Biomechanical properties of ruptured versus electively repaired abdominal aortic aneurysm wall tissue

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 h SEMI) 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/cm(2) vs 82 +/- 9.0 N/cm(2); 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. (J Vasc Surg 2006;43:570-6.) Clinical Relevance: Rupture of an abdominal aortic aneurysm is a deadly event that carries an overall mortality of more than 70%. Nonetheless, there exists no reliable criterion to determine the severity of an aneurysm. The wall of an aneurysm progressively weakens as a result of discordant repair/remodeling mechanisms, which can lead to changes in the mechanical properties of the tissue. We demonstrate here that a decrease in stiffness and an increase in thickness of the tissue (both noninvasively measurable) correlate with a decreased strength. Therefore, we believe that an improved risk prediction criterion could be drawn from our data.