Biaxial Tensile Properties of Thoracic Aortic Aneurysm Tissues

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 Hmi = (Hxi+Hyi)/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 = |Hxi-Hyi|/Hmi was not significantly different between the aneurysm (20 ± 3%) and the undilated tissues (12 ± 3%) for all specimens. For the specimens whose elastic modulus Hmi 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.