Abdominal Aortic Aneurysm Rupture Risk Prediction Based on Computer-aided Vascular Wall Stress Assessment Using Finite Element Method – The Future of Decision Making Process

Abstract

Introduction - Abdominal aortic aneurysm (AAA) is one of the most important conditions in vascular surgery1. Since in majority of cases AAA is asymptomatic often the first sign of AAA is its rupture with very high mortality, even when patient is urgently treated2. According the European guidelines3 when an asymptomatic AAA is identified the next course is directed only by its maximal diameter – if it is more than 5.5 cm an intervention is indicated. Despite the guidelines the literature says that 10-24 % of ruptured AAAs is less than 5.5 cm in diameter (small AAA)4. Clinical goal of this work was to determine a number of small ruptured AAAs and identification of possible risk factors leading to rupture small AAAs. Main goal of the project was to create an auxiliary diagnostic tool for AAA rupture risk assessment using 3D computer-aided modeling of AAA with vascular wall stress analysis using finite element method5 (FEM). Methods - The portion of small ruptured AAA was assessed in our center between years 2009-2016 together with possible risk factor for small AAA rupture. In the experimental part we describe the FEM and its use in vascular wall stress assessment of AAA. 3D computer model of AAA is created from CT angiographic images and wall stress is assessed using FEM and afterwards rupture risk is derived. Data gathered from mechanical tests of AAA wall specimens6 are used for making original mathematical more precise. This process was repeated with more than one hundred samples to get a model with the highest efficiency. Efficiency of the method was evaluated in blinded test confirming its ability to identify AAA with high risk of rupture (the set of CT images of both asymptomatic AAAs and ruptured AAAs from the time before the rupture was used). Results - Out of 90 patients with AAA rupture there were 15 with small AAA (16.7 %). Risk factors for AAA rupture were tested in our patient population and compared with literature. Female gender was not confirmed as a risk factor for small AAA rupture (contrary to literature)7, role of chronic obstructive pulmonary disease remains controversial and there was a significant correlation between number of ruptured AAA and the season of the year. Main outcome of the project is a functional diagnostic tool capable of AAA vascular stress analysis from CT findings and determination of rupture risk of particular AAA (Picture 1). Three criteria were compared according the efficiency and usefulness for clinical praxis (Peak Wall Rupture Risk, Probabilistic Rupture Risk Index and maximal diameter). Proving the Probabilistic Rupture Risk Index to be the best criterion according to efficiency and applicability. The discriminative powers of the predictors were 0.878, 0.859 and 0.789 respectively for PRRI, PWRR and the maximum diameter8. Conclusion - Portion of small ruptured AAA is clinically significant due to a high mortality of this condition. Current guidelines estimate the rupture risk based only on AAA diameter, but this criterion is not suitable for all AAA cases. Particular risk factors are describe in literature with controversial impact on AAA rupture risk, the same conclusion can be found in our patient population. New methods of vascular wall stress assessment using the 3D computer-aided modeling and FEM analysis have a strong potential to improve AAA rupture risk assessment and thus improve diagnostic and decision making process in clinical praxis. Supported by Ministry of Health of the Czech Republic grant no. 17-29701A. All rights reserved.