Mechanical properties of human aortic aneurysm tissues were measured with a biaxial tensile tester. Fifteen-mm-square specimens were obtained from thoracic aortic aneurysms of various origins and from undilated aortas adjacent to the aneurysms during aneurysmectomy, and were stored frozen until the measurement. Each specimen was stretched biaxially in physiological saline at room temperature at the rate of ~0.2 mm/sec. Although the ordered displacement was set equal for both directions, real strain applied to the specimens was not equibiaxial. The stress-strain curves under equibiaxial stretch were obtained by fitting measured curves with a strain energy function considering material anisotropy. Effects of freezing and ambient temperature on the mechanical properties were evaluated with porcine thoracic aortas. The mechanical properties of the frozen-stored specimens at 23°C were almost similar to those of the fresh specimens at 37 °C. Elastic modulus at zero load averaged for both directions H m i = (H x i +H y i)/2 was higher (P < 0.01) in the aneurysm tissues (1450 ± 250 kPa, mean ± SEM, n = 26) than in the undilated tissues (650 ± 140 kPa, n = 10). Anisotropy index K = |H x i-H y i |/H m i was not significantly different between the aneurysm (20 ± 3%) and the undilated tissues (12 ± 3%) for all specimens. For the specimens whose elastic modulus H m i was smaller than 1 MPa, however, the index K was significantly higher (P < 0.05) in the aneurysm specimens (23.1 ± 5.3%, n = 14) than the undilated tissues (9.5 ± 2.5%, n = 8). These results indicate aneurysm tissues are not only stiffer but also more anisotropic than the nonaneurysmal tissues.

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Rupture properties of thoracic aortic aneurysms (TAAs) were measured in vitro in a pressure-imposed test to predict the ultimate stress of TAAs from their mechanical behavior in a physiological pressure range. Each quadrilateral (ca. 20 9 20 mm 2 ) specimen of TAAs or porcine thoracic aortas (PTAs) was pressurized from the inner wall until rupture or up to 4500 mmHg, while its deformation was being monitored. In-plane stress r and strain e of the specimen were calculated using Laplace's law and deformations of the markers drawn on the specimen surface, respectively. Ultimate stress r max and tangent elastic modulus H were determined from the r-e curve as its maximum stress and slope, respectively. The tangent elastic modulus H of PTA specimens tended to increase with the increase in r, while that of TAA specimens tended to reach a plateau in a low-r region. This tendency was confirmed by fitting a function H = C r (1 À exp(Àr/s r )) to the HÀr relation of specimens: The yielding parameter s r was significantly lower in TAAs than PTAs. Furthermore, the logarithm of the parameter s r correlated significantly with r max , for all specimens. These results may indicate that s r is one of the candidate indices for rupture risk estimation.