Regional mechanical and microstructural variations in ascending thoracic aortic aneurysms influenced by tricuspid and bicuspid aortic valves

Ascending thoracic aortic aneurysms (ATAAs) exhibit regional biomechanical heterogeneity linked to microstructural changes and comorbidities. This study investigated microstructure and mechanical property variations across six regions of the ATAA wall from 10 patients after repair surgery. Planar biaxial tensile tests and two-photon microscopy were performed to determine material parameters for a microstructure-motivated constitutive model. These parameters describe the behavior of the aortic wall across a wide range of stretch values and ratios. Our results revealed significant inter- and intra-patient differences in mechanical properties , particularly apparent in bicuspid aortic valve (BAV) patients, impacting closely adjacent regions on the outer curvature in the longitudinal orientation. Tissue stiffness was mainly influenced by wall thickness. The thickness of the regional specimens of the wall was mainly affected by aortic valve phenotype and regional differences. Higher stiffness in the proximal-major region and isotropic mechanical behavior in mid-major and distal-major regions in ATAAs with BAV likely stem from altered hemodynamics post-BAV. The mainly isotropic behavior was supported and likely caused by multidirectional collagen reinforcement in these regions, unlike the consistent bi- or uni-directional distribution in other regions. Modeling anisotropy at discrete sections of the wall allowed for constitutive parameters reflecting the spatial distribution of collagen fibers and accurately predicting experimental data. This highlights the necessity of considering regional variations and microstructural changes for improved ATAA risk stratification using patient- or population-specific computational models in ATAA management. Statement of Significance Regional variations in the mechanical properties of aneurysmal ascending thoracic aortas are significant, particularly in bicuspid aortic valve (BAV) patients. To understand these differences, we analyzed six regions using a microstructure-motivated material model accounting for collagen fiber arrangement. Our analysis revealed that BAV is linked to increased stiffness and circumferential fiber alignment near the aortic root, alongside a more complex fiber reinforcement in distal regions of the outer curvature, likely due to altered hemodynamics. This model enables more accurate tissue behavior representation in personalized computational models. Notably, aortic valve phenotype, wall thickness, hypertension, and age were key stiffness determinants across different regions, emphasizing their importance for improved risk assessment and personalized computational models for ascending thoracic aortic aneurysm management.