Mechanical strength of aneurysmatic and dissected human thoracic aortas at different shear loading modes

Abstract

Rupture of aneurysms and acute dissection of the thoracic aorta are life-threatening events which affect tens of thousands of people per year. The underlying mechanisms remain unclear and the aortic wall is known to lose its structural integrity, which in turn affects its mechanical response to the loading conditions. Hence, research on such aortic diseases is an important area in biomechanics. The present study investigates the mechanical properties of aneurysmatic and dissected human thoracic aortas via triaxial shear and uniaxial tensile testing with a focus on the former. In particular, ultimate stress values from triaxial shear tests in different orientations regarding the aorta׳s orthotropic microstructure, and from uniaxial tensile tests in radial, circumferential and longitudinal directions were determined. In total, 16 human thoracic aortas were investigated from which it is evident that the aortic media has much stronger resistance to rupture under 'out-of-plane' than under 'in-plane' shear loadings. Under different shear loadings the aortic tissues revealed anisotropic failure properties with higher ultimate shear stresses and amounts of shear in the longitudinal than in the circumferential direction. Furthermore, the aortic media decreased its tensile strength as follows: circumferential direction >longitudinaldirection> radial direction. Anisotropic and nonlinear tissue properties are apparent from the experimental data. The results clearly showed interspecimen differences influenced by the anamnesis of the donors such as aortic diseases or connective tissue disorders, e.g., dissected specimens exhibited on average a markedly lower mechanical strength than aneurysmatic specimens. The rupture data based on the combination of triaxial shear and uniaxial extension testing are unique and build a good basis for developing a 3D failure criterion of diseased human thoracic aortic media. This is a step forward to more realistic modeling of mechanically induced tissue failure i.e. rupture of aneurysms or progression of aortic dissections.

Keywords: Aortic aneurysm; Aortic dissection; Connective tissue disorder; Thoracic aorta; Ultimate stress.

Fig. 1

(a) Representative photograph of a human ascending aortic aneurysm sample (CI) with a severely dilated diameter; (b) typical specimen with an incision of ∼ 1 mm for in-plane shear testing, which is glued to the upper specimen holder before insertion in the testing apparatus; (c) photograph of an ‘in-plane’ specimen subjected to simple shear loading; (d) ruptured into two parts and successfully tested ‘in-plane’ specimen.

Fig. 2

Sketches of six shear modes defined with respect to the radial (r-axis), circumferential (θ-axis), and longitudinal (z-axis) direction on an orthotropic tissue piece. Arrows indicate shear directions with corresponding shear stresses τ_ij and i; j ∈ {r;θ; z}, where i denotes the normal vector of the plane that is being sheared, and j denotes the direction in which the face is shifted. For example, (a) shows ‘in-plane’-shear modes in the zθ-plane with shear in z- and θ-directions, while (b) and (c) show ‘out-of-plane’-shear modes in the rz- and rθ-plane, respectively.

Fig. 3

Sketches of ‘in-plane’ shear test specimens in the longitudinal direction (rz-mode), (a), and in the circumferential direction (rθ-mode), (b), to obtain shear properties of the zθ-plane. The shaded surfaces are glued to the specimen holders of the apparatus and sheared. The specimen is longer in the direction in which it is being sheared (∼5 mm). On the shorter edge an incision of 1 mm parallel to the shearing direction is introduced (dashed lines). The remaining area (thick-lined black rectangular) is sheared until rupture occurs. Arrows indicate the shear directions.

Fig. 4

Sketches of ‘out-of-plane’ shear test specimens: (a) shear properties in the rz-plane; (b) shear properties in the rθ-plane obtained from these tests. The shaded surfaces are glued to the specimen holders of the apparatus and sheared. The specimen is longer in the direction in which it is being sheared (8 mm). On the shorter edge (3 mm), incisions parallel to the shearing direction are introduced from both sides (dashed lines). The thick-lined black parallelogram between the incised surfaces is the sheared surface, with the dimension of ∼1 mm, parallel to the longer edge. Arrows indicate the shear directions.

Fig. 5

Cauchy shear stress vs. amount of shear relationship during ‘in-plane’ shear tests of aneurysmatic human thoracic aortic tissues: (a), (b) ‘in-plane’ shear behavior within the rθ- and rz-modes of aneurysmatic tissues, respectively; (c), (d) ‘in-plane’ shear behavior within the rθ- and rz-modes of aneurysmatic tissues with connective tissue disorders (CTD), respectively.

Fig. 6

Cauchy shear stress vs. amount of shear relationship during ‘out-of-plane’ shear tests of aneurysmatic human thoracic aortic tissues: (a), (b) ‘out-of-plane’ shear behavior of aneurysmatic tissues within the zθ- and θz-modes, respectively; (c), (d) ‘out-of-plane’ shear behavior of aneurysmatic tissues with connective tissue disorders (CTD), respectively.

Fig. 7

Cauchy shear stress vs. amount of shear relationship during ‘in-plane’ shear tests of dissected human thoracic aortic tissues: (a), (b) in the rθ- and rz-modes, respectively.

Fig. 8

Representative SHG images of specimen AIX showing the collagen architecture: (a), (c) images taken from the rθ-plane and (b), (d) images from the zθ-plane of shear test samples in circumferential direction. Panels (a), (b) represent the planes normal and parallel to the plane of shearing of an ‘in-plane’ test sample, respectively; panels (c), (d) represent the planes parallel and normal to the shearing plane of an ‘out-of-plane’ test sample, respectively. White bars indicate 10 0 μm.