Hemodynamic evaluation of endovascular techniques of stenting and coiling for the treatment of internal carotid artery aneurysm: a computational study

Abstract

The selection optimum endovascular method for the treatment of different cerebral aneurysms has been the main challenge for surgeons. In the present article, the computational technique is used for the hemodynamic evaluation of two main endovascular techniques of stent and coiling for the reduction of the hemorrhage risk of ICA aneurysms. Comprehensive analyses of the blood flow are performed to detect potential regions with high risk at critical stages of the cardiac cycle. Two coiling porosity conditions and stent deformation have been investigated via computational study to reveal the main change related to hemodynamics when these techniques are implemented. The hemodynamic results of this study show that the endovascular technique is more efficient in small aneurysms rather than giant ones. Meanwhile, the stent treatment of the giant saccular aneurysm is effective when the parent vessel of this type of aneurysm is fully aligned and limited blood flow enters the sac area.

Introduction

The treatment of cerebral aneurysms has evolved significantly in recent decades, with the introduction of various endovascular techniques as alternatives to open surgical clipping^1,2, and³. Two prominent approaches are stent-assisted coiling and standalone coiling (without stent). Understanding the differences in hemodynamic effects between these techniques is crucial for selecting the optimal management strategy for patients with giant or small cerebral aneurysms^4,5.

The placement of an intracranial stent as an adjunct to coil embolization has been shown to improve aneurysm occlusion rates and long-term durability compared to coiling alone^6,7. Stents work by reconstructing the parent vessel at the aneurysm neck, which alters the local hemodynamics. This change in blood flow can promote thrombosis within the aneurysm sac and prevent further aneurysm growth or rupture^8–10. Additionally, the stent scaffolding provides a framework to retain coils within the aneurysm, especially for wide-neck or complex lesions^11–13.

The placement of a stent across the aneurysm neck modifies the intra-aneurysmal flow patterns, typically reducing the flow velocity, wall shear stress, and pressure within the aneurysm. These hemodynamic changes create a more favorable environment for aneurysm thrombosis and remodeling. Computational fluid dynamics studies have demonstrated that stents can redirect blood flow away from the aneurysm sac, effectively isolating it from the parent vessel^14–16.

In contrast, standalone coil embolization relies primarily on the volume-filling effect of the coils to disrupt blood flow within the aneurysm^17–19. While coiling can successfully occlude the aneurysm, it does not significantly alter the parent vessel hemodynamics. This can be a limitation, especially for large or wide-neck aneurysms, where residual flow within the aneurysm sac may persist and increase the risk of recurrence or future rupture^20–22.

In summary, the use of stents in the treatment of cerebral aneurysms provides additional hemodynamic benefits beyond standalone coiling, making it a valuable option for the management of complex or challenging aneurysm morphologies^23–25. The specific hemodynamic effects of stents can play a crucial role in the long-term stability and durability of the aneurysm treatment^26–28.

Evaluation of endovascular techniques for treatments of cerebral aneurysms offers important insight for the selection of the efficient method. The current study presents hemodynamic factors related to the hemorrhage of cerebral aneurysms when two primary endovascular treatments of coiling and stent are implemented distinctively. The computational methods of finite volume are used for the hemodynamic study of the blood flow through cerebrovascular aneurysms. The real geometry of ICA aneurysms is chosen to study the impacts of the endovascular techniques of coiling and stent in the reduction of hemorrhage risk. The main factors related to hemorrhage are investigated in a critical stage of the blood cardiac cycle. The impacts of stent and coiling on two geometries of ICA with different sac volumes are fully presented in this article. In addition, the blood flow contact with surface of aneurysm is also evaluated.

Aneurysm selection and computational modeling

The first step for evaluation of the aneurysm hemorrhage via hemodynamic analysis is real geometry selection of the ICA aneurysms. Two selected ICA aneurysms for this work are displayed in Fig. 1 with full geometrical specifications. The real geometry of these two models is obtained from the Aneurisk webpage³⁴ related to Emory University. The main difference between these two models is related to the volume of the sac and neck vessel angle. The size of the former for cases A and B is about 58 mm³ and 594 mm³, respectively. Consequently, the size of the ostium section area of these two models is proportional to the size of the sac volume. Besides, the sac neck vessel angle is also different for these two models. The angle of the sac neck vessel is about 23 degrees and 68 degrees for cases A and B, respectively.

Fig. 1.

selected saccular aneurysms (a) case A (b) case B.

The simulation of the pulsatile bloodstream in these cases is done via a simple algorithm while one-way FSI is chosen for the interaction of the blood flow and vessel. In this technique, the exerted force from blood could be considered for the possible deformation of the artery. The modeling of blood viscosity is done by the Casson formula while the hematocrit value for the selected male cases is 0.45. The Casson model is a commonly used rheological model for describing the flow behavior of blood. The Casson model can accurately capture the shear-thinning nature of blood, where the viscosity decreases as the shear rate increases. This reflects the behavior of blood, which contains red blood cells, platelets, and other cellular components that align and deform under shear, reducing the overall viscosity. The Casson model includes a yield stress term, which represents the minimum stress required to initiate flow in blood. This yield stress is caused by the aggregation of red blood cells at low shear rates, and the Casson model can account for this behavior. Compared to more complex rheological models, the Casson model has a relatively simple mathematical formulation, making it easier to implement and use in analytical and numerical studies of blood flow.

Depending on the Reynolds number, flow can be categorized as laminar or turbulent. For cerebral aneurysms, flow is often laminar, especially at low flow rates. For pulsatile flow studies, a transient solver is utilized to capture time-dependent changes in flow characteristics, while steady-state solvers are used for simpler cases. Define appropriate convergence criteria based on residuals and monitor variables (e.g., mass flow rate, pressure) to ensure simulation accuracy.

The simulation of the blood is done for three cardiac cycles to ensure the convergence of the results. The blood flow is laminar and transient and applied transient boundary conditions for the inlet and outlet are plotted in Fig. 2. The critical stages of the cardiac cycle are also demonstrated in Fig. 2. Due to the high mass flow of the blood at stage B (peak systolic), hemodynamic data of this stage is reported. However, OSI index is exempt from this rule and it is calculated at end of the cardiac cycle.

Fig. 2.

Applied boundary condition at inlet and outlet.

The endovascular coiling technique is applied via porous conditions inside the saccular region. Table 1 presents the main specifications for the implementation of the porosity inside the sac region. Two permeability values of the applied porous domain are investigated in this study. The diameter and length of the applied coil for the calculation of the porosity factors are 25 cm and 30 cm, respectively. Additional details regarding the implementation of the coiling inside the sac region are available in published articles^35,36. The stent is also modeled via its effect which is the alignment of the main vessel for reduction of the blood flow rate into the sac region^37–39. Two deformation stages are induced on the selected model via changing the sac neck vessel angle as demonstrated in Fig. 3.

Table 1.

The details of applied porousity.

Case	Aneurysm Volume (mm3)	Porosity	Permeability (m2)	1/Permeability (1/m2)
A	58.73	0.6	6.50E−09	1.538211E + 08
		0.8	1.54E−08	6.489328E + 07
B	594.47	0.6	6.66E−07	1.501458E + 06
		0.8	1.58E−06	6.334277E + 05

Fig. 3.

selected aneurysms and defamation status.

The production of the grid for such a complicated model is also done and the produced grid is displayed in Fig. 4. The saccular region which is purple colored requires high grid resolution due to its importance for the calculation of the hemodynamic factors. Grid study is also performed via comparison of average wall shear stress on sac wall for four produced grids. The range of generated grids for these two models is between 0.2e + 6 cells and le + 6 cells. The evaluation of the ave. WSS for these cases shows that grid numbers of 576,260 cells and 688,278 cells are adequate for Case A and Case B, respectively.

Fig. 4.

Generated grid cells for the selected aneurysm (case B).

Results and discussion

The contours of wall shear stress over the saccular surface after endovascular techniques of coiling and stent deformation are compared and illustrated in Figs. 5 and 6. The main critical region for the possible rupture by WSS value is near ostium section for both selected cases. Although the size and geometry of these cases are totally different, high value of WSS happens in the same region. However, the extension of the wall shear of these two models is different and it depends on how the aneurysm expands after the ostium section. Meanwhile, the sac neck vessel angle is also important for the efficiency of the deformation while coiling effects on the ostium section are limited. The deformation of the aneurysm on distribution of high WSS in the ostium section has limited impacts while usage of stent sufficiently reduces the risk of hemorrhage on the surface of an aneurysm in the middle of aneurysms as noticed in the results of WSS of case B. The quantitative evaluation of the maximum WSS on these two cases shows that wall shear stress in case B with higher size and neck vessel angle is double the of case A.

Fig. 5.

Wall shear stress change on the sac surface in original and deformed cases with coiling in Case A.

Fig. 6.

Wall shear stress change on the sac surface in original and deformed cases with coiling in Case B.

The quantitative comparison of the coiling porosity and stent deformation effects on the Maximum and mean AWSS on the sac surface is displayed in Fig. 7. While reducing the coiling porosity (or equivalent to increased permeability) meaningfully decreases the Maximum and Mean AWSS on the sac surface of Case A, these endovascular treatments are not effective in Case B due to giant specifications of the aneurysms. In case B, these hemodynamic factors are reasonably decreased after the second stage of deformation. In fact, in the giant ICA aneurysm, the coiling and deformation are effective when they are implemented thoroughly.

Fig. 7.

influence of the aneurysm deformation and coiling porosity on the AWSS.

The pressure change caused by the coiling and stent deformation is also demonstrated in Fig. 8. In case A, pressure changes indicate that the influence of the coiling on pressure on the sac surface happens gradually while the deformation significantly reduces the pressure on the sac wall. On the other hand, in the giant aneurysm, the change of the pressure value drops after 2nd deformation in Case B with giant aneurysm size.

Fig. 8.

Change of the pressure on the sac surface for different porosity and deformation conditions.

The hemodynamic feature of the jet after endovascular techniques could be evaluated via the blood flow structure of the iso-velocity surface. Figure 9 displays how coiling and stent techniques occlude the blood flow rate into the sac section. The value of the iso-velocity surface decreases considerably when the coiling porosity is implemented in case A. In contrast, induced deformation gradually decreases the velocity of the blood entering the sac region. In Case B, the usage of the stent and fully deformed aneurysm could sufficiently decline the velocity of the blood flow because of the large size of the aneurysm. It is also found that the main critical region is the section near the ostium where the incoming blood stream contacts the aneurysm wall in this case.

Fig. 9.

Blood flow structure inside aneurysms different porosity and deformation conditions.

The contour of OSI index, which is calculated at the end of the cardiac cycle, is displayed in Fig. 10. The critical segment on the sac surface for case A is near the dome region while the critical region varies in different locations on the sac surface of case B. In case A, the usage of either coiling or stent deformation reduces the size of the critical region and the value of OSI on the sac surface. However, the second stage of aneurysm is effective for the reduction of the OSI on the sac surface of giant case B. The coiling seems inefficient for the control of high OSI region on the sac surface.

Fig. 10.

distribution of OSI at early diastolic in different porosity and deformation conditions.

Figure 11 displays the influence of the coiling and deformation on the value of maximum OSI and mean OSI that occurred on the sac surface. A comparison of the maximum OSI value in cases A and b indicates that the reduction of OSI value is limited in Case A with a giant aneurysm. However, it seems that the deformations gradually decrease the Maximum OSI on the sac surface. The assessment of the change in mean OSI values for the chosen patients indicates that the porosity resulting from coiling visibly contributes to a decrease in the mean OSI for case A. In contrast, for the giant case, the second phase of deformation noticeably impacts the reduction of OSI on the sac’s surface. Additionally, the porosity in the giant case limits the decrease in the mean OSI value.

Fig. 11.

quantitative comparison of the coiling and stent effects on the hemodynamic factors of case A and case B.

Conclusion

This study presents comprehensive hemodynamic analyses of two main endovascular treatments of coiling and stent in two ICA aneurysms with different sizes. The pulsatile blood flow has been modeled inside two real ICA cases via computational fluid dynamics. A one-way FSI approach is employed for the interaction of the vessel with blood unsteady flow. The non-Newtonian characteristics of the blood are modeled via the Casson formula. The impacts of the coiling are modeled via porosity media inside the sac while the stent effect is modeled by the deformation of the parent vessel in which less blood flow enters to the sac section. Two coiling porosity conditions as well as two deformation models have been investigated in the present investigation. To evaluate the performance of these two methods, two selected saccular cerebral aneurysms have different sac volumes. The comparisons of the wall shear stress, pressure, and OSI index for these two cases are presented to assess the mechanism of these techniques for the reduction of possible hemorrhage of the aneurysm. Achieved hemodynamic data indicate that the performance of the stent is more effective for cases with lower aneurysm sac volume. Meanwhile, the control of the rupture risk for the giant aneurysm is done after the parent vessel is fully deformed via a stent. The effects of coiling for rupture risk control are limited when the size of the sac volume is large.

Author contributions

J.P. update simulations and analysis, X.X. edit the manuscript and English writing L.Z. wrote the main manuscript text and prepared figures. All authors reviewed the manuscript.

Data availability

All data generated or analysed during this study are included in this published article.

Declarations

Competing interests

The authors declare no competing interests.