The Nellix device (Endologix, Irvine, CA) is a novel endovascular aneurysm sealing system (EVAS) designed to address some of the issues with current infrarenal endovascular aneurysm devices. Persistent flow in the aneurysm sac after deployment of a stent graft can lead to aneurysm sac expansion and ultimately rupture. The Nellix device utilizes polymer-filled EndoBags to seal the aneurysm sac space. This is designed to decrease persistent sac flow and potential for endoleaks. First introduced in 2008, it has since been approved for commercial use in Europe and New Zealand. In the United States, it remains an investigational device, with refinement of the indications for use in 2016 based on complications observed in the first two years of data collection.
As this new device continues to be used and the EVAS procedure evolves, subsequent complications have become better understood. The unique design of the Nellix device means complications seen with both EVAS and EVAR, such as device migration and endoleak, cannot be addressed with the same conventional techniques that are used in EVAR. This chapter will address the structure of the Nellix device and how that structure, in addition to patient-specific factors, can lead to device migration, endoleaks, and aneurysm expansion. Prevention of these complications and available treatment modalities will be addressed.
Isolated Migration
Migration of the Nellix device (>10 mm) has been found to be caused by bending of the central stents. The Nellix device’s design means it is subject to downward flow forces not seen in traditional EVAR, ultimately leading to the foreshortening of the central stents. The polymer surrounding the stents creates a shelf against which blood flowing through the aorta impacts. This force of the blood contacting the polymer has been dubbed the “shelf force.” With each impact the shelf force must be resisted by a combination of the strength of the stents, the surrounding polymer, and that which surrounds the polymer, thrombus, and aortic wall. A second drag force is present in any stent not perfectly straight, caused by lateral force, which further bends the stent ( Fig. 10.1 ).
The strength with which the central stents resist the shelf and drag forces is not primarily caused by the rigidity of the stent itself, but from the surrounding polymer in the EndoBags. Understanding the aforementioned forces is paramount to prevention of device migration. Minimizing the distorting forces and maximizing the supporting forces can prevent this complication.
Prevention
Minimizing Distortion Forces
The shelf force is directly proportional to the area of the polymer shelf. As the diameter of neck in which the Nellix is deployed increases, so does the area of the polymer shelf. Therefore, limiting the aneurysm neck diameter decreases the area of the polymer shelf and thus the shelf force. Discovery of the shelf force as a primary cause of caudal migration led to refinement of the instructions for use (IFU) of the device, limiting neck diameter to 18–28 mm. Prior to the 2016 refinement of the IFU, aneurysms with neck diameters up to 32 mm had been included.
Maximizing Supporting Forces
The lateral force caused by drag slowly bends the stent into the surrounding thrombus, arresting only when coming in contact with the aortic wall. The stents themselves do not have the strength to resist this force and rely on the surrounding polymer for support. The amount of polymer able to be deployed in the EndoBags is limited by the amount of thrombus between the bag and the aortic wall. The refinement of IFU in 2016 created the “thrombus index,” a new method for evaluating this relationship. The thrombus index is defined as the ratio of the maximal aneurysm diameter to maximal flow lumen diameter. A thrombus index <1.40 has been seen to allow for sufficient polymer in the EndoBag to prevent device migration.
Treatment
Stent Relining
Those Nellix devices that have isolated migration, without endoleak or sac expansion, can undergo stent relining. This technique increases the overall stiffness of the system, thereby increasing the forces opposing the distortion forces. The stent-in-stent fix has the benefit of being a minimally invasive intervention, which does not preclude future interventions. Some technical considerations must be taken into account prior to relining the stent. It is the recommended technique in the absence of an endoleak and with at least 10 mm of remaining seal zone. The goal of treatment is to arrest migration and prevent the development of a Type 1A endoleak.
Relining can be achieved either with Nellix stents or an alternative balloon expandable covered stent. Balloon expandable stents are preferred because of the higher radial force during deployment. Using a Nellix to reline the Nellix has the benefit of requiring only a single stent, because of its long length. This avoids overlap required with multiple shorter stent deployment and prevents triple overlap, which optimizes lumen diameter.
Steps for Relining
- 1.
Obtain retrograde access via bilateral common femoral arteries.
- 2.
Use a standard technique to access the lumen of the Nellix stents.
- 3.
Pre-dilate the Nellix stents with noncompliant PTA balloons 12×40 mm throughout the entire length of the stents.
- 4.
Select a balloon expandable covered stent or Nellix stent.
- 5.
Deploy the new stents.
When selecting the Nellix stents, ensure an overlapping length of at least 35 mm between the original implanted Nellix stent and the new Nellix stents.
(Note: If using Nellix stents to reline, inject about 10 mL of saline through the EndoBag port of the console. Saline is used to reduce friction within the EndoBag, which will reduce the force required to detach the implant from the catheter.)
- 6.
Perform a post-dilation of the stents after deployment with noncompliant PTA balloons 12×40 mm.
- 7.
Perform a completion angiogram.
The new stents should reline the entire length of the previously implanted Nellix device.
Type 1A Endoleak
The overall incidence and persistence of a Type 1A endoleak with the Nellix device is extremely low. Published single-center studies as well as the Global Post Market Registry mark the incidence of Type 1A endoleak at 0%–4.3%, with early follow-up persistence of endoleak at 0%–0.4%. However, because of the unique design of the Nellix device, Type 1A endoleaks cannot be addressed as they are with traditional EVAR. The Nellix device is not compatible with proximal aortic cuffs or large balloon expandable stents, and therefore new technologies and adaptations of procedures currently in use can be used when this complication arises.
Understanding the anatomic and procedural risk factors that lead to Type 1A endoleaks can help the surgeon in the preoperative planning and during initial deployment of the Nellix device to prevent Type 1A endoleaks. Anatomic risk factors include: aneurysms with hostile infrarenal neck anatomy, including wide necks, severe calcification, significant thrombus, and neck angulation >60 degrees. “Stomach” shaped aneurysms, those with both angulation and large lateral diameters, can lead to stent bowing, loss of proximal stent alignment, and ultimately a Type 1A endoleak. Procedural factors can also contribute to the endoleak rate. During the initial deployment, low stent placement and stent misalignment forfeit available neck utilization and, therefore, increase the risk for this type of endoleak. These endoleaks may resolve spontaneously but should be closely surveilled every 3 to 6 months until sac stabilization or regression occurs, followed by annual check-ups. Those endoleaks associated with an increase in sac size may require intervention. Early diagnosis, continued monitoring, and, if necessary, endovascular treatment are key to prevent endoleak progression to aneurysm sac expansion and ultimately rupture ( Table 10.1 ).
| Intervention | Device migration? | Stent misalignment or low stent? | Distance between lowest renal artery and Nellix | Minimal proximal seal zone required |
|---|---|---|---|---|
| Coil and liquid embolics | No | No | n/a | 10 mm infrarenal seal zone |
| Proximal extension with covered stents plus coil and liquid embolization | No | Yes | n/a | 10 mm infrarenal seal zone |
| Proximal extension with Nellix system | Yes | Yes | ≥30 mm | 15 mm infrarenal seal zone |
| Nellix revisional ChEVAS: Proximal extension with Nellix system and parallel grafts | Yes | Yes |
| 15 mm seal zone at variable locations dependent on parallel grafts |
Prevention
During initial deployment of the Nellix device, performing a contrast-enhanced, saline prefill of the EndoBag and keeping the Nellix balloons inflated during prefill and polymer injection can mitigate stent bowing and proximal stent misalignment, which are risk factors for Type 1A endoleak.
Performing a prefill of the EndoBag allows for an accurate estimate of both the definitive polymer volume required to achieve a seal as well as the fill pressure. The surgeon must examine both apposition of the EndoBag to achieve a seal as well as stent location, which can move slightly during polymer filling. Performing a prefill allows the surgeon to make corrections to positioning prior to polymer injection.
After prefill, but prior to polymer fill, it is essential to ensure the Nellix stents have not moved. If the stent does move during prefill, once the saline has been aspirated from the EndoBag, the stents may be repositioned. After this, the Nellix balloons can remain inflated within the stent during polymer fill. This stabilizes the stents and allows the EndoBag to fill while maintaining stent position.
Perform prefill.
- 1.
Inject non-heparinized saline to the target pressure of 180 mmHg±10 mmHg.
- 2.
Contrast in the prefill can be of a maximum concentration of 15% of the non-heparinized saline volume.
- 3.
Perform an angiography in multiple views to assess aneurysm seal.
- 4.
Completely aspirate the saline once complete aneurysm sealing has been achieved.
Treatment
Unlike with EVAR, Type 1A endoleaks in Nellix devices are not treated with a proximal extension via either large balloon expandable stents or aortic cuffs. Four main minimally invasive options for intervention are available. Those not amenable to endovascular intervention can undergo conversion to open repair. The choice of treatment is dependent upon the anatomic characteristics of the leak and the aneurysm’s anatomic relationship to the Nellix graft. When deciding which intervention is appropriate, the surgeon must evaluate for the presence of device migration, the position of the Nellix stents, and the distance between the lowest renal artery and the Nellix device.
Coil and Liquid Embolics
For small to moderate Type 1A endoleaks without device migration and with correctly aligned stents, the surgeon can perform a minimally invasive deployment of a coil matrix with liquid embolics. To perform this, retrograde access via the bilateral femoral arteries and access from the brachial artery are obtained. Standard techniques are used to access each stent. Balloons are placed at each of the Nellix stents to allow partial or complete aortic occlusion proximal to the site of the endoleak and to avoid distal embolization of embolic material. A catheter is placed into the endoleak via brachial access. Angiography is performed to ensure placement. Coils are deployed taking care not to propagate the coils proximally. A scaffold is built in the endoleak with the coils. Liquid embolic agent is then deployed under continuous imaging into this scaffold, with Onyx being the most commonly reported embolic agent in the literature. A completion angiogram should be performed.
Reports of secondary intervention for proximal endoleak with Nellix are limited to case series. Harvey et al. and Ameli et al. described their institutions’ experience with coils and liquid embolics for Type 1A endoleak following placement of the Nellix graft with three and nine patients respectively. In both series, no residual endoleak or sac enlargement was seen on follow-up. In their series of 105 patients, Brownrigg et al. identified four patients with Type 1A endoleaks all treated with liquid and coil embolization. Following treatment there was complete endoleak resolution confirmed with postprocedure duplex.
A potential pitfall of this technique, as with all coil or liquid embolization, is the migration of the coil or reflux of liquid out of the endoleak site. Ameli-Renani et al. described two such cases in their series where Onyx refluxed from the endoleak during deployment into the limb of the Nellix graft causing stenosis. In both instances the stenosis was successfully treated with placement of a stent. To mitigate this risk, use of partial or complete aortic occlusion via balloon inflation within and/or proximal to the Nellix stents facilitate effective liquid embolic agent delivery with less risk of systemic embolization.
