Novel Radiological Classification of Basilar Artery Tip

The microneurosurgical management of basilar tip aneurysms presents significant challenges due to the deep-seated location of the basilar apex and the variability of available operative corridors. A retrospective analysis of 250 computed tomography angiograms (CTAs) was conducted. Inclusion criteria were excellent quality of the scan and absence of pathological process distorting skull base anatomy. The clinoidal line (CL) was defined in sagital reconstruction between the anterior and posterior clinoid processess. Then, by using multiplanar reconstruction, the vertical distance from this line to the basilar artery tip was measured in coronal reconstructions. The measurements were performed by two evaluators (GT and PPM), and the two senior authors established the classification system (SL and MP). The vertical position of the basilar bifurcation relative to the clinoidal line was categorized into four types: infraclinoidal (< -5 mm), clinoidal (-5 to +5 mm), supraclinoidal (+5 to +10 mm), and high-riding (> +10 mm). The presence of a fetal-type posterior cerebral artery (PCA) was documented, and statistical analyses were performed to assess correlations. The mean basilar artery tip to clinoidal line (BAT-CL) distance was +2.88 ± 4.8 mm. Type 2 (clinoidal) was the most frequent bifurcation type (55.6%), followed by Type 3 (24.0%), Type 1 (12.4%), and Type 4 (8.0%). A statistically significant correlation was observed between the presence of a fetal-type PCA and a lower basilar bifurcation position (p < 0.05). Interobserver reliability was excellent (ICC=0.92) for the BAT-CL measurements, ensuring the reproducibility of the current methodology. This study highlights the significant influence of a fetal-type PCA on the vertical position of the basilar bifurcation. Preoperative identification of this vascular variant is a reliable predictor of a lower bifurcation. While clinical outcome studies are necessary, recognizing this anatomical constraint during preoperative planning can help surgeons anticipate operative complexity and consider lower trajectories, such as the pretemporal trans-cavernous corridor, when appropriate.