Sterically Inhomogenous Viscoelastic Behavior of Human Saccular Cerebral Aneurysms

Abstract

To clarify the mechanism leading to the development and rupture of intracranial aneurysms, tensile strength and viscoelastic parameters of 22 human saccular aneurysms were investigated. Meridional and circumferential strips from the thin and the thick part of the aneurysm sack and 18 control strips from the basilar artery of 8 patients with pathologies not affecting the cerebral arterial system were studied. The length of the strips was increased in 200-µm steps, while distending force was recorded. Tensile strength and viscoelastic parameters were computed. In both directions, tensile strength of thick strips was significantly lower than that of controls. In the meridional direction, tensile strength of thin strips was significantly larger than that of thick ones (14.5 ± 4.1 × 10⁶ vs. 7.5 ± 2.0 × 10⁶ dyn/cm², p < 0.05). In the circumferential direction, thin strips tore at lower strain values than thick ones (29 ± 4 vs. 55 ± 16%, p < 0.05). Viscoelastic parameters changed in parallel. In circumferential direction, values of thick and thin strips were significantly lower than those of controls. In the meridional direction, values of thin strips were significantly higher than those of the thick ones. These observations show that characteristic mechanical deterioration and steric inhomogeneities accompany the loss of smooth muscle cells and the derangement of connective tissue elements in the wall of intracranial aneurysms, which may explain certain steps in their initiation, enlargement and rupture.

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

“…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). In either case, these layers do not seem to contribute significantly to the structural integrity of the aneurysmal wall.…”

“…Increased levels of collagenases have also been observed in aneurysmal tissue (AAssar et al, 2003;Anidjar et al, 1992;McMillan et al, 1995;Sluijter et al, 2004;Webster et al, 1991). The production of collagen (Type I) in cerebral aneurysms is mainly accomplished by fibroblasts (Eastwood et al, 1998;Espinosa et al, 1984;Kamphorst et al, 1991;Sluijter et al, 2004;Tóth et al, 1998), and these cells therefore play a key role in aneurysmal growth. Fibroblasts produce and organise the extracellular matrix (ECM).…”

“…The mobility of the fibroblast and its ability to contract the ECM are important properties in the maintenance of the ECM (Barocas et al, 1995;Dembo and Wang, 1999;Eastwood et al, 1998;Friedl and Bröcker, 2000;Friedrichs et al, 2007;Grinnell, 2003;Lo et al, 2000;Poole et al, 2005). The fibroblast also has the ability to align itself in the direction of existing collagen fibres and to produce new collagen that is aligned in the same direction (Birk et al, 1990;Cisneros et al, 2006;Friedrichs et al, 2007;Huang et al, 1993;Lin et al, 1999;Meshel et al, 2005;Tóth et al, 1998). Furthermore, the activity of adherent cells is strongly dependent upon mechanical stimuli from the surrounding ECM (Pelham and Wang, 1997;Weyts et al, 2003).…”

Mechanical behaviour of healthy and damaged human arteries and validation of parameters derived from experiments

“…We took over the measurement series from Gábor Raffai, who performed measurements under the supervision of Emil Monos. [2][3][4] He cut stripes out of the aneurysms parallel to the imaginary axis (meridional) and perpendicular ring-like direction (circumferential) for the uniaxial tests. We have performed the procedure shown for the internal carotid artery now for the total of 91 uniaxial measurement results, too.…”

Lattice Boltzmann method for modelling of biological phenomena

“…Hemodynamic parameters, such as velocity and Wall Shear Stress (WSS) have been estimated in-vivo and in-vitro to understand the underlying biomechanical reactions [23] [24].…”