Effect of Aneurysm and Bicuspid Aortic Valve on Layer-Specific Ascending Aorta Mechanics

Previous studies have not examined the participation of intimal, medial, and adventitial layers in providing mechanical strength to the ascending thoracic aortic aneurysm (ATAA) wall compared with the nonaneurysmal aorta. In this study we compared the mechanical properties of intact wall and its layers among ATAAs and nonaneurysmal aortas, with explicit consideration of the effects of valve morphology; that is, bicuspid aortic valve (BAV) versus tricuspid aortic valve (TAV), and aortic quadrant.

Whole ATAAs were taken from patients undergoing elective repair and nonaneurysmal aortas from age-matched autopsy subjects. These were cut into 2 circumferential and longitudinal tissue strips for the intact wall and its layers per quadrant, permitting examination of the aortic wall as a multilayered structure. Tissue underwent tensile testing for determination of failure properties.

Intact wall and layer-specific failure stretches (ie, extensibilities) were significantly greater in nonaneurysmal and BAV-ATAA than in TAV-ATAA, unaccounted for by elastin/collagen content changes. Intact wall failure stress (ie, strength) was significantly greater in BAV-ATAA than in TAV-ATAA, in analogy with medial failure stress. Failure stress and stretch associated negatively with age in most subject groups, layers, and intact wall, but failure stretch correlated positively with residual stretch (ie, structural bonds between layers).

No mechanical vulnerability of BAV-ATAA was found, corroborating current conservative guidelines regarding the management of bicuspid aortopathy. Weakening and added vulnerability was found in patients with valvular deficiency, aortic root aneurysm, hypertension, and hyperlipidemia. Aging led to increased susceptibility to dissection initiation or full rupture, or both, in both patient classes.