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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.
Tip Box 10.1
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.
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(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.
Tip Box 10.2
The new stents should reline the entire length of the previously implanted Nellix device.
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