The main findings of this study are as follows: 1) single or double bare-metal stents covering the neck of aneurysms can significantly change the flow in the parent artery and the haemodynamics of the aneurysm to achieve occlusion; 2) CAAs with a neck diameter of less than 5 mm and without adjacent stenosis can be treated with a single bare-metal stent; 3) CAAs with a neck diameter of 5-12 mm and no adjacent stenosis can be treated with double metal stents; and 4) no obvious ISR was found during follow-up.
CFD analysis can be used to simulate and analyse convective physicochemical problems, which helps to understand the haemodynamic changes in arteries resulting from abnormal structures. Therefore, it is widely used in cases of cerebral aneurysms[15,17] and thoracic and abdominal aortic aneurysms, while it is rarely used in the study of coronary aneurysms.
In this cerebral aneurysm intervention study, it was found that flow velocity, pressure and WSS were the most important parameters affecting the development of cerebral aneurysms. Flow velocity affected the formation of intracranial thrombi, and pressure and WSS were closely related to structural changes in the aneurysm wall. This study found that after stent implantation, the flow velocity and WSS in the CAA gradually declined, while the wall pressure increased(Table 2). The reason for this is due to the existence of the stent; the permeability of the flow inlet of the CAA was reduced, while a double stent makes this permeability even lower. In Case 1, after implantation of the second stent, the flow velocity in the CAA increased rather than decreased. This abnormal change may have been due to the small range in aneurysm neck diameter (<5 mm) and the change in permeability of the double-layer stents compared with the single-layer stents, which had a limited influence on the haemodynamics in the CAA. This indicates that for a CAA with a neck diameter of less than 5 mm, a single stent can effectively change the haemodynamics of the CAA and reduce the flow velocity.
In this study, the CAA pressure in the double-stent model was higher than that in the single-stent and stent-free models in Cases 2 and 3. Contrast retention was also found in the CAA after stent release. The reason for the increase in pressure was also due to the increased resistance of the outlet flow from the CAA due to stent implantation. This is consistent with the studies in cerebral aneurysms.
WSS is the tangential friction between the flow and the vessel wall per unit area. In Case 2, the WSS in the CAA after implantation of a single stent showed no significant difference compared with the stent-free condition, while after implantation of the second stent, the WSS was significantly reduced. This suggests that double-stent placement could influence the haemodynamics in the CAA significantly more than single-stent placement. In Case 3, a significant reduction in WSS was observed after the implantation of a single stent. After implantation of the second stent, the WSS decreased further. As shown in Figure 5, with stent implantation, the low WSS area in the CAA of the 3 cases was significantly increased. Malek found that WSS is an important indicator of CAA growth. When the WSS in the vessel wall is lower than 1.5 Pa, a macrophage-related inflammatory response can be induced, as can endothelial cell degeneration and apoptosis. By regulating the expression level of metalloproteinases, elastic fibres and collagen fibres in the artery wall are broken and absent, causing aneurysm growth and rupture.
Stent design and porosity are important factors in the outcome of aneurysm intervention[15,22]. Kim et al found that the “square mesh” stent shape was more effective in changing the velocity and WSS in cerebral aneurysms than the “ring mesh” stent shape. Lieber's study found that the haemodynamic changes in aneurysms were most pronounced when the porosity dropped to 76% but not noticeable when the porosity was lower than 70%. The range of metal coverage used for aneurysm treatment is generally between 60% and 86%, and the lower limit of this range is usually achieved by overlapping 2-layer stents[23,24]. However, metal coverage< 60% will lead to poor biocompatibility and high operational difficulty when stents are delivered through curved arteries. The H-stent had a square mesh design with metal coverage of 10-20%. When double-layer stents were superimposed, the metal coverage was approximately 71.3%, which was just within the range that has the greatest influence on haemodynamics.
In terms of material, the bare-metal stents were made of 316L stainless steel, which is suitable for the growth of new endothelial cells due to the absence of dug coatings. However, with the process of epithelialization, the permeability of the inlet is further reduced, and thrombosis in the CAA is promoted. OCT follow-up in Case 2 suggested that the stent surface was completely covered by neointima without excessive proliferation. Commercially covered stents contain polyfluortetraethylene (PTFE), which has poor biocompatibility and could affect epithelialization, leading to ISR. The PTFE study suggests that the incidence of subacute stent thrombosis in covered stents was 5.7%, while the incidence of ISR was as high as 31.6% (29.8% at the stent edge, 8.8% in the middle). RECOVERS studies have found similar results, and the incidence of subacute thrombosis within 30 days of PTFE stent placement was higher than that of traditional stent placement. Hachinohe et al performed a retrospective study of 190 patients implanted with PTFE-covered stents over 20 years and found that target vessel myocardial infarction, occlusion, revascularization and in-stent thrombosis events occurred at the early stage after stent implantation, and their incidence increased gradually in the long-term follow-up.
Stent compliance is one of the key factors of operation difficulty. Commercially covered stents, such as Graftmaster stents, generally have a sandwich structure, that is composed of a 50-μm thick PTFE layer between 2 stainless steel stents. Due to the poor compliance of covered stents, the operation difficulty is increased if there is obvious bending or narrowing of the vessels during delivery, and there are even reports that the operational strategy has been changed accordingly[27-28]. Covered stents are not suitable for segments with branches. Sequential bare-metal stent release can not only maintain good stent compliance without increasing the difficulty of the operation but also enable strategic adjustment according to the location of the branches to preserve the branch flow as much as possible. Finally, the longest covered stent is 28 mm, and the longest bare-metal stents can reach 30 mm, indicating a slight advantage of bare-metal stents in terms of stent length.
In terms of antiplatelet therapy, because covered stents are associated with a high incidence of ISR, physicians tend to choose a dual antiplatelet combination with a small dose of oral anticoagulants (such as warfarin), which increases the risk of bleeding. However, if patients are implanted with bare-metal stents without complications that they must use oral anticoagulants for (atrial fibrillation, deep venous thrombosis, etc.), they need to take dual antiplatelet drugs for only 6-12 months, which could reduce the risk of bleeding compared to covered stent therapy.