The Effect of Intraluminal Thrombus on Abdominal Aortic Aneurysm Wall Stress Using Nonlinear Elastic Membrane Analysis

Abdominal Aortic Aneurysm (AAA) is a multifactorial disease common in the elderly population (age > 60), the most common form of aortic aneurysms, and currently the 13th leading cause of death in United States. An AAA is defined as the abnormal, focal expansion of the abdominal (subdiaphragmatic) aorta to 50% of the normal diameter of its adjacent segment. Currently, AAA maximum diameter is the criterion used for clinical and treatment guidelines. However, many large AAA beyond the set diameter remain intact while some smaller ones rupture before reaching the critical diameter. Therefore, maximum diameter may not be the most adequate predictor of individual AAA rupture risk, as it does not take into account other patient-specific characteristics beyond the size of the aneurysm. Biomechanical AAA assessment reveals that material failure or rupture occurs when the stresses exceed the strength of the AAA wall. Therefore, mechanical parameters such as peak wall stress (PWS), spatially averaged wall stress (SAWS), and 99th percentile wall stress (99thWS) have been proposed to quantify the wall mechanics of AAA as one of the criteria to predict its rupture risk. These parameters are derived from AAA computational models subject to finite element analysis (FEA). FEA modeling is never performed with individual AAA wall and intraluminal thrombus (ILT) material properties, as these are unknown on a patient-specific basis; instead, population-averaged constitutive material models are used. ILT is a 3D fibrin structure formed between the flowing blood and the AAA wall in approximately 75% of AAAs. The objective of this work is to improve an existing nonlinear elastic membrane analysis (NEMA) algorithm by accounting for the presence of ILT in the AAA wall stress estimation. NEMA is a faster and requires less computational power for estimating AAA wall stress compared to FEA. It makes use of the patient-specific geometry and wall thickness derived from computed tomography angiography (CTA) images, without requiring a material model for the AAA wall. Moreover, NEMA assumes the arterial geometry is a thin membrane and, thus, disregards the presence of additional solid structures in the model, such as ILT. To accomplish the objective of this study, idealized AAA models with various maximum diameters and ILT thicknesses were created and meshed to estimate their wall stresses by FEA simulations using a uniform intraluminal peak systolic pressure of 120 mmHg. The FEA-estimated first principal stress distributions were used along with the local ILT thicknesses and radii to propose a spatially varying non-uniform pressure function that was applied to the AAA wall to account for the presence of ILT in the NEMA simulations. The pressure function was adjusted in NEMA to obtain a 99thWS within 1% of its FEA counterpart, thereby corroborating the validity of the pressure function for idealized AAA geometries. Additionally, for patent-specific AAA geometries, we assumed the stress ratio at the AAA wall with and without ILT follows the FEA results reported in previous literature, which is dependent on the ILT constitutive material model. In addition, we assumed there is an inverse linear relation between Cauchy stress and ILT thickness. To this end, a stiff ILT material results in NEMA-estimated first principal stresses considerably smaller than those calculated with a complaint ILT material. Since patient-specific AAA wall stress cannot be properly validated due to lack of individual wall and ILT material properties, the stresses were used in a backward approach to calculate the spatially varying non-uniform pressure acting on the AAA wall due to the effect of ILT, which was in turn used to re-calculate the first principal stresses. The stress distributions resulting from the forward and backward approaches were similar when using both stiff and compliant ILT materials. This study contributes to the development of a rapid method for estimating patient-specific AAA wall stress that can be used in individual rupture risk assessment, with the goal of improving the clinical decision-making criteria for recommending AAA repair.