Three-dimensional anatomical analysis of the horizontal part of the third segment of the vertebral artery in the low retrosigmoid approach

Iatrogenic injury to the horizontal part of the third segment of the vertebral artery (V3h) is a catastrophic complication of the low retrosigmoid approach. This study aimed to delineate three-dimensional (3D) anatomical variations of the V3h through quantitative analysis and to establish an anatomical risk-stratification framework to improve surgical safety. A multimodal study was performed using 10 cadaveric specimens and 217 clinical computed tomography angiography (CTA) datasets. Key measurements included the V3h-C1 and V3h-OC distances. A novel classification system was developed based on the spatial relationship between the V3h and two surgical reference lines. The mean V3h-C1 and V3h-OC distances were 8.77 ± 2.34 mm and 7.78 ± 2.72 mm, respectively. Only 11.1% of V3h segments were Type I (Closely Adherent), while 88.9% exhibited varying protrusions, with Type IV (Posteriorly Horizontal Projecting) being the most prevalent (55.6%). A high-position anatomical group, defined by V3h-C1 > 10 mm and V3h-OC < 5 mm, accounted for 6.2% of sides, and Type IIIb, characterized by prominent posterosuperior protrusion, accounted for 12.2%. These findings indicate substantial V3h variability and support individualized preoperative CTA-based 3D assessment. The proposed classification and measurements may help identify configurations requiring heightened caution during the low retrosigmoid approach.