The vertebral arteries provide the blood supply to the dorsal portion of the circle of Willis. The artery is commonly divided into four parts. The first part originates as the first branch of the subclavian artery, ascends between scalenus anterior and longus colli, passes ventral to C7 transverse process, and enters C6 foramen transversarium. The second part climbs the cervical spine within the foramen transversarium from C6 to C1. The artery travels slightly medial and dorsal as it ascends from C7 to C3. In its third part, the vertebral artery exits the C1 foramen transversarium, travels dorsally behind the superior articular process of the atlas, and enters the cranium via foramen magnum. The fourth part of the vertebral artery pierces the dura and unites with the contralateral vertebral artery to form the basilar artery (11, 12 and 13).

Anomalous course of the vertebral artery has been described from 2.3% to 2.7% in the subaxial cervical spine (4,13) to 20% in the atlantoaxial part of the spine (14,15). In this segment of population with evidence of a tortuous vertebral artery, the transverse foramen is at an average of 0.14 mm medial the foramen of Luschka (13) and has even been described to be less than 8 mm from midline in the lower cervical spine (16). When the vertebral artery takes such a tortuous cause, it may also result in erosions of the vertebral bodies and the pedicles; in such cases, the normal landmarks used for avoiding injury to the vertebral arteries cannot be used.

Several types of anomalies of the vertebral artery course have been noted in the atlantoaxial cervical spine. Paramore et al. (15) conducted a study using computed tomography to evaluate cervical spine anatomy; the authors found that 18% of the population had a high-riding C2 vertebral foramen. Such anomaly would likely predispose the patient to a vertebral artery injury in a C1-C2 transarticular screw placement. Other anomalies include an enlarged C2 vertebral artery groove resulting in thinning of the pedicle and the lateral mass (14), as well as an abnormal location of the bending point of the vertebral artery under the C2 superior articular facet (17).

The second part of the vertebral artery is at most risk during ventral cervical spine surgery. This risk is higher with procedures such as vertebrectomy than with discectomy and fusion (7). In the subaxial cervical spine, the vertebral artery is in danger in its position ventral to C7 transverse process as well as in the foramen transversarium in C3-C6. As the artery ascends, it travels in a slight medial and dorsal direction, possibly placing it at higher risk with surgery done on the more proximal levels.

During the decompression, care should be taken to avoid removing too much bone and disk laterally. The surgeon should also avoid angling off midline during the decompression, as this would lead to uneven removal of
bone and possible vertebral artery injury (7). The center of the cervical spine may be approximated as being halfway between the medial insertions of the longus colli (4,18). However, the medial insertion of the longus colli is more lateral in the lower cervical vertebrae than in the upper ones. If the medial insertion of longus colli used as a landmark for lateral dissection, excessive bone may be left in the upper cervical vertebrae (14,19). A better landmark is the uncovertebral joint; barring vertebral artery anomalies, it is approximately 5 to 6 mm medial to the medial edge of the foramen transversarium (14,19). Dissection and decompression should be done with caution in this area.

Vertebral artery injury as a result of an aberrant odontoid screw placement during closed reduction and internal fixation of a type 2 odontoid fracture has been recently reported (5). In this case, the injury was not identified until 4 days later, when the patient suffered a fatal brainstem infarction. Retrospectively the authors noted that the screw was protruding proximally from the proximal fragment; however, this was not identified during the procedure. Careful patient selection for operative treatment of a dens fracture with an odontoid screw is necessary for preventing such injuries; the authors recommend reconsidering this type of treatment in patients in whom the positioning required for fracture reduction precludes the use of the proper trajectory for screw insertion. This may be particularly difficult in patients with short necks, thick barrel chests, pronounced kyphosis of the thoracic spine, or morbid obesity (5).

In dorsal cervical spine procedures, the second and third part of the vertebral artery is at highest risk. While the artery is in most danger during the preparation of screw sites and insertion of screws, it may also be injured during dissection, particularly in the upper cervical spine (11,15,20,21). During the lateral extension of the exposure of C1, the dissection on the dorsal aspect of the dorsal ring should remain within 12 mm lateral to the midline, and dissection on the superior aspect of the dorsal ring should remain within 8 mm of the midline (21).