A study on the ultimate mechanical properties of middle-aged and elderly human aorta based on uniaxial tensile test

Abstract

Background: The mechanical properties of the aorta are particularly important in clinical medicine and forensic science, serving as basic data for further exploration of aortic disease or injury mechanisms.

Objective: To study the influence of various factors (age, gender, test direction, anatomical location, and pathological characteristics) on the mechanical properties and thickness of the aorta.

Methods: In this study, a total of 24 aortas (age range: 54-88 years old) were collected, one hundred and seventy-four dog-bone-shaped samples were made, and then the uniaxial tensile test was run, finally, pathological grouping was performed through histological staining.

Results: Atherosclerotic plaques were mainly distributed near the openings of blood vessel branches. The distribution was most severe in the abdominal aorta, followed by the aortic arch. Aortic atherosclerosis was a more severe trend in the male group. In the comparison of thickness, there were no significant differences in age (over 50 years) and test direction, the average thickness of the aorta was greater in the male group than the female group and decreased progressively from the ascending aorta to the abdominal aorta. Comparing the mechanical parameters, various parameters are mainly negatively correlated with age, especially in the circumferential ascending aorta (ε_p "Y = -0.01402*X + 1.762, R² = 0.6882", ε_t "Y = -0.01062*X + 1.250, R² = 0.6772"); the parameters of males in the healthy group were larger, while the parameters of females were larger in atherosclerosis group; the aorta has anisotropy, the parameters in the circumferential direction were greater than those in the axial direction; the parameters of the ascending aorta were the largest in the circumferential direction, the ultimate stress [σ_p "1.69 (1.08,2.32)"] and ultimate elastic modulus [E₂"8.28 (6.67,10.25)"] of the abdominal aorta were significantly larger in the axial direction; In the circumferential direction, the stress [σ_p "2.2 (1.31,3.98)", σ_t "0.13 (0.09,0.31)"] and ultimate elastic modulus (E₂ "14.10 ± 7.21") of adaptive intimal thickening were greater than those of other groups, the strain (ε_p "0.82 ± 0.17", ε_t "0.53 ± 0.14") of pathological intimal thickening was the largest in the pathological group.

Conclusion: The present study systematically analyzed the influence of age, sex, test direction, anatomical site, and pathological characteristics on the biomechanical properties of the aorta, described the distribution of aortic atherosclerosis, and illustrated the characteristics of aortic thickness changes. At the same time, new insights into the grouping of pathological features were presented.

Keywords: atherosclerosis; human aorta; material properties; pathology; uniaxial tensile test.

FIGURE 1

(A) Axial tensile test platform; (B) Clamp; (C) Buffer strip added inside the clamp; (D) Buffer strip material.

FIGURE 2

(A) Complete aorta; (B) Dog bone-shaped aorta sample; (C) Uniaxial tensile test process; the red arrow indicates the beginning of intimal rupture.

FIGURE 3

True stress-strain curve. Note: σ_p-ultimate stress; ε_p-ultimate strain; E₂-ultimate modulus of elasticity; σ_t-middle stress point; ε_t-middle strain point; E₀-starting modulus of elasticity. Zone 1, 2, 3, and four denote the starting, transition, strengthening, and failure phases of the aortic tensile test, respectively.

FIGURE 4

Different regions of a sample. (A) The sample’s broken end exhibits thickening of the intimal collagen fibers, calcium salt deposits, cholesterol crystals under the fibrous cap (HE staining reveals needle cracks), and foam cell formation within the necrotic core (red arrows), defined as fibrillary atherosclerosis; (B) The sample’s mid-section exhibits thickening of the intima of, with the presence of acellular lipid pools in the intima, and near the media where lipid deposition and proteoglycans are abundant but SMCs are absent (black arrows), defined as pathological intimal thickening. (HE staining, 40×, 80×).

FIGURE 5

Classification of the main pathological features in the region of the sample dissection. (A) Normal aorta (20×, 80×); (B) Adaptive intimal thickening is characterized by intimal thickening begins with an increase in smooth muscle cells (SMCs) and proteoglycan-collagen matrix with little or no infiltration of inflammatory cells (40×, 80×); (C) Pathological intimal thickening is characterized by the intima has many fusiform, foamy cells clustered beneath the arterial endothelium, and in some areas of the intima there are numerous proteoglycan and lipid deposition of lipid pools (80×, 200×); (D) Fibrillary atherosclerotic is characterized by calcium salt deposits and cholesterol crystals under the fibrous cap (HE staining shows needle cracks), foam cell formation and infiltration of inflammatory cells are seen within the necrotic core (40×, 80×). ★ indicates the main lesion area. Additional note: Figure 4 and Figure 5D are taken from the same sample.

FIGURE 6

The distribution of aortic atherosclerosis. The concentrated region of hardened plaques is indicated by the red arrow.

FIGURE 7

(A) Male, 58 years old; (B) Female, 58 years old.

FIGURE 8

Comparison of thickness in different directions of stretching. (A) Different genders; (B) Different anatomical parts.

FIGURE 9

Linear regression analysis between age and strain. (A) Ascending aorta; (B) Aortic arch; (C) Thoracic aorta; (D) Abdominal aorta.

FIGURE 10

Comparing the effect of gender on mechanical parameters. (A), (B), (C), (D), (E), and (F) denote the ultimate stress (σ_p), ultimate strain (ε_p), ultimate modulus of elasticity (E₂₎, middle stress point (σ_t), middle strain point (ε_t), and initial modulus of elasticity (E₀), respectively.

FIGURE 11

Comparison of the effects of different test orientations on mechanical parameters. (A–F) denote the ultimate stress (σ_p), ultimate strain (ε_p), ultimate elastic modulus (E₂), middle stress point (σ_t), middle strain point (ε_t), and initial elastic modulus (E₀), respectively.

FIGURE 12

Comparison of mechanical parameters at different anatomical parts. (A–F) denote the ultimate stress (σ_p), ultimate strain (ε_p), ultimate modulus of elasticity (E₂), middle stress point (σt), middle strain point (ε_t), and initial modulus of elasticity (E₀), respectively.

FIGURE 13

Comparison of parameters for different pathological features. (A–F) denote the ultimate stress (σ_p), ultimate strain (ε_p), ultimate modulus of elasticity (E₂), middle stress point (σ_t), middle strain point (ε_t), and initial modulus of elasticity (E₀), respectively.