Finite element based predictions of preferred material symmetries in saccular aneurysms

Over the years, various hypotheses have implicated the role of structural i nstabilities in the expansion of intracranial saccular aneurysms. Recent no nlinear analyses suggest, however, that particular subclasses of aneurysms are structurally stable (in the mechanics sense) and that we must consider different hypotheses. Indeed, based on an ever-increasing database, it appe ars that aneurysms may well expand via the remodeling of their constituents . Although more data and a kinetics-based formulation of remodeling are nee ded to examine this hypothesis, we present results from quasistatic finite element analyses of 12 subclasses of lesions that support the remodeling hy pothesis. Briefly, we identify regional variations in material symmetry, fo r a class of noncomplicated axisymmetric lesions subjected to a uniform dis tension pressure, that minimize local maxima in multiaxial stress and tend to homogenize the stress field. Such symmetries are termed preferred. It is shown that the numerical predictions are consistent with the teleological concept that some intracranial saccular aneurysms will seek to become spher ical, since the sphere is an optimal geometry for resisting a distension pr essure. To achieve this, however, different subclasses must develop differe ntly. Lesions having an initially large neck:height ratio must increase in height and therefore may seek to become increasingly stiffer circumferentia lly from the fundus to the neck. Conversely, lesions having an initially sm all neck:height ratio must increase in breadth and therefore may seek to be come increasingly stiffer meridionally from the fundus to the neck. We subm it that these results demonstrate the need for a detailed histological exam ination of regional variations in collagen organization in human lesions, f or it is upon data that an analysis of remodeling must be founded. (C) 1999 Biomedical Engineering Society. [S0090-6964(99)00405-1].