One-Year Outcomes Following Repair of Thoracoabdominal Aneurysms With the Multilayer Flow Modulator

Report From the STRATO Trial

Claude D. Vaislic, MD1; Jean Noel Fabiani, MD, PhD2; Sidney Chocron, MD, PhD3; Jacques Robin, MD4; Victor S. Costache, MD5; Jean-Pierre Villemot, MD, PhD6; Jean Marc Alsac, MD2; Pascal N. Leprince, MD, PhD7; Thierry Unterseeh, MD8; Eric Portocarrero, MD6; Yves Glock, MD, PhD9; and Herve´ Rousseau, MD, PhD10 on behalf of the STRATO Investigators Group.

1. Department of Cardiac, Vascular and Thoracic Surgery, Centre Hospitalier Prive Parly 2, Le Chesnay, France.
2. Department of Cardiovascular Surgery, Hopital Europeen Georges-Pompidou, Paris, France.
3. Department of Thoracic and Cardiovascular Surgery, Hopital Jean Minjoz, Besancon, France.
4. Department of Cardiovascular Surgery, Hopital Louis Pradel, Lyon-Bron, France. 5Department of Cardiac Surgery, Centre Hospitalier de la R´egion d’Annecy, Metz-Tessy, France.
6. Department of Cardiovascular Surgery and Transplantation, CHU-Nancy, Hˆopital d’adultes de Brabois, Nancy, France.
7. Department of Thoracic and Cardiovascular Surgery, Hopital Pitie-Salpétrière, Pierre and Marie Curie University, Paris, France.
8. Department of Interventional Cardiology, Institut Cardiovasculaire Paris Sud, Hopital Prive Claude Galien, Quincy-sous-S´enart, France.
9. Department of Cardiovascular Surgery, CHU Toulouse, Hopital Rangueil, Toulouse, France.
10. Department of Radiology, CHU Toulouse, Hopital Rangueil, Toulouse, France.

Purpose:

To evaluate endovascular repair of type II and III thoracoabdominal aortic aneurysms (TAAA) using the Multilayer Flow Modulator (MFM) in patients with contraindications for open surgery and fenestrated stent-grafts.

Methods:

In this prospective, multicenter, nonrandomized trial (EudraCT registration: 2009- 013678-42; ClinicalTrials.gov identifier NCT01756911), 23 patients (19 men; mean age 75.8 years) with Crawford type II (43.5%) and III (56.5%) TAAA (mean diameter 6.5 cm) were treated with the MFM between April 2010 and February 2011. The primary efficacy outcome measure was stable aneurysm thrombosis with associated branch vessel patency at 12 months; the primary safety endpoint was 30-day and 12-month all-cause mortality.

Results:

The rate of technical success was 100%. In 20 patients with computed tomography scans at 12 months, the primary efficacy outcome was met in 15 patients. The rate of primary patency of covered branch vessels was 96% (53/55); 1 patient with 2 occluded visceral branches underwent successful surgical reintervention. Endoleaks were identified in 5 patients (3 attachment site and 2 at device overlap), 4 of whom underwent reintervention (3 additional MFMs and 1 stent-graft implanted). At 12 months, aneurysm diameter was stable in 18 of 20 patients; the mean ratio of residual aneurysm flow volume to total volume had decreased by 28.9%, and the mean ratio of thrombus volume to total lumen volume had increased by 21.3% (n¼17). There were no cases of device migration, loss of device integrity, spinal cord ischemia, or aneurysm rupture.

Conclusion:

At 1 year, endovascular repair with the MFM appears to be safe and effective while successfully maintaining branch vessel patency. Follow-up is ongoing.

The authors declare no association with any individual, company, or organization having a vested interest in the subject matter/products mentioned in this article.

Corresponding author: Claude D. Vaislic, MD, Centre Hospitalier Priv ´e Parly 2, 21 Rue Moxouris, 78150 Le Chesnay, France. E-mail: claudevaislic@hotmail.com

Keywords: thoracoabdominal aneurysm, flow modulation, stent, endovascular repair, visceral artery, branch artery, patency, endoleak, mortality, reintervention, aneurysm thrombosis

The introduction of thoracic endovascular aortic repair (TEVAR) with dedicated stentgrafts has permitted less invasive treatment of aneurysms localized to the descending thoracic aorta,1–3 and new endovascular technologies have been developed to support treatment of more complex anatomies, such as thoracoabdominal aortic aneurysms (TAAA) involving visceral branch vessels.4 A hybrid approach combined open visceral debranching with endovascular TAAA exclusion. 5–7 Deployment of custom-made fenestrated aortic stent-grafts with selective stenting of visceral branch vessels has been effective, with 1-year survival over 80%.8–10 Alternatively, to circumvent the high costs and long manufacturing delays for fenestrated grafts, chimney stent-grafts have been deployed into aortic visceral branches to maintain perfusion in coordinated conjunction with aneurysm exclusion using standard thoracic stent-grafts.

The Multilayer Flow Modulator (MFM) is an uncovered, self-expanding stent with high radial force and flexibility (Fig. 1). It is made of braided cobalt-alloy wire (Phynox) that is biocompatible and resistant to fatigue and corrosion. The unique design of the MFM features multiple interconnecting layers of alloy wire, which give the device a porosity of ~65%. The 3-dimensional wire layering of the MFM alters blood flow within the aneurysm sac, changing turbulent to laminar flow that supports the formation of organized, stable thrombus inside the sac. According to the MFM design principle, as blood flows through the 3-dimensional geometry toward a branch and then exits at the outermost layer of the device, it is organized into a laminar flow channel through the sac that perfuses the branch vessel.12 The local peak wall shear stress (PWSS) is immediately reduced, thus protecting against rupture. Where there is no collateral involvement, the dynamic shear vortex within the aneurysm is eliminated, and the flow is then laminated and redirected along the aortic wall in the same direction as the systemic pressure.

The clinical benefits of this flow modulating technology in the treatment of TAAA, type B dissection, juxtarenal aortic aneurysm, and peripheral and visceral artery aneurysm have been suggested in case reports and early registry data.13–26 We now present 12-month follow-up from the prospective multicenter STRATO Trial of the MFM in high-surgical-risk patients with TAAA.

Figure 1 : The Multilayer Flow Modulator.

METHODS

Trial Design and Patient Cohort The prospective, nonrandomized STRATO Trial (EudraCT registration: 2009-013678-42; ClinicalTrials.gov identifier NCT01756911) was conducted to assess the safety and efficacy of the MFM in high-surgical-risk patients presenting with Crawford types II and III TAAA. Ten centers in France with no prior experience using the MFM participated after receiving approval from their local ethics committee.

Patients were eligible for the study if they had a TAAA with maximum diameter .5 cm involving at least one visceral branch vessel, a life expectancy .12 months, and were classified as ASA (American Society of Anesthesiologists) class 3 by both a surgeon and an anesthesiologist. Patients were excluded from the study if their life expectancy was ,12 months or if they had an active infection or an allergy to aspirin, clopidogrel, or contrast agents.

The trial protocol was approved by the French Health Authority; data analysis and statistical reporting were performed through the European Cardiovascular Research Center. An independent clinical events committee reviewed all safety events in follow-up and adjudicated any concerns. Informed written consent was obtained from all enrolled patients. Between April 2010 and February 2011, 23 patients (19 men; mean age 75.8 years) with Crawford type II (10, 43%) or III (13, 56%) TAAA having a mean 6.560.9 mm (range 4.6– 8.5) diameter were treated with the firstgeneration MFM under this protocol. The patients had significant comorbidities and risk factors (Table 1).

Device Specifications and Implantation Procedure
The MFM was available in diameters ranging from 25 to 45 mm and in lengths ranging from 80 to 150 mm. The stent was oversized 10% to 25% compared to the transverse aortic diameter at the proximal landing zone; its 100- cm-long (usable length) 18-F Teflon-coated delivery system was delivered over a 0.035- inch stiff guidewire. The device was deployed by means of a pullback mechanism. The specific details of the index implantation and any reintervention procedures were at the discretion of the individual investigators. Patients were required to have iliac/ femoral artery access compatible with the device delivery system and healthy landing zones of at least 20 mm. Patients were prescribed dual antiplatelet therapy according to usual practice at each center.

Surveillance
Prior to hospital discharge, all enrolled patients underwent physical examination and imaging evaluation of branch vessel patency. Patients were routinely followed at 1, 3 (optional), 6, and 12 months after implantation with physical examination and computed tomography (CT) or magnetic resonance (MR) imaging. Throughout the 12 months of follow-up, all adverse events were noted and adjudicated. Assessments of device integrity, migration, aneurysm sac dimensions, and branch vessel patency were recorded in a dedicated database that also included procedure details.

Endpoints and Definitions
The primary safety endpoint was all-cause mortality at 30 days and 12 months. The primary efficacy outcome measure was defined as (1) no circulating flow within the aneurysm sac (except for residual flow adjacent to any covered branches) and (2) patency of all covered side branches at 12 months. Secondary endpoints included the presence of endoleaks, the occurrence of secondary interventions, spinal cord ischemia (SCI), device migration, loss of device integrity, aneurysm rupture, and major adverse events. Other outcome measures were the change in aneurysm sac size, change in the ratio of aneurysm flow volume to total volume, and change in the ratio of thrombus volume to total lumen volume. Aneurysm expansion was defined, per protocol, as a .10-mm increase in the maximum diameter compared to discharge imaging (or 1-month imaging when discharge imaging was not available). Since the MFM is a porous stent, endoleak was defined for this device as persistent blood flow into the aneurysm due to incomplete or ineffective sealing at either the proximal or distal end of the stented segment (type I) or due to inadequate overlapping of multiple devices (type III). Types II and IV endoleaks are not applicable to the MFM.

RESULTS

Figure 2 : Patient flow and determination of the population available for endpoint analysis.

Technical success was achieved in all patients. The device was introduced through a surgical cutdown of the common femoral artery in 15 patients; 1 patient required an iliac conduit, and 7 patients had a percutaneous approach (Table 2). Overall, 53 devices were implanted in the 23 index procedures. Post-implantation balloon angioplasty was performed in 7 cases. The mean duration of the implantation procedure was 84 minutes. The disposition of the trial cohort is diagrammed in Figure 2. With 1 patient lost to follow-up within the first month, 1 death occurring at 11 months, and 1 patient declining 12-month imaging follow-up, there were 20 patients with imaging available at 12 months. Complete aneurysm repair (no circulating flow in the sac and patent branches maintained) was achieved in 13 of 20 patients for whom CT/MR imaging data were available at 6 months. At 12 months, complete sac thrombosis was achieved in 15 of 20 patients and was ongoing in 3 other patients. In the 2 remaining patients, the aneurysms were not thrombosed due to persistent endoleaks. In the 20 patients with imaging available at 12 months, primary patency had been achieved for 53 (96%) of 55 covered branch vessels: 12 of 13 celiac arteries, 14 of 15 superior mesenteric arteries (SMA; Fig. 3), all 26 renal arteries (Fig. 4), and the 1 left subclavian artery (LSA) that was covered.

Occlusions were detected in 2 covered visceral branches (the celiac trunk and SMA with splenic and mesenteric infarction) in 1 patient 14 days after the index procedure. This patient was not administered post-procedure dual antiplatelet therapy but was receiving only aspirin. The patient underwent successful surgical thrombectomy and bypass of both visceral branches; at 12 months, the celiac trunk and the SMA were patent through the grafts.

The only death occurred at 11 months in a patient with atrial fibrillation who had experienced a stroke during reintervention for type III endoleak at 3 months (Table 3). There were no cases of SCI, aneurysm rupture, or device migration or fracture, and there were no reported respiratory, renal, or peripheral complications.

Endoleaks (3 type I and 2 type III) requiring correction were identified in 5 (22%) of the 23 patients, and all were determined to be due to technical issues. The 3 type I endoleaks (Fig. 5) were induced by device misplacement (within a graft in the aortic arch; in a stenosed area at the top of a gothic aortic arch; within a previously implanted graft), which contributed to incomplete MFM opening in each case.

One of the type III endoleaks was due to placement of a small-diameter MFM into a larger MFM; in the other case, there was insufficient overlapping of 2 MFM devices (Fig. 6). Reintervention with implantation of one or more additional MFM devices was performed before 12-month follow-up in two of these cases and after 12-month follow-up in another. One type I endoleak was managed by combining stent-graft implantation and LSA exclusion. Reintervention was declined by the remaining patient with endoleak; he died 18 months after MFM implantation.

At discharge, mean maximum aneurysm diameter was 6.8 cm (range 5.3–8.1 cm) for the 22 patients with measurements. In the 20 evaluable patients at 1 year, mean maximum diameter was 7.2 cm (range 5.4–9.0). Maximum aneurysm diameter had remained stable (,10-mm change) for 18 of the 20 patients (Table 4). Two patients had diameter increase >10 mm (from 70.3 to 89.9 mm and from 73.5 to 86.5 mm, respectively). These 2 patients had proximal type I endoleaks induced by device misplacement. In the 17 patients with volume data at baseline and 12 months, the mean ratio of aneurysm flow volume to total volume had decreased by 28.9% (from 14.2% to 10.1%); simultaneously, the mean ratio of thrombus volume to total volume (Fig. 7) had increased by 21.3% (from 45.5% to 55.2%).

Figure 4 : A 73-year-old patient with a 77-mm Crawford type III TAAA (A) and a history of smoking, coronary artery disease, previous percutaneous coronary intervention, congestive heart failure, myocardial infarction, hyperlipidemia, and previous open surgical graft repair of infrarenal abdominal aortic aneurysm had an MFM implanted. At 6 months (B), the maximum aneurysm diameter was 80 mm and (C) both covered renal arteries were patent.

Figure 5 : The appearance of endoleak type I due to improper wall apposition of a proximal MFM (lower panel) implanted to treat a Crawford type IV thoracoabdominal aneurysm in a shaggy aorta (right images).

DISCUSSION

The rupture of an aortic aneurysm is conditioned by increasing local pressure induced by turbulence of blood flow in the aneurysm sac (expressed by the PWSS) relative to weakening of the aortic wall. Endovascular repair of challenging TAAAs that span the ostia of visceral arteries used covered stents to exclude blood flow into the aneurysm sac maintaining flow into branch vessels via fenestrated/branched or parallel grafts. The alternative solution represented by the uncovered mesh of the MFM is based on modulation of blood flow dynamics to create organized laminar flow into the branch vessels without the need for the extra steps involved in cannulation. At the same time, the laminated flow encourages thrombosis of the sac. The time arc for sac shrinkage may be extended beyond 12 months, particularly in large TAAAs involving visceral branches, but the significant shifts in blood flow dynamics seen with CT follow-up can confirm the reduction of local PWSS, virtually eliminating rupture risk. The aortic and branch vessel patency outcomes speak for themselves.12,26

Figure 6 : A type III endoleak due to inadequate and faulty overlap between devices (arrow).

Compared to stent-grafts, the MFM technology has numerous potential advantages for treating TAAA. The off-the-shelf device is available in a wide range of sizes, and less preoperative planning is required. It is not necessary to perform extensive measurements and calculations regarding visceral side branch location, diameters, and angulation, although thorough evaluation of landing zones is important.

The MFM allows less invasive treatment of TAAA; in particular, vascular access can be limited to a single sheath in one femoral artery for introduction of an angiographic catheter and another sheath in the other femoral artery for introduction of the device, without the need for additional upper extremity access. The 18-F sheath size is smaller than that of many TEVAR delivery systems. Without the need to selectively cannulate branch vessels, there is much less total fluoroscopy time, and less total contrast is administered. Facilitated by the device’s pin-and-pull mechanism, deployment of each MFM usually requires ,5 minutes. The total procedure time is shorter (and the cost can be less) than that for more complex endovascular devices. The midterm outcomes in the STRATO trial patients contraindicated for open surgery and fenestrated stent-grafts appear to demonstrate many of the potentials of the MFM: 100% successful device deployment, 75% with complete aneurysm thrombosis, and 96% primary patency of covered branch vessels. Through 12 months, there was just 1 death, and there were no cases of SCI, aneurysm rupture, or device migration or fracture, nor any reported systemic complications.

Figure 7 : Change in (A) the ratio of aneurysm flow volume to total aortic lumen volume and (B) the ratio of thrombus volume to total aortic lumen volume for 17 patients with volume data at baseline and 12 months.

By contrast, in a systematic review of 7 studies encompassing 155 patients with TAAA treated between 2003 and 2009 with custom-designed branched/fenestrated stent-grafts, mortality was 7.1% at 30 days and 16.1% through a mean follow-up of 11.8 months.10 Adverse events included SCI (n¼6), stroke (n¼3), renal failure (n¼9), myocardial infarction (n¼10), pneumonia (n¼7), postoperative hemorrhage (n¼4), and aortic dissection (n¼3). Primary endoleaks were reported in 23 (18.4%) patients (9 type I, 8 type II, and 6 type III), and there were 26 reinterventions.10 In a prospective single-center cohort of 89 TAAA patients unfit for open repair, technical success with fenestrated and branched endografts was 96.6%, median procedure duration was 220.5 minutes with median contrast volume of 182 mL. The rate of SCI was 7.8%, and mortality was 8.9% at 30 days and 14.6% at 1 year.8

In our patients, 5 type I and III endoleaks were identified through 12 months; all were adjudicated as being due to failure of placement or device overlapping, respectively. In the case in which two covered branch vessels were occluded, the patient was not taking dual antiplatelet therapy. The Italian multicenter registry of peripheral and visceral aneurysms treated with the MFM emphasized the importance of adherence to dual antiplatelet therapy and attention to proper landing zones and adequate overlapping.25 We would also stress the importance of pretreating concomitant stenosis identified in the branch arteries to be covered by the device.

Our per-protocol definition of aneurysm expansion as an increase in the maximum diameter of .10 mm at 12 months, rather than the more customary 5-mm cutoff, is supported by research findings of a 65-mm interobserver variability in the measurement of maximum aneurysm diameter27 and of an approximate variation of 10% to 18% in aortic diameter during the cardiac cycle.28 The changes in the volume flow data available for 17 of the 23 patients in this trial are consistent with the other clinical and CT findings indicating aneurysm stabilization. Through 12 months of follow-up, aneurysm sac size was stable in 90% of patients. Bearing in mind that these were all patients with extended TAAA, our experience beyond the 12-month time frame supports the assessment by others25,26 that complex aneurysms take longer to reduce than simple lesions.

There is a report of potential concern when using the MFM associated with a surgical anastomotic pseudoaneurysm. Lazaris et al.29 described one aortic rupture where the MFMs were implanted in shaggy aorta with a uncorrected type I endoleak. However, the authors reported that the rupture might be related to type IV endoleak, which is not applicable to the MFM.

Limitations
Since this trial enrolled only nonsurgical candidates per protocol, direct comparison with an open surgical arm was not possible. The fact that our current follow-up extends to only 12 months is a particular limitation given the expected longer time interval to sac shrinkage in patients with covered branches, even when the volume data indicate that sac thrombosis has been achieved. In addition to longer follow-up for the patients in this trial, outcome data are needed for other patients treated with the MFM in multicenter registries.

Conclusion
This series of nonoperative patients with types II and III TAAA involving branch vessels underwent successful endovascular treatment involving the 3-dimensional uncovered MFM. Deployment of the device was technically successful in all patients, with no device failures. Through 12 months of follow-up, primary covered branch vessel patency was 96.4%. There was no aneurysm rupture, no device migration or fracture, and no incidence of SCI or systemic complications. There was radiographic evidence of progressive sac thrombus formation in the treated patients throughout the duration of the trial. Surveillance of the patients in this study continues so that we can provide longer-term analysis of clinical outcomes, sac morphology, and thrombus volume to assess the durability of this novel type of aneurysm repair.

Acknowledgments:

The STRATO Trial was sponsored by Cardiatis (Isnes, Belgium) and was conducted under the auspices of the French Ministry of Health. The contract research organization was the European Cardiovascular Research Center. Members of the independent Clinical Events Committee were Alain Carpentier (HEGP, Paris), Daniel Guilmet (Hˆopital Foch, Suresnes, France), Olivier Varenne (Hˆopital Cochin, Paris), Francine Leca (M´ec´enat Chirurgie Cardiaque, Paris), and Jean Mani (Clinique Paris V, Paris). The validation of measurement methodology was performed by P. Garot, MD, at the independent core laboratory. The authors acknowledge the contributions of co-investigators Patrice de Cassin, MD (Centre Hospitalier Priv´e Parly 2, Le Chesnay, France); Camille Durst, MD (Hˆopital Jean Minjoz, Besan¸con, France); Didier Revel, MD, PhD (Hˆopital Louis Pradel, Lyon-Bron, France); and Marc Sapoval, MD (Hopital Europeen Georges-Pompidou, Paris, France). The investigators extend their appreciation to Grayson H. Wheatley III, MD, for his significant contribution to the preparation of the final manuscript.

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