THE DIRECTIONAL DEPENDENCY OF ASCENDING AORTA BIOMECHANICS DISAPPEARS WITH SEVERE MEDIAL DEGENERATION

Abstract

The study of biomechanics can potentially provide insight into the pathogenesis of aortic complications and improve risk assessment. Aortas are described to have anisotropy, the directional dependency of mechanical properties. We hypothesize that disease-related remodelling of the aortic wall microstructure found in ascending aortic aneurysms results in changes in anisotropy. Excised aneurysmal ascending aortic tissue was obtained at the time of surgery and control tissue was obtained at organ donation or autopsy. The tissue was subjected to biaxial tensile testing. Stress-strain relationships were collected in both the circumferential and axial directions; from this data, both the apparent modulus of elasticity and energy loss were derived. Energy loss describes normalized hysteresis between aortic loading and unloading curves. Movat pentachrome histological staining was performed on each sample and aortic wall composition was quantified colorimetrically. Energy loss was greater in the circumferential direction than in the axial direction for both aneurysmal and control aortas (p<0.0001), while the modulus of elasticity was the same in both directions. This directional dependency in energy loss became diminished in larger aortas (r2=0.15, p=0.01), especially when indexed to body surface area (r2=0.29, p=0.002), aortas with greater energy loss (r2=0.44, p<0.0001), and aortas associated with tricuspid valves (p=0.004). An inflection point was noted in disease severity separating two populations: aortas without any directional dependency in energy loss (isotropic), and those with varying degrees of anisotropy. Aortas with indexed aortic size greater than 3.5 cm/m2 or energy loss greater than 35% were uniformly isotropic. Evaluating the microstructure, aortas with elevated collagen-to-elastin ratios (greater than 2) reflecting severe medial degeneration, were also uniformly isotropic. Aneurysmal aortas with higher collagen-to-elastin ratios tended towards isotropy (r2=0.29, p=0.001). Energy loss detects the anisotropic nature of aortic tissue unlike the apparent modulus of elasticity; this is further evidence that it may be a more sensitive and physiologically relevant metric in the evaluation of aneurysmal disease. Isotropy is associated with severe medial degeneration indicating that destruction of microstructure can be captured by global biomechanics, thereby identifying energy loss isotropy as a marker of disease severity.