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In 1979, the author introduced three rules related to the anatomy of saccular aneurysms that should be considered when planning the operative approach to these lesions. These three aspects of anatomy are reviewed in this chapter in relation to each of the common aneurysm sites. First, these aneurysms arise at a branching site on the parent artery. This site may be formed either by the origin of a side branch from the parent artery, such as the origin of the posterior communicating artery from the internal carotid artery, or by subdivision of a main arterial trunk into two trunks, as occurs at the bifurcation of the middle cerebral or basilar arteries. Second, saccular aneurysms arise at a turn or curve in the artery. These curves, by producing local alterations in intravascular hemodynamics, exert unusual stresses on apical regions that receive the greatest force of the pulse wave. Saccular aneurysms arise on the convex, not concave, side of the curve. Third, saccular aneurysms point in the direction that the blood would have gone if the curve at the aneurysm site were not present. The aneurysm dome or fundus points in the direction of the maximal hemodynamic thrust in the preaneurysmal segment of the parent artery. Since the original introduction of the three rules, our anatomic studies have revealed a fourth rule. The fourth rule is that there is a constantly occurring set of perforating arteries situated at each aneurysm site that need to be protected and preserved to achieve an optimal result.
Aneurysms are infrequently encountered on a straight, nonbranching segment of an intracranial artery. The aneurysms occurring on straight, nonbranching segments are more often found to have sacs that point longitudinally along the wall of the artery in the direction of blood flow and to project only minimally above the adventitial surface. Aneurysms having these characteristics are of a dissecting type, rather than of the congenital saccular type, and their development is heralded more frequently by the onset of ischemic neurological deficits than by the subarachnoid hemorrhage associated with congenital saccular aneurysms. It is rare to find an aneurysm on the concave side of an arterial curve or to find one that points in a direction opposite to that of the flow in the parent artery.