Histology and Biaxial Mechanical Behavior of Abdominal Aortic Aneurysm Tissue Samples

Abstract

Abdominal aortic aneurysms (AAAs) represent permanent, localized dilations of the abdominal aorta that can be life-threatening if progressing to rupture. Evaluation of risk of rupture depends on understanding the mechanical behavior of patient AAA walls. In this project, a series of patient AAA wall tissue samples have been evaluated through a combined anamnestic, mechanical, and histopathologic approach. Mechanical properties of the samples have been characterized using a novel, strain-controlled, planar biaxial testing protocol emulating the in vivo deformation of the aorta. Histologically, the tissue ultrastructure was highly disrupted. All samples showed pronounced mechanical stiffening with stretch and were notably anisotropic, with greater stiffness in the circumferential than the axial direction. However, there were significant intrapatient variations in wall stiffness and stress. In biaxial tests in which the longitudinal stretch was held constant at 1.1 as the circumferential stretch was extended to 1.1, the maximum average circumferential stress was 330 ± 70 kPa, while the maximum average axial stress was 190 ± 30 kPa. A constitutive model considering the wall as anisotropic with two preferred directions fit the measured data well. No statistically significant differences in tissue mechanical properties were found based on patient gender, age, maximum bulge diameter, height, weight, body mass index, or smoking history. Although a larger patient cohort is merited to confirm these conclusions, the project provides new insight into the relationships between patient natural history, histopathology, and mechanical behavior that may be useful in the development of accurate methods for rupture risk evaluation.

On Constitutive Modeling of Arteries

“…Substituting the critical shear (21) into (17) and (18) yields the maximum stable stretch (which also coincides with the maximum stable principal stretch) and the minimum stable stretch (which also coincides with the minimum stable principal stretch), respectively and…”

“…Special (λ 2 = 1) simple normal tests of human abdominal aorta aneurysm (AAA) walls with cruciform specimens have been conducted by Pancheri et al (2017) [21]. Experimental curves of AAA averaged over thirteen patients in both circumferential and axial directions have been used to fit the special simple normal CSE models (38) and (39).…”

Pulsatile flow measurements and wall stress distribution in a patient specific abdominal aortic aneurysm phantom

“…Material parameters in the isochoric part of the energy function were determined using prior strain‐controlled, planar biaxial measurements on tissue samples from an AAA resected during elective repair surgery . In that study, each specimen was assumed to include two families of parallel fibers oriented at angles relative to the circumferential direction, with the unit vector directions denoted by and and given by …”

Computational Evaluation for Age-Dependent Material Nonlinear Behavior of Aortic Wall Tissue on Abdominal Aortic Aneurysms

“…Therefore, the mechanical analysis in identifying the precise material behavior and degradation of abdominal aortic properties according to the major factors should be performed in order to solve these problems. Experimental and computational analyses of the extracted tissues from carcasses, cadavers, and patients have been conducted to identify the precise material behaviors and properties based on various factors, such as vascular location [3][4][5][6][7], layer [8][9][10][11], diameter [12], and smoking preference [5] in leading overseas research institutions for many decades [13][14][15][16][17].…”

“…Moreover, they reviewed the failure stress, strain, and tension based on the location of the aneurysm, orientation of fiber, patient's age and smoking preference, and presented the average stress and strain data and relationship between stress and strain in healthy and diseased tissues. Pancheri et al [7] provided new insights on the relationships between patients' natural histories, histopathologies, and mechanical behavior that may be useful in identifying the rupture risk evaluation method. Tong et al [8] examined the biaxial extension behavior of the intraluminal thrombus and thrombus-covered wall in abdominal aortic aneurysms according to the patient's age, and compared this with the proposed material models.…”