Intra-aneurysmal hemodynamic alterations by a self-expandable intracranial stent and flow diversion stent: high intra-aneurysmal pressure remains regardless of flow velocity reduction.

OBJECT: Little is known about how much protection a flow diversion stent provides to a non-thrombosed aneurysm without the adjunctive use of coils.
METHODS: A three-dimensional anatomically realistic computation aneurysm model was created from the digital subtraction angiogram of a large internal carotid artery-ophthalmic artery aneurysm which could have been treated with either a neck bridging stent or a flow diversion stent. Three-dimensional computational models of the Neuroform EZ neck bridging stent and Pipeline embolization device were created based on measurements with a stereo-microscope. Each stent was placed in the computational aneurysm model and intra-aneurysmal flow structures were compared before and after placement of the stents. Computational fluid dynamics were performed by numerically solving the continuity and Navier-Stokes momentum equations for a steady blood flow based on the finite volume method. Blood was assumed as an incompressible Newtonian fluid. Vessel walls were assumed to be rigid, and no-slip boundary conditions were applied at the lumens. To estimate the change in the intra-aneurysmal pressures we assumed that, at the inlets, the intra-arterial pressure at peak systole was 120 mm Hg both before and after stent placement
RESULTS: Without any stent, the blood flow entered into the aneurysm dome from the mid to proximal neck area and ascended along the distal wall of the aneurysm. The flow then changed its direction anteriorly and moved along the proximal wall of the aneurysm dome. In addition to the primary intra-aneurysmal circulation pattern, a counterclockwise vortex was observed in the aneurysm dome. The placement of a Neuroform EZ stent induced a mean reduction in flow velocity of 14% and a small change in the overall intra-aneurysmal flow pattern. The placement of a Pipeline device induced a mean reduction in flow velocity of 74% and a significant change in flow pattern. Despite the flow velocity changes, Neuroform EZ and Pipeline devices induced reductions in intra-aneurysmal pressure of only 4 mm Hg and 8 mm Hg, respectively.
CONCLUSIONS: The flow diversion effects of both stents were limited to flow velocity reduction. In a non-thrombosed aneurysm or an aneurysm with delayed thrombosis, the intra-aneurysmal pressure remains essentially unchanged regardless of the level of the intra-aneurysmal flow velocity reduction induced by the stents.