Directional wall strength in saccular brain aneurysms from polarized lightmicroscopy

The aneurysm wall, which must withstand arterial blood pressure, is compose d of layered collagen. Wall strength is related to both collagen fiber stre ngth and orientation. When the aneurysm enlarges, the amount and organizati on of the collagen fibers change, potentially increasing the risk of ruptur e. We studied the directional organization and molecular strength of the co llagen fibers layer by layer across the walls of four aneurysms in order to measure their mechanical integrity. The technique incorporates the birefri ngent properties of collagen, enabling us to use linearly polarized light f or measuring the orientation of the fibers, and the Senarmont compensator t o measure the birefringence and thus mechanical strength. Intact aneurysms were obtained at autopsy, fixed at physiological pressure, sectioned at 4 m u m, 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 fr om 0.73 to 1.9 MPa. Comparing the weakest to the strongest direction, the b reaking strength varied by a factor of up to 2X, implying a significant deg ree of mechanical anisotropy. (C) 2000 Biomedical Engineering Society. [S00 90-6964(00)01005-5].