RESUMEN
It is generally acknowledged that rupture of an abdominal aortic aneurysm (AAA) occurs when the stress acting on the wall over the cardiac cycle exceeds the strength of the wall. Peak wall stress computations appear to give a more accurate rupture risk assessment than AAA diameter, which is currently used for a diagnosis. Despite the numerous studies utilizing patient-specific wall stress modeling of AAAs, none investigated the effect of wall calcifications on wall stress. The objective of this study was to evaluate the influence of calcifications on patient-specific finite element stress computations. In addition, we assessed whether the effect of calcifications could be predicted directly from the CT-scans by relating the effect to the amount of calcification present in the AAA wall. For 6 AAAs, the location and extent of calcification was identified from CT-scans. A finite element model was created for each AAA and the areas of calcification were defined node-wise in the mesh of the model. Comparisons are made between maximum principal stress distributions, computed without calcifications and with calcifications with varying material properties. Peak stresses are determined from the stress results and related to a calcification index (CI), a quantification of the amount of calcification in the AAA wall. At calcification sites, local stresses increased, leading to a peak stress increase of 22% in the most severe case. Our results displayed a weak correlation between the CI and the increase in peak stress. Additionally, the results showed a marked influence of the calcification elastic modulus on computed stresses. Inclusion of calcifications in finite element analysis of AAAs resulted in a marked alteration of the stress distributions and should therefore be included in rupture risk assessment. The results also suggest that the location and shape of the calcified regions--not only the relative amount--are considerations that influence the effect on AAA wall stress. The dependency of the effect of the wall stress on the calcification elastic modulus points out the importance of determination of the material properties of calcified AAA wall.
Asunto(s)
Aorta Abdominal/fisiopatología , Aneurisma de la Aorta Abdominal/fisiopatología , Calcinosis/fisiopatología , Modelos Cardiovasculares , Aneurisma de la Aorta Abdominal/complicaciones , Calcinosis/complicaciones , Simulación por Computador , Elasticidad , Humanos , Resistencia al Corte , Estrés MecánicoRESUMEN
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/cm(2), 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/cm(2), 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.
Asunto(s)
Aneurisma de la Aorta Abdominal/patología , Rotura de la Aorta/patología , Fenómenos Biomecánicos , Simulación por Computador , Humanos , Imagenología Tridimensional , Factores de RiesgoRESUMEN
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 x 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/cm(2) vs. 67.6 +/- 8.1 N/cm(2), 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.
Asunto(s)
Aneurisma de la Aorta Abdominal/patología , Biopsia , Femenino , Humanos , Masculino , Caracteres Sexuales , Resistencia a la TracciónRESUMEN
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 x 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.