Correlation of Wall Microstructure and Heterogeneous Distributions of Strain in Evolving Murine Abdominal Aortic Aneurysms

Abstract

Purpose A primary deficiency in predicting the progression and rupture-risk of abdominal aortic aneurysms (AAAs) is an inability to assign patient-specific, heterogeneous biomechanical properties to the remodelling aortic wall. Toward this end, we investigated possible correlations between three quantities having the potential for non-invasive measurement (diameter, wall thickness, and strain) and local wall microstructure within evolving experimental AAAs. Methods AAAs were initiated in male C57BL/6J mice via in situ adventitial application of elastase and allowed to progress for 1–4 weeks. Regional in vitro Green strain was assessed using custom panoramic digital image correlation and compared to local geometry and histology. Results Diameter correlated mildly with elastin grade and collagen, when considering all circumferential locations and remodeling times. Normalized wall thickness correlated strongly with normalized collagen area fraction, though with outliers in highly cellular regions. Circumferential Green strain correlated strongly with elastin grade when measured over the range of 20–140 mmHg, though the correlation weakened across a physiologic range of 80–120 mmHg. Axial strain correlated strongly between in vitro and physiologic ranges of pressures. Conclusions Circumferential heterogeneities render diameter a poor predictor of underlying regional microstructure. Thickness may indicate collagen content, though corrections are needed in regions of increased cellularity. In vitro circumferential strain predicts local functional elastin over large ranges of pressure, but there is a need to extend this correlation to clinically relevant pressures. Axial strain in the aneurysmal shoulder region may reflect the elastic integrity within the apical region of the lesion and should be explored as an indicator of disease severity.

Development and Realization of an Experimental Bench Test for Synchronized Small Angle Light Scattering and Biaxial Traction Analysis of Tissues

“…In Figure 8c, an example of the distribution of fiber orientation at a given tension set-point and the E and φ average trend are shown. (6) and laser source (7). View (b): gripper (8), gripper fingers (9), LM linear actuator (10), collimator-focusing lens cage (11) and annular light (12).…”

“…The biomechanical degradation of the tissue is highly correlated with the microstructural components' status: aneurysms are an example of pathologies caused by these alterations. The main components of biological tissues are given by fibers and extracellular matrix [5,6], which are the principal factors responsible for their macroscopic behavior. The strict connection between microstructural components and macrostructural mechanics holds for a wide number of biological actors [7].…”

Should We Ignore What We Cannot Measure? How Non-Uniform Stretch, Non-Uniform Wall Thickness and Minor Side Branches Affect Computational Aortic Biomechanics in Mice

“…In this respect it is important to keep in mind that for many cardiovascular applications it is much more important to correctly estimate the location of strain/stress hotspots than to get the absolute values right. Moreover, previous experimental studies highlighted that aortic wall stiffness shows an inverse correlation with wall thickness so that the global structural stiffness does not change along the aorta (38). This highlights the need for a mouse-specific heterogeneous constitutive model which, to our knowledge, has not been performed on any in vivo simulation of a complex vascular network.…”

(Peak) Wall Stress as an Indicator of Abdominal Aortic Aneurysm Severity

“…The mortality of AAA rupture is increasing, while also expanding more toward younger populations [1–4]. If rupture happens, 50% of patients will die before arriving to the emergency room; moreover, there is up to 80% 30-days risk of mortality for patients undergoing AAA repair surgery [5–7].…”

“…However, from a biomechanical point of view, rupture occurs if the overall effective stress developed inside the aortic wall exceeds the mechanical strength of the vessel tissue [1, 7, 15]. Therefore, the models which utilize peak wall stress (PWS) as an indicator for severity of rupture are more promising.…”