Region-specific biomechanical characterization of ascending thoracic aortic aneurysm of hypertensive patients with bicuspid aortic valves

Hypertension and bicuspid aortic valve (BAV) are key clinical factors that may affect local biomechanical properties of ascending thoracic aortic aneurysms (ATAAs). This study sought to investigate regional differences in biaxial mechanical properties of the ATAAs for the hypertensive patients with BAV. Fresh ATAA samples were harvested from 16 hypertensive patients (age, 66 ± 9 years) undergoing elective aortic surgery. Biaxial extension tests were employed to characterize region-specific biaxial mechanical behaviors of the hypertensive BAV-ATAAs. A material model was used to fit biaxial experimental data to obtain model parameters in different regions. Histological analysis was performed to investigate the underlying aortic microstructure and to determine percentages of elastic and collagen fibers. Mechanical behaviors of the hypertensive BAV-ATAAs were nonlinear and anisotropic for most specimens from anterior, lateral and posterior regions. Under the equibiaxial stresses, the ATAA tissues in the lateral region had significantly lower extensibility and significantly higher stiffness in both circumferential and longitudinal directions when compared with the posterior and medial regions. The material model was able to fit regional biaxial data well. Histology showed that laminar structures of elastic fibers were mainly disrupted in the anterior and lateral regions in which, however, pronounced collagen fiber hyperplasia was observed. Moreover, there was a strong positive correlation between circumferential aortic stiffness and patient age in the anterior and lateral regions. Our results suggest that elastic properties in the lateral and anterior regions are more deteriorated than those in the posterior and medial regions for the hypertensive BAV-ATAAs. Thus, the outer curvature of the ATAA wall should be regarded as special quadrants that may be highly susceptible to microstructural changes and may have a substantial impact on aneurysm growth.