An inelastic multi-mechanism constitutive equation for cerebral arterial tissue

Intracranial aneurysms (ICA) are abnormal saccular dilations of cerebral arteries, commonly found at apices of arterial bifurcations and outer walls of curved arterial segments. Histological evidence suggests the stages in ICA development include the deformation of a segment of arterial wall into a "bleb" with no identifiable neck region followed by the development of an aneurysm with a clear neck. Afterwards, the aneurysm may undergo further enlargement, possibly with significant biological response including calcification and thrombosis. Past studies of the biomechanics of cerebral aneurysm tissue have been directed at modeling elastic deformations of pre-existing aneurysms. Taking this approach, the aneurysm wall is treated as a different entity than the arterial tissue from which it developed. In the current work, a nonlinear, inelastic, dual-mechanism constitutive equation for cerebral arterial tissue is developed. It is the first to model the recruitment of collagen fibers and degradation of the internal elastic lamina, two important characteristics of early stage aneurysm formation.