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To investigate geometrical determinants of target vessels instability in fenestrated endovascular aneurysm repair (FEVAR), using a computed tomography angiogram postimplantation analysis.We retrospectively reviewed single-center data on consecutive patients undergoing FEVAR (2014-2021). The geometrical analysis consisted in the assessment of bridging stent lengths and diameters, stent conformation, and graft misalignment. Bridging stent length was categorized in three components: protrusion length (PL) into the main endograft, bridging length (BL) between the fenestration and the origin of the target vessel, and sealing length (SL) of apposition in the target vessel. The conformation was measured as the flare ratio (the ratio of maximum to minimum bridging stent diameter within the PL). Horizontal misalignment was measured as the angle between the fenestration and the target vessel ostium on computed tomography angiography axial cuts. The primary end point was freedom from target vessel instability; secondary end points were target vessels primary patency and freedom from related endoleaks. Time-dependent outcomes were estimated as Kaplan-Meier curves; Cox proportional hazards were used to identify the predictors of target vessel instability.There were 46 patients (juxta/pararenal: n = 34 [74%]; thoracoabdominal: n = 11 [26%]), with 147 target arteries incorporated through a bridging stent. Freedom from target vessel instability was 87% (95% confidence interval [CI], 80-94) at 42 months. Primary patency was 98% (95% CI, 96-100) and freedom from endoleak was 85% (95% CI, 76-93). PL (hazard ratio [HR], 1.08; 95% CI, 0.22-5.28; P = .923), sealing length (HR, 0.95; 95% CI, 0.87-1.03; P = .238), and flare ratio (HR, 4.66; 95% CI, 0.57-37.7; P = .149) were not associated with target vessel instability. By multivariate analysis, a BL of more than 5 mm (HR, 4.98; 95% CI, 1.13-21.85; P = .033) was significantly associated with instability. Patients with a BL 5 mm or more had a significantly greater degree of horizontal misalignment (21 ± 12° vs 9 ± 13°; P = .011).An optimal geometrical conformation between the bridging stent and the main endograft at the level of target vessels is warranted to improve the midterm outcomes of FEVAR. A BL of more than 5 mm was associated with a greater risk of target vessel instability, likely as a result of a less accurate endograft alignment. The sizing and planning of FEVAR should be performed to maintain a BL of less than 5 mm.