Yielding Phenomena of Aortic Wall and Intramural Collagen Fiber Alignment: Possible Link to Rupture Mechanism of Aortic Aneurysms

Abstract

Estimation of wall strength of the aortic aneurysms is necessary for the prediction of their rupture risk. We previously found a significant correlation between their tensile strength σ MAX and a yielding parameter τ σ , which is the stress when tangent elastic modulus reaches at 63% of the plateau level. This may indicate that the wall strength is estimated from their pressure-diameter relationship. Here we show a possible mechanism of the correlation between τ σ and σ MAX by focusing on alignment of collagen fibers. Thin (150-µm) slices of porcine thoracic aortas were uniaxially stretched in circumferential direction until failure under a microscope, and a retardance, a phase shift when polarized light passes through a birefringent material, was measured as the degree of collagen fiber alignment. Strength σ MAX correlated significantly with τ σ as obtained previously. The retardance increased with the increase in the stress and reached a plateau at the stress σ Ret-plateau , indicating that σ Ret-plateau is the stress at which most of the intramural collagen fibers have aligned. The stress σ Ret-plateau correlated significantly with τ σ and both parameters has similar values. This may indicate that the aortic wall yields when all of collagen fibers become straight. Smaller σ Ret-plateau means that most collagen fibers are stretched and loaded at smaller stress, resulting in failure at smaller stress. This seems to be a reason for the significant correlation between τ σ and σ MAX .

Machine learning–aided exploration of relationship between strength and elastic properties in ascending thoracic aneurysm

“…The same correlation was also reported for aorta root aneurysms . Sugita et al found that the strength correlates significantly with a “yield point,“ namely, a particular stress at the early phase of response . The importance of the latter finding lies in that the yield point is more likely in in vivo stress range.…”

“…It is remarkable that the tension at the maximum curvature location correlates significantly with the strength (Figure ). Sugita et al reported that the ATAA strength correlates strongly with the stress at which tangent elastic modulus reaches 63% of the plateau level. The present finding is in congruence with these reports.…”

“…The exponential response at the later stage is almost entirely due to collagen, which becomes taut at this stage . Since elastin is significantly reduced in ATAA, collagen fibers may be recruited earlier, resulting in an early transition and possibly a short transition period . An early recruitment may lead to a quick buildup of stress and consequently, an early rupture.…”

Characteristics of thoracic aortic aneurysm rupture in vitro

“…Martin et al [22] developed a predictive model to assess the in vivo rupture risk of ATAAs based on the measured strength and estimated wall stress. Sugita et al [31] used an inflation test to measure the bi-biaxial properties, and reported a strong correlation between strength and a characteristic stiffness modulus [30]. On the related subject of abdominal aortic aneurysm (AAA) strength, Vorp's group [28,33] measured the tensile strength of AAA tissues.…”

Local properties and rupture characteristics of thoracic aortic aneurysm tissue

“…The same correlation was also reported for aorta root aneurysms [64]. Sugita et al found that the strength correlates significantly with a 'yield point', namely a particular stress at the early phase of response [117,116]. The importance of the latter is that the yield point is more likely in in vivo stress range.…”