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To determine the renal artery to aorta relationship in the setting of thoracoabdominal aortic aneurysm (TAAA) in order to help strategize preoperative stent-graft planning, device design, and deployment technicalities.The preoperative computed tomography (CT) studies of 147 patients who underwent TAAA repair between 2005 and 2008 were retrospectively reviewed. The Crawford classification of the TAAA, the renal artery implantation angle (RAIA), and the maximal aortic diameter were determined using 3-dimensional imaging analysis (centerline of flow). RAIAs were determined to be positive or negative as a function of their relative position above or below the plane perpendicular to the centerline of flow at the level of the renal ostia. RAIAs and maximum aortic diameters were compared between types II/III TAAA (n=72) and type IV TAAA (n=75), stratified by side, and examined for correlation.Maximal aortic diameter was not significantly different between the 2 groups: 67.5±13.4 mm for type II/III versus 65.3±12.5 mm for type IV (p=0.3). There was no correlation between the maximal aortic diameter and the RAIAs. RAIAs in type II/III TAAAs were commonly orthogonal to the aortic centerline (mean -5.7°±19.1° on the right and -2.8°±22.4° on the left, respectively), while type IV TAAAs had downward pointing renal arteries (mean -24.1°±18.4° and -20.4°±18.8° for the right and left, respectively). There was a significant difference between the two groups regarding RAIAs on both sides (p<0.00001).The primary location of longitudinal aortic growth will drive the RAIA in a cranial or caudal direction. When the disease process is largely located below the renal ostia, infrarenal aortic lengthening drives the renal ostia cranially, forcing the implantation angle of the renal ostia to be caudally directed. The opposite occurs in type II or III TAAAs, where the bulk of disease is above the renal arteries, driving the ostia down to create RAIAs that are nearly orthogonal to the centerline of flow. Utilization of this data could result in endovascular grafts designed with branches replacing fenestrations for renal artery perfusion.