Ascending aortic geometry and its relationship to the biomechanical properties of aortic tissue.

Objective: The objective of this study was to evaluate the relationship between ascending aortic geometry and biomechanical properties. Methods: Preoperative computed tomography scans from ascending aortic aneurysm patients were analyzed using a center line technique (n = 68). Aortic length was measured from annulus to innominate artery, and maximal diameter from this segment was recorded. Biaxial tensile testing of excised tissue was performed to derive biomechanical parameters energy loss (efficiency in performing the Windkessel function) and modulus of elasticity (stiffness). Delamination testing (simulation of dissection) was performed to derive delamination strength (strength between tissue layers). Results: Aortic diameter weakly correlated with energy loss (r 2 = 0.10; P < .01), but not with modulus of elasticity (P = .13) or delamination strength (P = .36). Aortic length was not associated with energy loss (P = .87), modulus of elasticity (P = .13) or delamination strength (P = .90). Using current diameter guidelines, aortas >55 mm (n = 33) demonstrated higher energy loss than those <55 mm (n = 35; P = .05), but no difference in modulus of elasticity (P = .25) or delamination strength (P = .89). A length cutoff of 110 mm was proposed as an indication for repair. Aortas >110 mm (n = 37) did not exhibit a difference in energy loss (P = .40), modulus of elasticity (P = .69), or delamination strength (P = .68) compared with aortas <110 mm (n = 31). Aortas above diameter and length thresholds (n = 21) showed no difference in energy loss (P = .35), modulus of elasticity (P = .55), or delamination strength (P = .61) compared with smaller aortas (n = 47). Conclusions: Aortic geometry poorly reflects the mechanical properties of aortic tissue. Weak association between energy loss and diameter supports intervention at larger diameters. Further research into markers that better capture aortic biomechanics is needed.

Biomechanical properties of the aortic root are distinct from those of the ascending aorta in both normal and aneurysmal states.

Background: Although aneurysms of the ascending aorta and the aortic root are treated similarly in clinical guidelines, how biomechanical properties differ between these 2 segments of aorta is poorly defined. Methods: Biomechanical testing was performed on tissue collected from the aortic root (normal = 11, aneurysm = 51) and the ascending aorta (normal = 21, aneurysm = 76). Energy loss, tangent modulus of elasticity, and delamination strength were evaluated. These biomechanical properties were then compared between (1) normal ascending and normal root tissue, (2) normal and aneurysmal root tissue, (3) normal and aneurysmal ascending tissue, and (4) aneurysmal root and aneurysmal ascending tissue. Propensity score matching was performed to further compare aneurysmal root and aneurysmal ascending aortic tissue. Clinical and biomechanical variables associated with decreased delamination strength in the aortic root were evaluated. Results: The normal aortic root demonstrated greater viscoelastic behavior (energy loss 0.08 [0.06, 0.10] vs 0.05 [0.04, 0.06], P = .008), and greater resistance against delamination (93 [58, 126] mN/mm vs 54 [40, 63] mN/mm, P = .05) compared with the ascending aorta. Delamination strength was significantly reduced in aneurysms in both the root and the ascending aorta compared with their normal states. Aneurysms of the aortic root matched to the ascending aortic aneurysms in terms of baseline characteristics including size, were characterized by a larger decrease in delamination strength from baseline (Δ -59 mN/mm vs Δ -24 mN/mm). Aging (P = .003) and the presence of hypertension (P = .02) were associated with weakening of the aortic root, while diameter did not have this association (P = .29). Conclusions: The normal aortic root was found to have distinct biomechanical properties compared with the ascending aorta. When aneurysms form in the aortic root, there is less strength against delamination, without other biomechanical changes such as increased energy loss observed in aneurysmal ascending aortas. Age and hypertension were associated decreased aortic wall strength in the aortic root, whereas diameter had no such association.

Ascending aortic aneurysm haemodynamics are associated with aortic wall biomechanical properties.

OBJECTIVES: The effect of aortic haemodynamics on arterial wall properties in ascending thoracic aortic aneurysms (ATAAs) is not well understood. We aim to delineate the relationship between shear forces along the aortic wall and loco-regional biomechanical properties associated with the risk of aortic dissection. METHODS: Five patients with ATAA underwent preoperative magnetic resonance angiogram and four-dimensional magnetic resonance imaging. From these scans, haemodynamic models were constructed to estimate maximum wall shear stress (WSS), maximum time-averaged WSS, average oscillating shear index and average relative residence time. Fourteen resected aortic samples from these patients underwent bi-axial tensile testing to determine energy loss (ΔUL) and elastic modulus (E10) in the longitudinal (ΔULlong, E10long) and circumferential (ΔULcirc, E10circ) directions and the anisotropic index (AI) for each parameter. Nine resected aortic samples underwent peel testing to determine the delamination strength (Sd). Haemodynamic indices were then correlated to the biomechanical properties. RESULTS: A positive correlation was found between maximum WSS and ΔULlong rs=0.75, P = 0.002 and AIΔUL (rs=0.68, P=0.01). Increasing maximum time-averaged WSS was found to be associated with increasing ΔULlong (rs=0.73, P = 0.003) and AIΔUL (rs=0.62, P=0.02). Average oscillating shear index positively correlated with Sd (rs=0.73,P=0.04). No significant relationship was found between any haemodynamic index and E10, or between relative residence time and any biomechanical property. CONCLUSIONS: Shear forces at the wall of ATAAs are associated with local degradation of arterial wall viscoelastic hysteresis (ΔUL) and delamination strength, a surrogate for aortic dissection. Haemodynamic indices may provide insights into aortic wall integrity, ultimately leading to novel metrics for assessing risks associated with ATAAs.

Dependency of energy loss on strain rate, strain magnitude and preload: Towards development of a novel biomarker for aortic aneurysm dissection risk.

Dissection is the most common mode of failure for ascending aortic aneurysms. Currently, failure risk is assessed by measuring aortic diameter, which is insufficient as it misses many dissection patients. This motivated the search for a new biomarker that captures intrinsic tissue material properties related to failure. Energy loss is promising in this regard as it is correlated with microstructure degradation and failure of aneurysms. However, for energy loss to be used clinically, its dependency on in vivo loading conditions, which vary from patient-to-patient, must be determined. In this study, the sensitivity of energy loss to physiological strain rate, magnitude, and preload was examined. Energy loss was found to be relatively insensitive to loading conditions while maintaining a significant correlation with delamination strength as a surrogate for dissection except at low strains. These results can be used for clinical translation of in vivo measurements of energy loss to evaluate aortic dissection risk.

Ascending Aortic Length and Its Association With Type A Aortic Dissection.

Background The aim of this study was to determine the role of ascending aortic length and diameter in type A aortic dissection. Methods and Results Computed tomography scans from patients with acute type A dissections (n=51), patients with proximal thoracic aortic aneurysms (n=121), and controls with normal aortas (n=200) were analyzed from aortic annulus to the innominate artery using multiplanar reconstruction. In the control group, ascending aortic length correlated with diameter (r2=0.35, P<0.001), age (r2=0.17, P<0.001), and sex (P<0.001). As a result of immediate changes in aortic morphology at the time of acute dissection, predissection lengths and diameters were estimated based on models from published literature. Ascending aortic length was longer in patients immediately following acute dissection (median, 109.7 mm; interquartile range [IQR], 101.0-115.1 mm), patients in the estimated predissection group (median, 104.2 mm; IQR, 96.0-109.3 mm), and patients in the aneurysm group (median, 107.0 mm; IQR, 99.6-118.7 mm) in comparison to controls (median, 83.2 mm; IQR, 74.5-90.7 mm) (P<0.001 all comparisons). The diameter of the ascending aorta was largest in the aneurysm group (median, 52.0 mm; IQR, 45.9-58.0 mm), followed by the dissection group (median, 50.3 mm; IQR, 46.6-57.5 mm), and not significantly different between controls and the estimated predissection group (median, 33.4 mm [IQR, 30.7-36.7 mm] versus 35.2 mm [IQR, 32.6-40.3 mm], P=0.09). After adjustment for diameter, age, and sex, the estimated predissection aortic lengths were 16 mm longer than those in the controls and 12 mm longer than in patients with nondissected thoracic aneurysms. Conclusions The length of the ascending aorta, after adjustment for age, sex, and aortic diameter, may be useful in discriminating patients with type A dissection from normal controls and patients with nondissected thoracic aneurysms.

Biomechanics of Aortic Dissection: A Comparison of Aortas Associated With Bicuspid and Tricuspid Aortic Valves.

Background Current methods for aortic dissection risk assessment are inadequate for patients with ascending aortic aneurysms associated with either bicuspid aortic valves (BAVs) or tricuspid aortic valves (TAVs). Biomechanical testing of aortic tissue may provide novel insights and biomarkers. Methods and Results From March 2017 to August 2019, aneurysmal ascending aortas (BAV=23, TAV=23) were collected from elective aortic surgery, normal aortas from transplant donors (n=9), and dissected aortas from surgery for aortic dissection (n=7). These aortas underwent delamination testing in simulation of aortic dissection. Biaxial tensile testing was performed to determine modulus of elasticity (aortic stiffness), and energy loss (a measure of efficiency in performing the Windkessel function). Delamination strength (Sd) was lowest in dissected aortas (18±6 mN/mm) and highest in normal aortas (58±16 mN/mm), and aneurysms fell in between, with greater Sd in the BAV group (37±10 mN/mm) than the TAV group (27±10 mN/mm) (P<0.001). Bicuspid aortopathy was associated with greater stiffness (P<0.001), while aneurysms with TAV demonstrated greater energy loss (P<0.001). Sd decreased by 7.8±1.2 mmol/L per mm per decade of life (r2=0.45, P<0.001), and it was significantly lower for patients with hypertension (P=0.001). Sd decreased by 6.1±2.1 mmol/L per mm with each centimeter increase in aortic diameter (r2=0.15, P=0.007). Increased energy loss was associated with decreased Sd (r2=0.41), whereas there was no relationship between Sd and aortic stiffness. Conclusions Aneurysms with BAV had higher Sd than those with TAV, suggesting that BAV was protective. Energy loss was lower in aneurysms with BAV, and inversely associated with Sd, representing a potential novel biomarker.