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Abstract: An intraluminal thrombus (ILT) forms in the majority of abdominal aortic aneurysms (AAAs). While the ILT has traditionally been perceived as a byproduct of aneurysmal disease, the mechanical environment within the ILT may contribute to the degeneration of the aortic wall by affecting biological events of cells embedded within the ILT. In this study, the drained secant modulus (E 5∼modulus at 5% strain) of ILT specimens (luminal, medial, and abluminal) procured from elective open repair was measured and compared using unconfined compression. Five groups of fibrin-based thrombus mimics were also synthesized by mixing various combinations of fibrinogen, thrombin, and calcium. Drained secant moduli were compared to determine the effect of the components’ concentrations on mimic stiffness. The stiffness of mimics was also compared to the native ILT. Preliminary data on the water content of the ILT layers and mimics was measured. It was found that the abluminal layer (E 5=19.3kPa) is stiffer than the medial (2.49kPa) and luminal (1.54kPa) layers, both of which are statistically similar. E 5 of the mimics (0.63, 0.22, 0.23, 0.87, and 2.54kPa) is dependent on the concentration of all three components: E 5 decreases with a decrease in fibrinogen (60–20 and 20–15mg/ml) and a decrease in thrombin (3–0.3 units/ml), and E 5 increases with a decrease in calcium (0.1–0.01M). E 5 from two of the mimics were not statistically different than the medial and luminal layers of ILT. A thrombus mimic with similar biochemical components, structure, and mechanical properties as native ILT would provide an appropriate test medium for AAA mechanobiology studies. [Copyright &y& Elsevier]

Abstract: The rupture risk of abdominal aortic aneurysms (AAAs) is thought to be associated with increased levels of wall stress. Finite element analysis (FEA) allows the prediction of wall stresses in a patient-specific, non-invasive manner. We have recently shown that it is important to include the intra-luminal thrombus (ILT), present in approximately 70% of AAA, into FEA simulations of AAA. All FEA simulations to date assume an isotropic, homogeneous material behavior for this material. The purpose of this work was to investigate the multi-axial biomechanical behavior of ILT and to derive an appropriate constitutive relation. We performed planar biaxial testing on the luminal layer of nine ILT specimens obtained fresh in the operating room (9 patients, mean age 71±4.5 years, mean diameter 5.9±0.4cm), and a constitutive relation was derived from this data. Peak stretch and maximum tangential modulus (MTM) values were recorded for the equibiaxial protocol in both the circumferential (θ) and longitudinal (L) directions. Stress contour plots were used to investigate the presence of mechanical anisotropy, after which an appropriate strain energy function was fit to each of the specimen datasets. The peak stretch values for the luminal layer of the ILT were (mean±SEM) 1.18±0.02 and 1.13±0.02 in the θ and L directions, respectively (). The MTM values were 20±2 and 23±3N/cm2 in the θ and L directions, respectively (). From these results and our observation of the symmetry of the stress contour plots for each specimen, we concluded that the use of an isotropic strain energy function for ILT is appropriate. Each specimen data set was then fit to a second-order polynomial strain energy function of the first invariant of the left Cauchy–Green strain tensor, resulting in an accurate fit (average ; range 0.80–0.99). Comparison of our previously reported, uniaxially derived constitutive relation with the biaxially derived relation derived here shows large differences in the predicted mechanical response, underscoring the importance of the appropriate experimental methods used to derive constitutive relations. Further work is merited in an effort to produce more accurate predictions of wall stresses in patient-specific AAA, and viscoelastic behaviors of the ILT. [Copyright &y& Elsevier]

Abdominal aortic aneurysms (AAAs) can typically remain stable until the strength of the aortic wall is unable to withstand the forces acting on it as a result of the luminal blood pressure, resulting in AAA rupture. The clinical treatment of AAA patients presents a dilemma for the surgeon: surgery should only be recommended when the risk of rupture of the AAA outweighs the risks associated with the interventional procedure. Since AAA rupture occurs when the stress acting on the wall exceeds its strength, the assessment of AAA rupture should include estimates of both wall stress and wall strength distributions. The present work details a method for noninvasively assessing the rupture potential of AAAs using patient-specific estimations the rupture potential index (RPI) of the AAA, calculated as the ratio of locally acting wall stress to strength. The RPI was calculated for thirteen AAAs, which were broken up into ruptured ( n= 8 and nonruptured ( n= 5) groups. Differences in peak wall stress, minimum strength and maximum RPI were compared across groups. There were no statistical differences in the maximum transverse diameters (6.8 ± 0.3 cm vs. 6.1 ± 0.5 cm, p= 0.26) or peak wall stress (46.0 ± 4.3 vs. 49.9 ± 4.0 N/cm2, p= 0.62) between groups. There was a significant decrease in minimum wall strength for ruptured AAA (81.2 ± 3.9 and 108.3 ± 10.2 N/cm2, p= 0.045). While the differences in RPI values (ruptured = 0.48 ± 0.05 vs. nonruptured = 0.36 ± 0.03, respectively; p= 0.10) did not reach statistical significance, the p-value for the peak RPI comparison was lower than that for both the maximum diameter ( p= 0.26) and peak wall stress ( p= 0.62) comparisons. This result suggests that the peak RPI may be better able to identify those AAAs at high risk of rupture than maximum diameter or peak wall stress alone. The clinical relevance of this method for rupture assessment has yet to be validated, however, its success could aid clinicians in decision making and AAA patient management. [ABSTRACT FROM AUTHOR]

Objective: The purpose of this study was to evaluate and compare the biomechanical properties of abdominal aortic aneurysm (AAA) wall tissue from patients who experienced AAA rupture with that of those who received elective repair. Methods: Rectangular, circumferentially oriented AAA wall specimens (approximately 2.5 cm × 7 mm) were obtained fresh from the operating room from patients undergoing surgical repair. The width and thickness were measured for each specimen by using a laser micrometer before testing to failure with a uniaxial tensile testing system. The force and deformation applied to each specimen were measured continuously during testing, and the data were converted to stress and stretch ratio. The tensile strength was taken as the peak stress obtained before specimen failure, and the distensibility was taken as the stretch ratio at failure. The maximum tangential modulus and average modulus were also computed according to the peak and average slope of the stress-stretch ratio curve. Results: Twenty-six specimens were obtained from 16 patients (aged 73 ± 3 years [mean ± SEM]) undergoing elective repair of their AAA (diameter, 7.0 ± 0.5 cm). Thirteen specimens were resected from nine patients (aged 73 ± 3 years; P = not significant in comparison to the electively repaired AAAs) during repair of their ruptured AAA (diameter, 7.8 ± 0.6 cm; P = not significant). A significant difference was noted in wall thickness between ruptured and elective AAAs: 3.6 ± 0.3 mm vs 2.5 ± 0.1 mm, respectively (P < .001). The tensile strength of the ruptured tissue was found to be lower than that for the electively repaired tissue (54 ± 6 N/cm2 vs 82 ± 9.0 N/cm2; P = .04). Considering all specimens, no significant correlation was noted between tensile strength and diameter (R = −0.10; P = .55). Tensile strength, however, had a significant negative correlation with wall thickness (R = −0.42; P < .05) and a significant positive correlation with the tissue maximum tangential modulus (R = 0.76; P < .05). Conclusions: Our data suggest that AAA rupture is associated with aortic wall weakening, but not with wall stiffening. A widely accepted indicator for risk of aneurysm rupture is the maximum transverse diameter. Our results suggest that AAA wall strength, in large aneurysms, is not related to the maximum transverse diameter. Rather, wall thickness or stiffness may be a better predictor of rupture for large AAAs. [Copyright &y& Elsevier]

A recent study investigated the association of gender with the growth rate of AAAs and found a significant increase in the growth rate of AAAs in women than in men. On the basis of these observations, we hypothesize that there are gender-associated differences in AAA wall integrity and mechanical strength. The purpose of this study was to explore this hypothesis by comparing the tensile strength of freshly resected AAA tissue specimens between women and men. Seventy-six rectangular specimens (20 mm long × 5 mm wide) from 34 patients (24 male, 10 female) were excised from the anterior wall of patients undergoing open repair of their abdominal aortic aneurysm and tested in a uniaxial tensile tester. Ultimate tensile strength (UTS) was taken as the peak stress obtained before specimen failure. While there were no statistical differences in strength between specimens taken from male and female patients, there was a trend toward a decrease in strength in females as compared to males (87.6 ± 6.7 N/cm2 vs. 67.6 ± 8.1 N/cm2, p = 0.09). To the authors knowledge this work represents the first report of differences in biomechanical properties as a function of gender. The nearly significant decrease in UTS in women versus men reported here may be important in assessing the risk of rupture in AAA. Further testing is warranted to confirm the current trends. [ABSTRACT FROM AUTHOR]