This study investigated how human ascending thoracic aortic aneurysm biomechanics change across the clinically important 5 cm surgical threshold and whether aneurysm diameter adequately reflects the mechanical condition of the aortic wall. Ascending aortic tissue was collected from 71 patients undergoing elective ATAA repair and 31 controls obtained in the heart-transplant setting. Patients with aneurysms were divided into groups with maximum ascending aortic diameter below 5 cm or at least 5 cm. The authors combined planar biaxial mechanical testing with opening-angle and residual-stretch measurements to characterize nonlinear tissue stiffness, deformation, and circumferential residual stress. Opening angle increased progressively from 182 ± 10° in controls to 261 ± 12° in ATAA <5 cm and 310 ± 14° in ATAA ≥5 cm, indicating substantial residual-stress remodeling with aneurysm enlargement. In contrast, derived residual-stretch measures did not differ significantly among groups. Biaxial testing showed that aneurysmal tissue was generally stiffer than control tissue in both circumferential and longitudinal directions at membrane tensions of 60 and 120 N/m. However, stiffness did not show a discrete additional increase when aneurysm diameter crossed 5 cm. At 60 N/m, circumferential stiffness increased from 542 ± 22 N/m in controls to 625 ± 25 N/m in ATAA <5 cm and 696 ± 26 N/m in ATAA ≥5 cm, while longitudinal stiffness increased from 497 ± 25 N/m to 595 ± 26 N/m and 676 ± 33 N/m, respectively. At 120 N/m, both aneurysm groups also generally remained stiffer than controls. Stiffness correlated positively with unloaded aortic radius, particularly at 60 N/m, but substantial overlap existed among the biomechanical distributions of controls and both aneurysm-size groups. Estimated circumferential deformation between unloaded ex vivo and loaded in vivo configurations increased with aneurysm size, with Green strain rising from approximately 0.12 in controls to 0.23 in ATAA <5 cm and 0.40 in ATAA ≥5 cm. Overall, the findings indicate that material stiffness and residual stress remodel in complementary but non-synchronous ways during ATAA enlargement and that the 5 cm diameter threshold does not define two mechanically distinct aneurysm populations. The authors therefore propose that patient-specific biomechanical phenotyping may eventually complement diameter-based assessment of aneurysm progression and risk.
CELLSCALE INSTRUMENT USED
BioTester
A CellScale BioTester 5000 with 5 N capacity was used for planar biaxial mechanical characterization of human ascending aortic wall specimens from control subjects and patients with ascending thoracic aortic aneurysms below and above the 5 cm surgical threshold. Up to four approximately 1 × 1 cm square specimens were harvested around the circumference of the tubular ascending aorta from 55 ATAA patients and 18 controls. Unloaded wall thickness was measured at three locations before testing. Specimens were mounted on the BioTester, preconditioned for 10 cycles, and then subjected to nine biaxial loading protocols using a previously established procedure. For the present analysis, the authors selected the equibiaxial protocol to provide equal loading in the circumferential and longitudinal directions and enable standardized comparisons among groups. Second Piola-Kirchhoff membrane tension versus Green strain curves were constructed from the BioTester data, and Green strain and local tangent stiffness were evaluated at membrane tensions of 60 and 120 N/m in both anatomical directions. The BioTester measurements showed greater aortic-wall stiffness in both ATAA groups than in controls at several loading conditions but no clear stiffness transition between aneurysms below and above 5 cm. At 60 N/m, circumferential stiffness was 542 ± 22 N/m in controls, 625 ± 25 N/m in ATAA <5 cm, and 696 ± 26 N/m in ATAA ≥5 cm; longitudinal stiffness was 497 ± 25, 595 ± 26, and 676 ± 33 N/m, respectively. At 120 N/m, circumferential stiffness was 1148 ± 85 N/m in controls, 1636 ± 138 N/m in ATAA <5 cm, and 1603 ± 84 N/m in ATAA ≥5 cm. These measurements demonstrated substantial mechanical remodeling before the 5 cm surgical threshold is reached and revealed considerable patient-to-patient heterogeneity within each diameter category, providing the experimental basis for the study's conclusion that aortic diameter and mechanical phenotype are related but not interchangeable.
