A New “Angle” on Aortic Neck Angulation Measurement

Abstract

Infrarenal aortic neck angulation has been identified as one of the most powerful predictors of endovascular aneurysm repair (EVAR) failure. Whereas the “gold standard” to measure this vital angle is three-dimensional (3D) reconstruction and centerline measurement, most surgeons rely on estimations of angulation based on coronal and sagittal views on computed tomography (CT) imaging. Unfortunately, neither of these two-dimensional views accurately represents the true angle (Fig 1). The importance of calculating a true angle based on perpendicular views has been emphasized in specialties such as orthopedics but not yet in vascular surgery. In response to this need, we have developed a novel trigonometry-based formula (the paired angle formula) that uses coronal and sagittal measured angles to calculate the true angle (Fig 2). The objective of this study was to compare the paired angle formula for estimating true aortic neck angulation with gold standard 3D centerline measurements. Fifty randomly selected patients treated by EVAR at The Ottawa Hospital between 2010 and 2015 were studied. The 3D centerline aortic neck angle measurements were made by radiology staff using Aquarius iNtuition (TeraRecon, Foster City, Calif). The paired angle formula was applied by a vascular surgeon, resident, and student using two-dimensional coronal and sagittal angles from CT imaging to estimate the true angle. Of the 50 patients selected, 47 had preoperative and postoperative CT imaging. The average age at time of procedure was 78 years; 74% of patients were male, and average preoperative aneurysm diameter was 5.73 cm. A total of 94 scans were analyzed, with all except one performed with contrast enhancement. Linear regression found that 3D centerline aortic neck angle measurements were significantly and strongly associated with paired angle formula estimates from all three users (R2 values range, 0.92-0.96; P < .0001). Average user estimate deviations from true angles ranged from −5% to +19%. The paired angle formula also accurately predicted severe angulation (>60 degrees) in pre-EVAR scans (P < .001). Interobserver agreement was very high (Pearson correlation coefficients of 0.87 and 0.88 for the resident and student, respectively, with surgeon set as standard). Similar to pre-EVAR measurements, there was a high correlation between 3D reconstruction measurements and the paired angle formula in post-EVAR measurements (Fig 3). The novel paired angle formula accurately estimates the true aortic neck angle. If validated in other populations with other users, this method could obviate the need for 3D imaging in patients undergoing elective EVAR. Furthermore, the importance of accurate angle measurement is not limited to vascular surgery and has direct relevance to several other specialties including orthopedics.Fig 2Paired angle formula to calculate the true angle of the aortic neck by inputting the angles measured on coronal (C) and sagittal (S) views.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Fig 3Comparison of three-dimensional (3D) centerline measurements, labeled Gold Standard, and the paired angle formula preoperatively (left) and postoperatively (right). Pearson correlation coefficient was very high in the preoperative group (ρ = 0.86; P < .0001) and postoperative group (ρ = 0.84; P < .0001).View Large Image Figure ViewerDownload Hi-res image Download (PPT)