Quantitative analysis of unusual entrance of the vertebral artery into the cervical foramen (V2 segment) and its clinical implications

Unusual entrance of the vertebral artery into the cervical transverse foramen (UE-V2S) is a relatively common clinical anomaly. Because spinal surgeons should understand the anatomical characteristics and clinical implications of this condition, we aimed to provide a description based on a very large number of UE-V2S anomaly cases. We retrospectively analyzed 2207 three-dimensional head-and-neck computed tomographic angiograms (CTAs) that did not have specific vascular abnormalities. After confirming which cases had an unusual vertebral artery (VA) entrance into the transverse foreman (TF), we measured their vertebral artery diameter (VAD), transverse foramen area (TFA) and bilateral pedicle width (PW) from C3 to C7. The shortest horizontal distance from the vertical line in the medial margin of the TF to VA (D-TFVA) was measured in the extra-osseous region to estimate the exact location of the extra-osseous VA, except at the C7 level. An unusual V2 entrance was observed in 11.4 % (252 patients) of all 2207 consecutive patients and 6.5 % (268 courses) of all 4414 courses. The prevalence rankings for the different measures were as follows: unilateral UE-V2S = E5 > E4 > E7 > E3; bilateral UE-V2S = E5 (bilateral) > E4 (Rt) with E5 (Lt) > E4 (bilateral). Generally, the VAD of the anomaly side was statistically smaller than the normal contralateral side, which can induce a smaller TFA value at all sub-axial levels. Cervical pedicle fixation was preferable in the adjacent lower segment of unusual VA entrance level than the normal side in this study due to a broader PW, which was evident on the anomaly side. However, there was no statistical evidence that the side of the C7 entrance of the VA had a narrow PW. The lowest D-TFVA value was -3.8, indicating that we should take care not to exceed 3.8 mm medially into the vertical line of the medial TF wall during dissection when taking an anterior cervical approach. To avoid an unexpected VA injury and to improve the efficiency of even routine cervical operations, spinal surgeons should determine whether the patient has a UE-V2S and have a full understanding of the clinical characteristics of this anomaly.