Quantitative evaluation and design optimization for stent embolization of newly developing micro-porous covered stent for cerebral aneurysm treatment
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Tsutomu TAJIKAWA*1, Shogo NISHI*2 and Yasuhide NAKAYAMA*3 *1 Kansai Univ. Dept. of Mechanical Engineering *2 Department of Neurosurgery, Neuro-Intervention, Spinal Surgery, National Cerebral and Cardiovascular Center Research Institute, |
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Abstract
The authors have been developing a micro-porous covered stent as a newly device for cerebral aneurysm treatment(1) as shown in Fig. 1. This stent has a porous film to control both embolization and rapid neointimal formation. By stenting this covered stent, thrombus formation is immediately occurred by blood flow stagnation in the aneurysm and its cavity is occluded. This covered stent has three key features. First is readily treatable with only placing a single stent at the parent vessel. Second, thrombus formation and its adhesion on the stent surface is prevented by early endothelial tissue formation on the luminal face of the covered stent. The last, the blood flow from the parent vessel to branching arteries can be maintained by flowing through the pores. Therefore, the risk of treatment for large and giant cerebral aneurysm with a broad neck and thinner aneurysm walls is much lower relative to another surgical and endovascular treatments such as clipping and coil embolization.
According to the previous studies, using a covered stent, of which pore diameter is 100µm and porosity is 23.6%, was able to be satisfied in both the immediate aneurysm embolization and the completion of neointimal formation within 3 months. And the result is expected that the neointimal formation becomes earlier by using much larger pores. But there is a possibility that the covered stent with large pore cannot embolize because increasing inflow from the parent vessel.
The purpose of this study is estimation of stent embolization and optimization of pore design to ensure compatibility between reliable embolization and controlling early neointimal formation. We has conducted in-vitro experiment under the steady flow condition based on flow similarity law by using a two dimensional parent vessel model with a saccular aneurysm model and various simplified micro-porous film models (4).
The flow visualization result shows that three kinds of flow pattern in the aneurysm model were observed namely, viscous shear induced swirl flow(5), friction pressure loss induced semi-swirl flow and those coexistence and transitional flow as shown in Fig. 2. According to flow visualization result, it is considered that viscous shear force caused by shear flow in the vicinity of the parent vessel wall drives the blood in the aneurysm cavity passing through the micro-pores on the film. The PIV results show that mean shear rate in the aneurysm after stenting, of which pore diameter is <200µm and porosity is 60%, was able to keep within the range of thrombus formation caused by blood stagnation as shown in Fig. 3.
The authors has calculated the moment of inertia of the swirl flow in aneurysm cavity and its driving force (accurately torque) due to wall shear stress on the surface of the parent vessel by using fundamental mechanics theory. As the result of the driving force estimating, it is indicated that ratio of the driving torque to the moment of inertia of the blood in the aneurysm cavity shows good agreement with the flow pattern change and amount of shear rate in the aneurysm as shown in Fig. 4. Therefore, it is suggested that our proposed parameter is one of the important hydrodynamical parameter to estimate the flow pattern in aneurysm and the stent embolization. This parameter and in-vitro experiment result can be introduced the optimized pore design and it is suggested that the covered stent with 60% porosity and 200µm in pore diameter is one of the best balance embolization and neointimal formation.
Finally, to confirm the result of this in-vitro experiment, follow-up animal experiment has been conducted. The result of stenting, a newly designed stent with our proposed porous film can be embolized immediately just after stenting as shown in Fig. 5. Therefore, these results indicated that in-vitro experiment and theoretical analysis based on mechanical engineering are effective tool for medical device development.
Keywords
Micro-Porous Covered Stents, Aneurysm, Embolization, Thrombus formation, Shear Rate, In-Vitro Experiment, Conservation of Angular Momentum
Fig. 1 A prototype of micro-porous covered stent.
Fig. 2 Typical visualized flow pattern in the aneurysm model.
Fig. 3 Effect of covered stent placement with shear rate as well-known as thrombus formation caused by flow stagnation.
Fig. 4 Relationship between observed flow pattern in the aneurysm model and counter map of the ratio of the viscous shear force to rotational moment of the blood.
| (a) Before stenting | (b) Just after stenting (d=200µm,  =60%) |
| Fig. 5 The result of follow-up animal exmeriment. | |
References