Loss of mechanical directional dependency of the ascending aorta with severe medial degeneration

Biomechanical characterization of the aortic wall may help risk stratify patients with aneurysms. We investigated the degree of anisotropy, the directional dependency of mechanical properties, in control and aneurysmal ascending aortic tissue. We hypothesized that medial degeneration and aortic wall remodeling as found in aneurysmal tissue alter energy loss in both the circumferential and longitudinal directions, thereby reducing anisotropy. Aneurysmal and control ascending aortic tissue excised during surgery was subjected to biaxial tensile testing. Stress-strain relationships were collected in the circumferential and longitudinal directions; from these data, the mechanical properties of energy loss and the apparent modulus of elasticity were derived, and the associated anisotropy indices were calculated. Movat pentachrome histological staining was performed, and aortic wall medial degeneration was quantified. Energy loss was greater in the circumferential than the longitudinal direction, demonstrating significant anisotropy in both normal and aneurysmal aortas (P<.0001). This directional dependency diminished in (a) larger aortas (r2=0.15, P=.01), especially when indexed to body surface area (r2=0.29, P=.002); (b) aortas with greater overall energy loss (r2=0.44, P<.0001); (3) aortas associated with tricuspid valves (P=.004); and (4) higher collagen-to-elastin ratio (r2=0.29, P=.001). Aortas with collagen-to-elastin ratios greater than 2 were uniformly isotropic. Furthermore, the greatest energy loss anisotropy was found on the inner curvature of the aorta (P=.01). Energy loss demonstrates the directional dependency of aortic tissue. Energy loss isotropy is associated with medial degeneration, indicating that microstructural changes can be captured by global biomechanics, thereby identifying it as a marker of disease severity.