Local distribution of collagen fibers determines crack initiation site and its propagation direction during aortic rupture

Abstract

Although elucidation of the mechanism of aortic aneurysm rupture is important, the characteristics of crack initiation and propagation sites remain unknown. To determine the microscopic properties of these sites, the characteristics of local strains and constituents at crack initiation and propagation sites were investigated during biaxial stretching of porcine thoracic aortas (PTAs). PTAs were sliced into approximately 50-$\upmu \hbox {m}$-thick sections, and the center of the sections was made especially thin using our previously developed technique. Alpha-elastin and cell nuclei were fluorescently labeled as indices of local elastin density and as a strain marker, respectively. Birefringence and second harmonic generation (SHG) light images were used to determine local collagen distributions. The specimens were then stretched biaxially with a laboratory-made tensile tester under a fluorescent microscope equipped with a birefringence imaging system. Local strains were calculated from the local displacement of the cell nuclei. The degree of alignment and density of local collagen fibers were measured from retardance and SHG images. The strain distributions, specifically the first and second principal, and maximum shear strains, fluorescent intensity of $\upalpha$-elastin, and degree of alignment of collagen fibers, showed insignificant differences between the crack initiation sites and other sites. The retardance and intensity of SHG light at the crack initiation sites were significantly lower than those at other sites for all ($n = 6$) specimens. Cracks tended to propagate along the local direction of the collagen fibers. These results indicate that the local density and direction of collagen fibers play an important role in aorta rupture.