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Research Papers

Histology and Biaxial Mechanical Behavior of Abdominal Aortic Aneurysm Tissue Samples

[+] Author and Article Information
Francesco Q. Pancheri

Department of Mechanical Engineering,
Tufts University,
Medford, MA 02155

Robert A. Peattie

Department of Surgery,
Tufts Medical Center,
Boston, MA 02111
e-mail: robert.peattie@tufts.edu

Nithin D. Reddy, Timothy D. Ouellette, Mark D. Iafrati

Department of Surgery,
Tufts Medical Center,
Boston, MA 02111

Touhid Ahamed, Wenjian Lin

Department of Civil and
Environmental Engineering,
Tufts University,
Medford, MA 02155

A. Luis Dorfmann

Department of Civil and
Environmental Engineering,
Tufts University,
Medford, MA 02155;
Department of Biomedical Engineering,
Tufts University,
Medford, MA 02155

1Corresponding author.

Manuscript received December 16, 2015; final manuscript received November 1, 2016; published online January 23, 2017. Assoc. Editor: Jonathan Vande Geest.

J Biomech Eng 139(3), 031002 (Jan 23, 2017) (12 pages) Paper No: BIO-15-1647; doi: 10.1115/1.4035261 History: Received December 16, 2015; Revised November 01, 2016

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.

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Figures

Grahic Jump Location
Fig. 1

(a) AAA tissue sample as received. Severe polymorphous atherosclerotic plaque with precipitated calcium agglomerates is clearly apparent. (b) Tissue specimen undergoing planar biaxial tension. Digital gage marks track the movement of the physical markers. The circumferential and longitudinal directions of the sample relative to the aorta are shown on the right. (c) Schematic diagram of the biaxial test apparatus, showing the sample, water bath, grips, actuator arms, and load cells. Not shown is the video extensometer directly above the specimen used to measure and control deformation in the gage region.

Grahic Jump Location
Fig. 2

Finite element calculation. (a) Mesh used to evaluate the deformation field in the gage region of the cruciform-shaped sample, with length scale. (b)–(d) Contour plots showing homogeneity of the deformation field, (b) axial stretch distribution, (c) circumferential stretch distribution, and (d) shear distribution.

Grahic Jump Location
Fig. 3

Representative histologic images characterizing the wall ultrastructure, 40× magnification, Masson's trichrome stain. Blue—collagen; red—smooth muscle and extravasated cells. All images oriented with the tunica intima facing down. (a) Patient 2, (b) patient 4, (c) patient 5, (d) patient 6, and (e) patient 7. (f) Fractional area of coverage for EvG (highlighting elastin fibers), TRI (highlighting collagen fibers), or anti-SMA (highlighting smooth muscle). dM—disrupted tunica media and dA—disrupted tunica adventitia.

Grahic Jump Location
Fig. 4

Measured nominal stress components in the circumferential and axial directions, Pθθ and Pzz, for patient 2 during the fifth loading cycle in each test. (a) λz = 1.0, λθ → 1.1, (b) λz = 1.1, λθ → 1.1, (c) λz → 1.1, λθ → 1.1, and (d) λz = 1.0, λθ → 1.15.

Grahic Jump Location
Fig. 5

Measured nominal stress components in the circumferential and axial directions, Pθθ and Pzz, during the fifth loading cycle in each test, averaged over all patient samples. (a) λz = 1.0, λθ → 1.1, (b) λz = 1.1, λθ → 1.1, (c) λz → 1.1, λθ → 1.1, and (d) λz = 1.0, λθ → 1.15. Data points—measured values; solid curves—corresponding model fitted curves.

Grahic Jump Location
Fig. 6

(a) Values of mean maximum stresses and (b) of mean maximum elastic moduli in equibiaxial tension in the circumferential, (θ), and longitudinal, (z), directions

Grahic Jump Location
Fig. 7

Representative failure sites of tissue samples failing during biaxial extension tests. Axial arrow indicates the original axial orientation of the tissue sample in the AAA wall. Red arrow indicates the site and orientation of failure. (a) Patient 3, (b) patient 4, (c) patient 5, and (d) patient 6.

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