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The Multilayer Stents— A New Progress of Endovascular Treatment of Aneurysms Authors: ZHANG Yong-xue1, LU Qing-sheng2, JING Zai-ping2 Department of Vascular Surgery, Changhai Hospital, Shanghai, China 1. First author: Dr. ZHANG Yong-xue, MD. Dept. of Vascular Surgery, Changhai Hospital, Shanghai, China. E-mail: [email protected] 2. Corresponding author: Dr. Prof. LU Qing-sheng, Dept. of Vascular Surgery, Changhai Hospital, Shanghai 200433, China. (Tel and Fax: 021-31161666, E-mail: [email protected]) Corresponding author: Dr. Prof. JING Zai-ping, Military Institute of Vascular Disease, Changhai Hospital, Shanghai 200433, China. (Tel and Fax: 021-31161670. E-mail: [email protected]). Classification: literature review Abstract: Objective: To review the recent progress of multilayer stents in treating arterial aneurysms and draw an initial conclusion about its paradigm. Data sources: PubMed database and ELSEVIER database were searched with keywords of “Cardiatis” or “multilayer stent” for relevant articles from Jan 2008 to Sep 2012 . Relevant websites (provided by Cardiatis) were also involved in the review process. Study selection: well-controlled, relatively large-scale, retrospective studies as well as meaningful individual cases were all selected as materials. Results: In total 23 articles were involved in this review. The newly introduced Cardiatis multilayer stent aim at creating an active flow-modulating barrier between normal blood flow and aneurismal sac, which can induce thrombosis within aneurismal sac and preserve collateral circulation at the same time. Meanwhile it retains the virtue of flexibility and low-profile of uncovered stents. Currently, it has been applied in complicated aneurysms located in different segments of arterial system. Conclusion: This new concept of multilayer uncovered stent offers a promising alterative in the treatment of arterial aneurysms. However, a further large-scale clinical and hemodynamic study is required to evaluate the long-term effect. Key words: multilayer stents; grafted stents; aneurysm ; hemodynamics An aneurysm is defined as a dilatation more than 50% in diameter of a normal artery [1], and has a natural propensity to rupture without surgical intervention. The treatment of aneurysms has experienced an evolution from complex open surgical repair to minimally invasive endovascular therapy since the first case of endovascular repair of an abdominal aortic aneurysm conducted by Parodi and his colleagues in 1991 [2]. With rapid development of endovascular technique as well as instruments in the recent decade, this minimally invasive therapy has been applied in more and more cases [3]. The current endovascular approach for treatment of aneurysms depends on the use of grafted stents that mechanically block the aneurysm, prevent blood flow from entering the aneurismal sac, and thereafter lead to a reduction in wall pressure and shear stress of an aneurysm. Despite its advantages of lower mortality rate and less complications, the application of covered stents is restricted in some complex aneurysms located at aortic arch or next to vital visceral arteries. Meanwhile, the inflexibility and high-profile of its deliver system impedes its utilization in small-diameter arteries such as superior mesenteric artery and renal artery. In 2008, Henry M reported a renal artery aneurysm successfully treated with a new type of 3-dimensional multilayer stent [4], in which the multilayer stent was deployed in front of the aneurysm neck with the inferior renal artery covered. Angiogram showed blood flow inside the sac was immediately and significantly reduced and all the renal artery branches remained patent. At 6-month follow-up, angiography showed complete shrinkage of the aneurysm wall while all the inferior renal artery branches remained patent. This accidental discovery offered an attractive and promising alternative to covered stents in complex aneurysms’ management, and attracted attention of vascular surgeons in the study of its hemodynamic mechanism. We briefly review the recent progress of this new concept of uncovered stents, in terms of its mechanism, features, indications and contraindications. METHODS A comprehensive literature review was performed in PubMed database and ELSEVIER database through Sep 2012, using keywords of "cardiatis" or "multilayer stent". Same reports from the two databases were identified and excluded. Relatively large-scale investigations, as well as meaningful case reports were included in the analyse process. Relevant websites (provided by Cardiatis) were also studied to draw a comprehensive conclusion. RESULTS A total of 15 relevant articles were identified in PubMed. A search in ELSEVIER database found 66 articles, of which 25 were relevant. 2 articles were literature review, 2 was commentary, and 15 were reduplicative reports when compared with articles from PubMed. The remaining 6 articles were therefore selected for analyze. Another 2 articles were found on website provided by Cardiatis. In total 23 relevant articles were involved in our review. DISCUSSION Hemodynamic mechanisms To understand the mechanism of the multilayer stents, one has to be familiar with basic theories of hemodynamics. When a multilayer stent is placed across the aneurysm, flow velocity within the aneurysm is reduced while laminar flow in the main artery is improved [4]. Computational fluid dynamics (CFD) study indicates that a stent with an overall porosity of 50% to 70% will significantly reduce the inflow rate into an aneurismal sac [5]. Optimal flow modulation effect is reached with a 65% mean porosity. Secondary to this porosity level, when an aneurysm is covered, the stent laminates the blood flow by pressure drop. Using particle image velocimetry, Augsburger et al. demonstrated that velocity in the aneurismal sac decreases by an average of 88% when a multilayer stent is placed [6]. Unique features: Unlike grafted stents, the multilayer uncovered stent is a 3-dimensional interlocked porous stent system which has several unique features. The first characteristic is its porosity, which means it is incapable of preventing blood flow from entering the aneurismal sac. However, due to a hemodynamic flow-modulating effect, , the flow velocity within the aneurismal sac is reduced, causing an organized thrombus to form [7]. Therefore, if we consider the mechanism of covered stents as “passive mechanical barrier”, this multilayer stent, on the other hand, creates an “active physiological barrier”, which means that it causes the body to form thrombus actively within the aneurysm sac. Exactly because of its new concept of exclusion mechanism, some inevitable drawbacks of traditional covered stents such as side branches obstruction and the requirement of branch embolization to avoid type 2 endoleak might be overcome. The second feature is its capacity of preserving vital collaterals. In fact, a hemodynamic study conducted by C. Wailliez and G. Coussement with help of CFD software has confirmed that the collateral circulation is few or with difficulty supplied with blood when a local aneurysm forms, and when there is a multilayer stent, the collateral is correctly supplied and has a more regular flow [8]. In comparison with traditional covered stents which may cause problems like endoleaks and don’t permit blood flow of collateral arteries, this new uncovered stent has obvious advantages. The third feature of the multilayer stent lies in its flexible low-profile deliver system. Traditional stent-graft applied in aortic aneurysms requires a deliver system with a diameter of 18 French to 26 French, the inflexibility and high profile impedes its application in small-diameter arteries like renal artery and superior mesenteric artery. The uncovered stent, on the contrary, can be compressed into 6 French to 10 French deliver system and therefore could be applied in these small arteries. Finally, due to its active exclusion mechanism, this new multilayer stent requires less landing zone than covered stents that rely on mechanical block of an aneurysm. Applications in aneurysms: A 3-dimensional interlocked multilayer stent is now commercially available (Cardiatis, Isnes, Belgium) and has been tested successfully in patients with hard-to-treat aneurysms at various anatomical locations. A large-scale clinical study is now ongoing and the outcome has been encouraging so far. Its application has been reported in different complex aneurysms. 1. Application in peripheral aneurysms. Popliteal artery aneurysms (PAAs) account for approximately 70% of all peripheral aneurysms [9, 10]. Prior to the occurrence of endovascular techniques, open surgical repair including ligation and bypass had been the only valid solution to prevent clinical consequence of arterial embolization, thrombosis and rupture. However, there are significant rates of peri-operative complications including neurologic event, infection, seroma and hematoma [11], and there remains a 30% chance of aneurismal expansion over time in spite of open repair [12,13]. Given the associated surgical morbidity and risk, the endovascular therapy was introduced in clinical practice [14-18]. A retrospective cohort study conducted in 2007 consisting of 41 cases of open and 15 cases of endovascular repair has documented similar 1 to 2 years patency rate, and the latter approach has been considered as the first treatment option in high-risk patients. Raffaele Pulli reported in 2012 a retrospective study including 64 PAAs in 59 patients, 21 cases were treated with endovascular repair, including 1 case in which Cardiatis multilayer stent was utilized, whereas in the remaining 43 cases open repair was performed. Estimated primary patency rates at 24 months were 59.4% in endovascular group and 78.1% in open group (P=0.1); estimated 24-month secondary patency rates were 78.4% and 81.6% respectively. The result indicated no significant difference in terms of patency rate in two groups but the need for reintervention to maintain the patency of graft was significantly higher among patients endovascularly treated [19]. Michel Henry et al. reported 4 PAAs treated with the multilayer stents in 2011, all of which thrombosed after surgery. Mid-term follow-up is now ongoing [20]. So far the largest sample of PAAs with multilayer stents came from Antoniou GA et al. who reported 6 patients underwent multilayer stents placement, with a primary and secondary patency rate at 6 months of 67 % and 100 %, respectively [21]. Meanwhile, attempts were made to treat aneurysms at other location including iliac artery and femoral artery with this bare stent by Michel Henry and the results obtained seemed promising [22]. In 2010 he reported a short-term follow-up of a multicenter prospective study to assess the safety, feasibility and efficacy of Cardiatis multilayer stents for treatment of common iliac artery aneurysms (n=8). Technical success (defined as successful deployment of the stent) was achieved in all cases and at a mean follow-up time of 4 months no post-procedural complications were observed. The occlusion rate was 100%, with all collateral branches patent [23 ]. Although there has been so far no paradigm of its indication in peripheral artery, the clinical outcome so far has been promising and has presented an effective alternative to surgical and grafting approaches. 2. Application in visceral aneurysms. Visceral artery aneurysms (VAAs) are a relatively rare pathological lesion with a prevalence thought to be approximately 0.01% to 0.2% in general population [24]. Currently, there is a general consensus in the literature for the treatment of asymptomatic VAAs of size > 2 cm, and for symptomatic ones, regardless of size [25-27]. The reference standard for the treatment is considered to be open surgical repair [28, 29], especially in emergency cases. Nevertheless, when it is technically feasible, the endovascular approach is indicated in high-risk patients with suitable anatomy [30]. Modern endovascular technique in treating VAAs comprises coil embolization, stent-graft mechanical occlusion and the recent multilayer stent physiologic exclusion. A. Balderi et al. retrospectively analyzed the endovascular treatment of 30 patients affected by VAAs (26/31 were treated metal coil, 1/31 with covered stent, 1/31 with coils and Amplatzer plug and 3/31 with Cardiatis multilayer stents). During follow-up all VAAs remained excluded, with parenchymal ischemia observed in 8/31 cases. One of the three patients treated with multilayer stents died 10 days after surgery as a result of new-onset hemorrhage; In the remaining two cases a reduction of sac diameter from 81mm to 71mm (13%) and from 40mm to 36mm (10%) respectively was documented [24]. Meanwhile, in another center, Emanuele Ferrero et al. repoted their experience of visceral artery aneurysms with treatment strategies ranging from surgery to multilayer stents. His retrospective study included a series of 32 patients with different locations of VAAs (18/32 of splenic artery, 5/32 of hepatic artery, 3/32 of superior mesenteric artery, 3/32 of pancreaticoduodenal artery, 2/32 of celiac axis and 1/32 of gastroduodenal artery). The difference of morbidity/mortality rate between open and endovascular group was of no statistical significance (9/24 versus 4/8; p=0.6597). The multilayer stents was applied in 4 cases (all were hepatic artery aneurysms). In three cases the multilayer stent successfully slowed down blood flow within the aneurysm, along with consequent thrombus formation and preservation of collateral circulations. However, in another case of multilayer stent, a technical complication with hepatic artery dissection was reported. The reason was considered to be improper stent deployment [30]. But to some extent, this case indicated lack of experience rather than methodological mistake. The largest series with multilayer stents was reported by Ruffino et al. including 19 patients with VAAs. In 6-month follow-up, a stent patency rate of 87.5%, a collateral artery patency rate of 100%, a complete sac thrombosis rate of 87.5%, and a sac shrinkage rate of 75% were reported [31]. Beside the above mentioned relatively large series of retrospective studies of VAAs, there were in addition several successful cases of multilayer stents reported. Ferrero E et al. analyzed two cases with hepatic artery aneurysms (34 mm and 48 mm in diameter, respectively) which were treated with multilayer stents. At 12 months, a computed tomography scan showed thrombosis of the aneurysmal sac and patency of all the branches of the hepatic artery [32]. Similarly, Balderi A et al. presented a case of hepatic artery aneurysm successfully excluded by multilayer stents [33]. Carsten Meyer reported in 2010 a complex renal artery aneurysm of 26mm in diameter with large middle-branch artery arising from the aneurismal sac. Immediate post-interventional angiogram documented decreased blood flow inside the aneurismal sac, and angiography performed at 3 month follow-up demonstrated a complete exclusion of the aneurysm with preservation of the middle-artery [34]. Ahmed M. Elsharkawy et at. used the multilayer stent for the treatment of a hepatic artery pseudoaneurysm after liver transplantation, the multilayer stent was placed from the native hepatic artery into the transplant right hepatic artery, and the result was satisfactory: Computed tomography angiography (CTA) performed 2 weeks after surgery demonstrated patent arterial flow to both lobes of the liver and a thrombosed pseudoaneurysm [35]. Giampaolo Carrafiello et al. made attempts in 2011 to treat a celiac trunk aneurysm with the uncovered stents, and CTA at 12 month follow-up showed the patency of the stents without signs of stenosis or myointimal hyperplasia, as well as the complete thrombosis of the sac, and direct perfusion of the liver and spleen [36]. The above mentioned series and cases have shown an encouraging and promising prospect of the utilization of multilayer stents. However, despite all these successful cases with multilayer stents, there is still lack of large scale of studies concerning its long-term outcome. And already, there have been unsuccessful cases reported by Balderi and Emanuele in their relatively large series. Further clinical studies are needed to evaluate its reliability. 3. Application in aortic aneurysms. Similar to peripheral and visceral artery aneurysms, aortic aneurysms (including thoracic aortic aneurysms and abdominal aortic aneurysms, TAAs and AAAs) are treated conventionally with open surgical repair. But in more and more cases endovascular repair has been applied and in term of short-term outcome, it seems superior to open approach [37, 38]. However, it has been demonstrated that various complications including endoleaks, limb occlusion, and device failure can develop during long-term follow-up [39-44]. The braided multilayer stent has been applied in several TAAs/AAAs’ treatment with purpose of avoiding some major complications. In Michel Henry’s center, 10 aortic aneurysms (6 TAAs and 4 AAAs) were experimentally treated with multilayer stents, all of which were thrombosed during follow-up. In some of them shrinkage of aneurismal sac was observed [45]. A longer-term evaluation concerning the patency of collaterals, the time needed for complete thrombosis, and pressure change within the aneurismal sac is required to draw a final conclusion of its reliability. In addition, other individual cases were also reported which seemed to be in favor of multilayer stents’ therapeutic effect. Wulf Euringer et al. reported a case of HIV-related multi-aneurysms successfully treated with the multilayer stents. The patient was a 45-year-old male with AIDs who was diagnosed by computed tomography with multiple aneurysms at the following locations: bilateral subclavian arteries (right side:39×42mm,left side:28×25mm ); suprarenal abdominal aorta (20×26mm); and infrarenal abdominal aorta (51×49×46mm). The proximity of the subclavian artery aneurysms to the sprouting vertebral artery, as well as the immediate vicinity of aortic aneurysms to the orifices of the renal and visceral arteries prohibited the application of grafted stents, and therefore the multilayer stents were chosen. In total 4 multilayer stents were implanted (2 for the left subclavian artery aneurysm, 1 for the right subclavian artery aneurysm, and 1 for the two abdominal aortic aneurysms). Immediate post-intervention angiogram demonstrated a flow decrease in all aneurysms and patency of all collateral circulations. A follow-up CTA at 18 months revealed only the suprarenal aneurysm was persistent but with minimal perfusion. However, a reduction in size was achieved in all aneurysms [46], which indicated an aneurismal pressure decrease occurred. Benjelloun A, et al. reported a case of multiple thoracoabdominal aneurysms of tuberculous origin treated with an endovascular procedure with the multilayer stent. At 18 months, serial imaging studies showed disappearance of some aneurysms and regression of others [47]. Tolva VS et al. presented a thoracoabdominal aortic aneurysm which was discovered in a 57-year-old female with severe comorbidity, three multilayer stents were implanted in the thoracoabdominal aortic tract, obtaining at a 20-month follow up, a complete exclusion of the TAAA, with normal patency of visceral vessels [48]. Natrella M et al. recently reported a complicated juxtarenal aortic aneurysm successfully treated with multilayer stents, in which aneurismal shrinkage was observed [49]. Another case reported by Sidney Chocron et al. was a thoracic-abdominal residual type B dissection, in which two multilayer stents were implanted. A CT at 1-month follow-up showed the false lumen was not completely thrombosed. However, at 3 months, CT documented the thoracic false lumen was no longer patent, leaving only a false lumen in the abdominal aorta with a maximal diameter of 36mm [7]. The results of successful cases of multilayer stents seem convincing to support its application in aortic diseases. However, Lazaris et al. reported a case of ruptured aortic aneurysm which was previously treated with multilayer stents, suggesting that the issue of blood flow persistence inside the aneurismal sac had not completely been solved, keeping the aneurysm enlargement and rupture a probability [50]. In France, the Cardiatis multilayer stents are currently under massive evaluation in the treatment of TAAs/AAAs (AFSSAPS, protocol# 2008-A01396-49/A). An outcome of large-scale clinical studies is expected in the near future. Preliminary experiences: In term of aneurysm exclusion, the Cardiatis multilayer stent is a new concept of active, physiological barrier that has several unique features in comparison with traditional grafted stent. The main advantage lies in its capacity of excluding the aneurysm and preserving the vital collateral circulation at the same time. Meanwhile it retains the virtue of flexibility and low-profile of uncovered stents. At present, a lot of successful cases have indicated the safety and reliability in a preliminary experience. However, because its concept is newly introduced, there has been so far no paradigm of its utilization. A further large-scale clinical and hemodynamic study is required to evaluate the long-term effect of multilayer stents. Here we try to draw a conclusion in following fields according to the initial experiences from different centers. 1. Indications and contraindications Due to lack of large series of clinical study, it is too early to determine the indications and contraindications of multilayer stents. However, according to the above mentioned series of studies and individual cases, the application in aneurysms with vital collateral circulations or with complicated anatomy is indicated, especially among high-risk patients with multiple co-morbid conditions. Ruptured aneurysms with hemorrhage seem to be a contraindication with regards to the failed case reported by A. Balderi, in which the patient died of new-onset bleeding. In mechanism, a bleeding aneurysm may have a different hemodynamic status within the sac. However, this conclusion is far from accurate and requires further verification. 2. Criterion of success In the literature, most authors choose to utilize the multilayer uncovered stents with the intention to maintain vital collateral circulations while achieve the exclusion of aneurysms. During follow-up period, the following effects are expected to achieve: The patency of stent; The patency of collateral arteries; The complete exclusion and thrombosis of aneurismal sac; The shrinkage and pressure reduction of sac. The endovascular repair of an aneurysm aims at the prevention of rupture, therefore, the pressure reduction within the aneurismal sac is considered as gold standard of success. 3. The time needed to achieve complete thrombosis In most reported cases, CTA at 3 to 6 months is most likely to demonstrate a complete thrombosis of the aneurysm sac. The length of time needed may be associated with the morphological feature of the aneurysm, with/without branches sprouting from the aneurysm, and the coagulation system of the individual. A failed case is considered if the aneurismal sac remains patent 6 months after intervention. 4. Post-surgery medication For aortic aneurysms, an antiplatelet therapy after implantation of traditional stent-graft is not necessary in most situations. For peripheral and visceral artery aneurysms, however, anticoagulation or antiplatelet medication is often recommended after endovascular stent deploymen. After multilayer stents’ approach, In Wulf Euringer’s opinion, an antiplatelet therapy is needed to preserve side branch patency. On the other hand, the thrombosis process might be impeded by aggressive antiplatelet or anticoagulation medications. There seems to be a paradox situation here, and given the few data available on multilayer stents, the optimal post-surgery medication remains uncertain [46]. However, according to previous experience in traditional stents and the few available initial experience in multilayer stents, a antiplatelet therapy including 100mg/d acetylsalicylic acid and 75mg/d clopidogral is recommended in peripheral and visceral aneurysms treated with multilayer stent in order to prevent parenchymal ischemia caused by stent stenosis. CONCLUSION In conclusion, the recently introduced multilayer uncovered stent is a promising therapeutic option for the treatment of aneurysms in different locations from aorta to peripheral and visceral arteries. The results obtained so far demonstrate its safety and reliability. But a further large-scale clinical study with long-term follow-up is warranted to draw a robust conclusion. References: 1. Johnstone KW, Rutherford RB, Tilson MD, Shah DM, Hollier L, Stanley JC. Suggested standards for reporting on arterial aneurysms. J Vasc Surg 1991; 13:444-450. 2. Parodi JC, Palmaz JC, Barone HD. 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