Investigation on the Regional Loss Factor and Its Anisotropy for Aortic Aneurysms

Abstract

An aortic aneurysm is a lethal arterial disease that mainly occurs in the thoracic and abdominal regions of the aorta. Thoracic aortic aneurysms are prevalent in the root/ascending parts of the aorta and can lead to aortic rupture resulting in the sudden death of patients. Understanding the biomechanical and histopathological changes associated with ascending thoracic aortic aneurysms (ATAAs), this study investigates the mechanical properties of the aorta during strip-biaxial tensile cycles. The loss factor-defined as the ratio of dissipated energy to the energy absorbed during a tensile cycle-the incremental modulus, and their anisotropy indexes were compared with the media fiber compositions for aneurysmal (n = 26) and control (n = 4) human ascending aortas. The aneurysmal aortas were categorized into the aortas with bicuspid aortic valves (BAV) and tricuspid aortic valves (TAV). The strip-biaxial loss factor correlates well with the diameter of the aortas with BAV and TAV (for the axial direction, respectively, R² = 0.71, p = 0.0022 and R² = 0.54, p = 0.0096). The loss factor increases significantly with patients' age in the BAV group (for the axial direction: R² = 0.45, p = 0.0164). The loss factor is isotropic for all TAV quadrants, whereas it is on average only isotropic in the anterior and outer curvature regions of the BAV group. The results suggest that loss factor may be a useful surrogate measure to describe the histopathology of aneurysmal tissue and to demonstrate the differences between ATAAs with the BAV and TAV.

Keywords: age; aortic aneurysm; aortic diameter; collagen and elastin; loss factor; strip-biaxial tensile test.

Figure A1

Variation of incremental modulus: (a) axial and (b) circumferential directions of the TAV group, and (c) axial and (d) circumferential directions of the BAV group, and anisotropy of the incremental modulus for the (e) TAV and (f) BAV groups. Only the axial modulus of the TAV group shows dependency on the region (p = 0.0305) (one-way ANOVA with repeated measure and Bonferroni’s multiple comparisons post-test). Asterisk denotes significant difference: * p < 0.05.

Figure A2

Figure presents data for all available control tissue. (a) Axial and (b) circumferential loss factor; (c) axial and (d) circumferential incremental modulus; and (e) loss factor anisotropy and (f) incremental modulus anisotropy for the control group (the mean values and standards deviations are shown).

Figure A3

Average circumferential loss factor of the BAV group with respect to (a) diameter; and (b) age.

Figure 1

(a) Illustration of the ascending thoracic aorta (ATA) and the aortic quadrants; (b) A tissue sample in its initial load-free configuration (left), and when it is under the biaxial tensile loads (right); (c) Typical non-linear stress-strain behavior of a tissue sample subjected to a loading-unloading tensile cycle and the definition of loss factor.

Figure 2

Average loss factor for the three tissue groups in the axial and circumferential directions. Average axial loss factor depends on groups (p = 0.0419) (Kruskal-Wallis and Dunn’s multiple comparisons post-test). Asterisk shows significant difference: (*) p < 0.05. The figure also presents the histology images for the aortic quadrants of a 76-year-old patient from the TAV group and an 81-year-old patient from the BAV group.

Figure 3

Variation of axial loss factor: (a) TAV; and (b) BAV groups. Variation of circumferential Loss Factor: (c) TAV group; and (d) BAV group; Variation of loss factor Anisotropy: (e) TAV: No regional effect; (f) BAV: significant regional effect (p = 0.0021) (one-way analysis of variance (ANOVA) with repeated measure and Bonferroni’s multiple comparisons post-test). Asterisks denote significant difference: * p < 0.05 and ** p < 0.01.

Figure 4

(a) Average collagen is dependent on groups (p = 0.0257); (b) Average elastin is dependent on groups (p = 0.0271); (c) The average content of collagen and elastin varies amongst the groups (p = 0.0260) (Kruskal-Wallis and Dunn’s multiple comparisons post-test); the images depict the median and interquartile. For the BAV group: (d) collagen depends on region (p = 0.0075); and (e) Col/Ela (p = 0.0458) (one-way ANOVA with repeated measure and Bonferroni’s multiple comparisons post-test). Asterisks denote significant difference: * p < 0.05.

Figure 4

(a) Average collagen is dependent on groups (p = 0.0257); (b) Average elastin is dependent on groups (p = 0.0271); (c) The average content of collagen and elastin varies amongst the groups (p = 0.0260) (Kruskal-Wallis and Dunn’s multiple comparisons post-test); the images depict the median and interquartile. For the BAV group: (d) collagen depends on region (p = 0.0075); and (e) Col/Ela (p = 0.0458) (one-way ANOVA with repeated measure and Bonferroni’s multiple comparisons post-test). Asterisks denote significant difference: * p < 0.05.

Figure 5

Average axial loss factor with respect to (a) aortic diameter for the TAV group; (b) age for the TAV group; (c) aortic diameter for the BAV group; and (d) age for the BAV group.

Figure 6

Col/Ela with respect to diameter for (a) the TAV; and (b) the BAV groups; collagen with respect to diameter for (c) the TAV; and (d) the BAV groups.

Figure 7

(a) Axial loss factor with respect to Col/Ela for the BAV group; (b) Col/Ela with respect to age for this group.

Figure 8

Loss factor anisotropy with respect to incremental modulus anisotropy for (a) the TAV; and (b) the BAV group.