Directional Wall Strength in Saccular Brain Aneurysms from Polarized Light Microscopy

Abstract

The aneurysm wall, which must withstand arterial blood pressure, is composed of layered collagen. Wall strength is related to both collagen fiber strength and orientation. When the aneurysm enlarges, the amount and organization of the collagen fibers change, potentially increasing the risk of rupture. We studied the directional organization and molecular strength of the collagen fibers layer by layer across the walls of four aneurysms in order to measure their mechanical integrity. The technique incorporates the birefringent properties of collagen, enabling us to use linearly polarized light for measuring the orientation of the fibers, and the Sénarmont compensator to measure the birefringence and thus mechanical strength. Intact aneurysms were obtained at autopsy, fixed at physiological pressure, sectioned at 4 microm, and stained with 0.05% picrosirius red. By combining birefringence and orientation data we estimated tensile strength as a function of direction on the aneurysmal wall. The average breaking strength of the wall ranged from 0.73 to 1.9 MPa. Comparing the weakest to the strongest direction, the breaking strength varied by a factor of up to 2X, implying a significant degree of mechanical anisotropy.

A theoretical model for fibroblast-controlled growth of saccular cerebral aneurysms

“…Again, solutions for different values of λ pre are included. The ulti- Critical region mate stress of cerebral aneurysmal tissues have been obtained experimentally and falls in a range of about 0.5-2.0 MPa MacDonald et al, 2000). This critical range is also indicated in Fig.…”

“…Most bifurcations of the cerebral vasculature are structurally stable, but a small number develop a weakness that causes the wall to expand outwardly in the region near the flow divider of the branching artery (Austin et al, 1993;MacDonald et al, 2000;Rowe et al, 2003). Some measurements of the macroscopic mechanical properties of cerebral arteries and aneurysms exist (Coulson et al, 2004;Monson et al, 2003Monson et al, , 2005Scott et al, 1972;Steiger, 1990;Tóth et al, 1998Tóth et al, , 2005 and the structural organisation of these tissues is fairly well documented (Canham et al, 1991b(Canham et al, ,a, 1992(Canham et al, , 1996(Canham et al, , 1999Finlay et al, 1991Finlay et al, , 1995Finlay et al, , 1998Hassler, 1972;MacDonald et al, 2000;Rowe et al, 2003;Smith et al, 1981;Whittaker et al, 1988). In the aneurysmal wall, the tunica media and the internal elastic lamina have often disappeared or are severely fragmented (Abruzzo et al, 1998;Sakaki et al, 1997;Stehbens, 1963;Suzuki and Ohara, 1978;Tóth et al, 1998).…”

Intracranial Aneurysmal Pulsatility as a New Individual Criterion for Rupture Risk Evaluation: Biomechanical and Numeric Approach (IRRAs Project)

“…In the present study, the aneurysm wall was considered as isotropic and homogeneous, but the anisotropy of this kind of biologic material is well known. 34 Nevertheless, isotropy and homogeneity were assumed because they most probably do not change the trends observed when comparing soft and stiff materials in this range of physiologic solicitation. To characterize the behavior of blood, more sophisticated models can be used to account for non-Newtonian effects, especially in the aneurysmal sac (Cebral et al 35 and Sforza et al 36 ).…”

Cardiovascular Mechanics

“…The stress, however, is also influenced by the thickness increase of the aneurysmal wall, which is larger for higher axial stretches and thereby reduces the stress to some extent. The strength of cerebral aneurysmal tissue has been experimentally estimated to 0.5-2.0 MPa [103,104]. The peak stresses in our model for the various axial stretches and the medial collagen fiber angles are 0.58-0.63 MPa, and are of the same order.…”

“…For the lowest axial stretch considered (λ L = 1.0), the thickness was 122 µm, and for the largest (λ L = 1.4) 137 µm. The thickness increases are in the range of experimentally determined values [104], where the thickness of larger cerebral aneurysms is between 116 and 212 µm.…”