk during C4 corpectomy using normal anatomic landmarks.

Extraforaminal anomalies refer to a situation in which the vertebral artery runs anterior to the transverse foramen of C6 to C1. An analysis of computed tomography (CT) angiograms showed that the vertebral artery enters through the C6 transverse foramen 94.9% of the time. However, anomalous entry sites at C4, C5, and C7 occurred at 1.6%, 3.3%, and 0.3%, respectively. In their MRI-based study, Eskander and associates found that only 92% of arteries entered at C6 ⁷ ( Fig. 50-4 ).

A to C, Sagittal T2-weighted magnetic resonance images of a patient with cervical stenosis. The left parasagittal image ( A ) shows the left vertebral artery passing anterior to the transverse foramen of C7 and into the transverse foramen of C6. The right parasagittal image ( C ) shows anomalous anatomy with the artery coursing anterior to the transverse foramen at C6 and into the transverse foramen at C5. The midsagittal image ( B ) is included for orientation.

Arterial abnormalities are varied, but they include such findings as dual and triple lumen arteries or the presence of a hypoplastic vertebral artery. Although most of these findings have few surgical implications, vertebral artery hypoplasia affects treatment options and potential neurologic sequelae in the case of inadvertent injury. Hypoplasia occurs in approximately 10% of the population.

At the atlanto-occipital joint, variations occur with greater regularity. Erosion of the C2 transverse foramen has been reported to have an incidence of 33%, occurring more commonly on the left side. Of these anomalies, 20% are severe enough to preclude the safe placement of C2 instrumentation. Similarly, arcuate foramina of C1 have a reported prevalence of 15.5%, with implications on exposure for C1 lateral mass screw placement.