| The motivation for this work is the need to understand the mechanics of expansion and rupture of intracranial saccular aneurysms, and in particular, to identify an improved predictor of rupture-potential which will help physicians better manage patients harboring these lesions. Although saccular aneurysms are a well-known clinical and pathological entity, the natural history of unruptured lesions remains an enigma, controversy exists over the 'critical size' beyond which surgical intervention is deemed necessary, and rupture of these lesions remains the most common cause of non-traumatic subarachnoid hemorrhage. Two reasons for the poor understanding of the pathogenesis and rupture appear to be the lack of appropriate animal models which yield saccular lesions large enough for mechanical as well as biochemical tests, and the lack of an appropriate theoretical framework to guide experiments for quantifying the immunochemistry, histology, and mechanics of these aneurysms.; The purpose of this research, therefore, is threefold: to perform new static and dynamic analyses of the mechanics based on the best data available, to identify new experimental guidelines, and to collect new data on human and animal lesions. Specifically, using a custom finite element program, we show that it is the lesion shape (local curvatures), not size, which dictates the multiaxial distribution of wall stress, and that Laplace's law is reasonable for quasi-static analyses of a small sub-class of axisymmetric lesions. We then propose a nonlinear governing differential equation to examine the wall dynamics of this sub-class of aneurysms surrounded by an inviscid (cerebro-spinal) fluid. We show that inertial effects are significant only at forcing frequencies greater than 10 Hz, below which quasi-static analyses can be employed to characterize the behavior of these lesions.; As for experimental guidelines, we postulate a form of strain energy function which exploits histological data and can be used to calculate material parameters following experimental 'measurement' of multiaxial tensions and collagen orientation. We also propose a robust method to remove high frequency noise from experimental data obtained from cyclic inflations of rubber membranes, aneurysms and human intracranial arteries.; Finally, experimental data are reported for four human cerebral arteries, one saccular aneurysm, and six canine vein pouch lesions. Forms of strain energy functions commonly used to describe multiaxial behavior of systemic arteries could not characterize well the mechanical behavior of intracranial vertebral arteries, which are much stiffer in the axial than the circumferential direction. Furthermore, we also show that a canine vein pouch model, which is commonly used to evaluate the efficacy of endovascular treatment modalities, does not mimic the mechanical response of human saccular aneurysms; the development of an appropriate animal model thus remains as an important challenge in aneurysm research. |
