Middle Cerebral Artery Bifurcation Geometry and Aneurysm Formation: A Propensity Score-Matched 3D Rotational Morphometry Study

Background/Objectives: The middle cerebral artery (MCA) bifurcation is a common site of intracranial aneurysms. We evaluated associations between MCA bifurcation geometry and aneurysm presence and explored relationships between rupture and aneurysm size using a standardized 3D morphometry pipeline. Methods: This single-center, retrospective, propensity score-matched case–control study included 133 MCA bifurcation aneurysms and 266 matched controls, with bifurcation and sac-level geometry quantified using 3D rotational DSA. Associations with aneurysm presence were evaluated using generalized estimating equation (GEE) logistic regression clustered by patient ID. The discriminatory ability of individual geometric markers was assessed using ROC-AUC. Results: After matching, baseline covariates were balanced. In univariate comparisons, aneurysm-bearing bifurcations had a wider angle between the two M2 branches (opening angle), smaller dominant and non-dominant M1–M2 branch angles, smaller parent diameters and cross-sectional areas, a higher opening angle-to-parent diameter ratio (opening-to-caliber index), and lower parent-to-dominant-branch and daughter-branch diameter ratios. In multivariable GEE analysis, a larger opening angle, smaller dominant-branch angle, and lower D1/D2 ratio were independently associated with aneurysm presence; each 5° increase in opening angle corresponded to approximately 40% higher odds. The opening-to-caliber index showed the numerically highest single-marker ROC-AUC but did not remain independently associated with aneurysm presence after adjustment. Among the aneurysms, rupture was not associated with bifurcation geometry; ruptured aneurysms were larger. Across size strata, the opening angle and opening-to-caliber index increased, and the dominant-branch angle decreased. Conclusions: MCA bifurcation aneurysm presence was associated with a wider, more asymmetric bifurcation geometry, whereas rupture appeared more closely related to sac-level morphology.