The Endurant Stent Graft

Since endovascular aneurysm repair (EVAR) was first described by Parodi near the end of the 20th century, there has been a paradigm shift from open surgical repair to EVAR for most patients. Anatomical constraints and instructions for use currently limit EVAR application to approximately 40% to 60% of patients.^1-3 First-generation devices have been characterized by a number of design-related issues, limiting the overall effectiveness of EVAR in certain patients.1-3 These limitations have included the size of the delivery sheath with associated small-access vessels, obtaining seal in angulated and short proximal necks, and lack of conformability of early generation devices. Often, in patients with anatomic limitations, multiple adjuvant procedures would be required, including the need for conduits or placement of Palmaz stents (Cordis Corporation, Bridgewater, NJ) or extender cuffs proximally to achieve seal.^4-6

Distally within the iliac system, tortuosity and ectasia can be associated with limb thrombosis and type Ib endoleaks due to kinking and poor seal/undersizing, respectively. Although safe, hypogastric artery coil embolization, when required to extend the stent graft to the external iliac artery, is associated with buttock claudication. Other limitations include stent and graft fatigue with the risk of a type III endoleak.^5-9

To address and overcome the issues of earlier-generation stent grafts, the Endurant® stent graft (Medtronic, Inc., Minneapolis, MN) (Figure 1) was designed to allow more patients to undergo EVAR while making implantation easier and improving long-term outcomes.

DESCRIPTION OF THE ENDURANT STENT GRAFT SYSTEM

The Endurant stent graft is a self-expanding nitinol device designed for the endovascular treatment of abdominal aortic aneurysms. All stent graft components are composed of nitinol metal stents, which are sewn onto the multifilament polyester (PET [polyethylene terephthalate]) graft fabric. The suprarenal stents with anchor pins on the proximal end are laser cut from a nitinol tube. The remaining stents include seal, body, contour, and limb stents formed from nitinol wire and sewn onto the graft material. The suprarenal stents are sewn to the graft fabric using an ultra-high molecular-weight polyethylene suture that is designed to aid in better stent-to-graft attachment strength and provide a more durable proximal attachment. Radiopaque markers are sewn onto each component of the stent graft, including limbs and extensions, to aid in visualization and to facilitate placement of each component.

ENDURANT DESIGN PROCESS

The Endurant stent graft design team, which incorporated input from more than 250 physicians, focused on creating a reliable and robust endovascular solution that focused on the following key areas:

• treatment of short and highly angulated necks
• treatment of tortuous anatomies without kinking the device
• long-term durability
• improvements in profile and deliverability.

The design optimized key performance attributes such as sealing and fixation, durability, conformability in tortuous vessels, and delivery profile. A multidisciplinary process including physician expertise, clinical imaging, computational modeling, and in vitro bench testing was utilized in the design. The subsequent sections discuss how the Endurant stent graft system was designed to meet the previously mentioned performance goals. Table 1 highlights how the design features of the Endurant stent graft system have translated to meaningful and consistent clinical outcomes.

Sealing

To treat a variety of neck anatomies and achieve maximum seal, the M-shaped proximal sealing stent of Endurant was designed to work with the suprarenal stent to enhance wall apposition and minimize the risk of graft material infolding. By maximizing the support within the seal region, a continuous circumferential coverage of the seal zone is maintained. The design of the stent seal was selected to maximize outward radial force while ensuring that the device could be constrained to a low-profile delivery system (18 or 20 F). To address highly angulated anatomies, stent spacing and heights were optimized to achieve a high degree of conformability while maintaining the required outward radial force.

A similar design philosophy was applied to the distal limb sealing regions. The development and assessment of sealing performance included clinical imaging analysis, computational analysis, and simulated use testing. Component testing, such as radial force and simulated use testing, which was designed to represent challenging physiological conditions has shown that the seal region of the Endurant device performs with excellent results.

Fixation

Active fixation was included in the Endurant stent graft to reduce the risk of device migration. This is especially critical for the treatment of aneurysms because device migration can result in loss of sealing integrity. The fixation mechanism consists of a dualanchor- pin configuration (two anchor pins per stent crown), which provides redundancy in the prevention of device migration. The one-piece laser-cut suprarenal stent design was selected for its enhanced structural integrity and durability. To optimize engagement of the suprarenal stent, several design attributes, such as pin angulation, pin length, tip geometry, location along the suprarenal stent, and takeoff angulation, were evaluated and optimized.

Use of the anchor pins in highly angulated neck anatomies was also taken into account. The dual-pin design allows for robust fixation in cases in which not all anchor pins may be fully engaged in the aortic wall. Evaluation of the effectiveness of aortic tissue engagement and strength (in both straight and angulated anatomies) was performed using benchtop and cadaveric tissue testing. Long-term durability testing simulating challenging physiologic loading conditions was also conducted.

Durability

Stent grafts undergo a variety of cyclic loading forces as a result of hemodynamic conditions in the aorta. Musculoskeletal motions can also result in repeated deformation of the device, potentially resulting in wear. Additionally, modular endografts can present an opportunity for overlap-related abrasion. In order to minimize the risk of fatigue and to design a graft that is extremely durable, the Endurant stent graft design underwent a rigorous battery of fatigue testing, including the evaluation of cyclic loading forces, aortic deformations, and overlapping components. Clinical imaging and physiologic data assessing neck angulation, vessel tortuosity, and cardiac and vessel wall motion were incorporated into computational analyses, as well as benchtop evaluation.

Durability testing was performed on stent graft components, including the anchor pins, stent-to-graft attachment, and nitinol stents, as well as whole device testing, to evaluate radial fatigue, aneurysm pulsatile fatigue, and overlap-related interactions.

Conformability

Incorporating conformability while eliminating the risk of kinking was one of the key components in the design of the graft. To accomplish this, the spacing between stents was optimized, and the peaks with the unique M-shaped design were offset to allow the graft to be highly conformable, even in tortuous anatomy.

Delivery System Performance

Unique anatomical challenges to EVAR are seen in women and patients with severe aortoiliac occlusive disease. The use of a conduit, with its own related morbidities, or iliac artery injury was associated with early generation devices. Lowering the profile of the stent graft delivery system is key to increasing the number of patients who are suitable for EVAR, as well as simplifying the procedure. Graft material selection, stent design, and graft cover design and materials have made significant improvement in the delivery system profile. The Endurant stent graft component designs were also optimized, such as the creation of stent geometries that allowed for the lowest crimp profile and graft material properties. Furthermore, the reduced crossing profile and careful selection of materials allowed the Endurant delivery system to be very flexible. For example, the addition of hydrophilic coating to the surface of the sheath of the Endurant delivery system has improved the system's ability to track through challenging anatomy. The reduced force required to track the system is attributed to this lubricious hydrophilic coating.

The two-stage delivery process of the Endurant system relies on the tip-capture mechanism to facilitate accurate and reliable alignment of the proximal edge of the stent graft. The tip-capture system ensures that fixation is precisely achieved. This new design has been evaluated in multiple configurations via preclinical in vivo evaluation and in vitro benchtop testing in challenging physiological models.

ANATOMIC CRITERIA FOR THE ENDURANT SYSTEM'S INSTRUCTIONS FOR USE

Suitable candidates for EVAR require a proximal neck of ≥ 10 mm in length with nonsignificant calcification and/or nonsignificant thrombus with ≤ 60° infrarenal and ≤ 45° suprarenal angulation and a vessel diameter of approximately 10% to 20% smaller than the labeled Endurant diameter. Neck diameters between 19 and 32 mm can be treated, and distal fixation can be achieved in vessels between 8 and 25 mm. Distal fixation of > 15 mm is required.

CURRENT RESULTS

Current results with the Endurant stent graft have been very encouraging given the challenging anatomy set forth in the clinical trials. In a variety of studies assessing the Endurant stent graft, the number of patients enrolled ranged from 80 to 1,200.

ENGAGE (Endurant Stent Graft Natural Selection Global Postmarket Registry)* is a global, prospective, real-world postmarket registry evaluating more than 1,200 patients treated with the Endurant stent graft system. Thirty-day data have been reported on the full 1,200 patients, and 1-year data are available on 350 patients. The technical success rate of the Endurant stent graft system was 99.1%. Through 30 days, there were no instances of stent graft kinking, twisting, or bare stent fracture.¹⁰

The device received US Food and Drug Administration approval based on the Endurant US regulatory trial, which was a nonrandomized multicenter study consisting of 26 sites. A total of 150 patients were enrolled, and 2-year follow-up has been completed. Inclusion criteria included a neck length of ≥ 10 mm with < 60° of angulation. The mean aneurysm size was 57 mm, and the procedure was successful in 99.3% of patients. The overall operative 30-day mortality rate was 0%, and major adverse events occurred in only 4%. At 1 year, there were no type I/III endoleaks, and type II endoleaks were seen in only 8.9% of patients. No patients had sac enlargement, 50.4% of patients had a stable sac, and 49.6% of patients had sac shrinkage. Furthermore, there were no fractures, migrations, or postoperative ruptures.¹¹

As described elsewhere in this publication, the Endurant stent graft has demonstrated consistent clinical results at 2 years, with no type 1 endoleaks, migration, conversion to surgery, aneurysm-related mortality, and postoperative rupture.¹⁰

DISCUSSION

The Endurant stent graft is the newest-generation stent graft based on collaboration between engineers and physicians, designed to overcome the limitations of first-generation devices. As demonstrated by the studies to date, the device is highly effective in overcoming adverse anatomy with excellent clinical results.

The Endurant stent graft offers a low-profile delivery system to navigate and overcome significant disease of the aortoiliac segment. The smaller French sizes (18 F for 23- and 25-mm devices; 20 F for larger devices up to 36 mm) are currently available for patient implants. The addition of a hydrophilic coating should limit the need for the use of conduits, especially in women. The flexibility of the device, based on the construction of the wireformed M-shaped body stents and the sinusoidal design distally, allows for effective access through calcified, narrowed, elongated, and kinked vessels.

Adverse proximal neck anatomy and angulation are associated with migration and type I endoleak. The Endurant stent graft appears amenable to overcoming these limitations, at least in the short term, based on current literature. The highly conformable body, which was one of the key design features, allows for sealing in a variety of neck shapes, even with significant angulation.

Results from the Endurant US IDE and CE Mark studies have been encouraging. Additional results from the ENGAGE real-world registry are also promising, showing safety and effectiveness of the device.

Frank R. Arko, MD, is with the Sanger Heart and Vascular Institute in Charlotte, North Carolina. Financial interest disclosure information was not available at the time of publication.

Tim Roush, MD, is with the Sanger Heart and Vascular Institute in Charlotte, North Carolina. Financial interest disclosure information was not available at the time of publication.

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