The Nellix EVAS system was a new concept that targeted the problem of type II endoleaks. Additionally, due to a required proximal neck length of only 10 mm in the original IFU, compared to 15 mm with most of the standard EVAR systems, potentially more patients could be treated using an endovascular approach.
Consistent with the current literature our technical success rate was 100% and no immediate post-interventional complication occurred.
However, there were six patients with complications during follow up, all of them with limb separation and caudal migration of the stent graft, often with concurrent type Ia endoleak. The early migrations occurred after just 12 months in patients with both short and long neck (4 mm and 19 mm), the late migrations after 36 months, again in patients with both short and long necks (6 mm, 8 mm, 20 mm, and 22 mm), indicating that neck length per se might not be the major contributing factor to migration.
The Nellix device had no hooks or other anchoring system to prevent limb separation or caudal migration. In addition, the stent grafts were balloon-expandable stents which could not adapt to any conformational changes of the aorta. Presumably due to better fixation of the distal parts of the stent grafts in the iliac arteries, migration appears to mainly affect the proximal parts of the stent grafts with caudal displacement leading to type 1a endoleaks. In fact, in the study from Dorweiler et al., (2017) there was 1 patient out of 24 that experienced deviation of the proximal limb segments. That patient was the only one with type Ia endoleak and aneurysm growth at 12 months. Hence it can be concluded that conformational changes in form of limb separation in the proximal graft segments are important indicators of an impending type Ia endoleaks while changes in the iliac segments are probably benign. Whether limb separation precedes or comes after caudal migration cannot be concluded from our data, but in our study they were always concomitant and more importantly highly associated with type Ia endoleaks.
Possible endovascular therapy options for migration and type Ia endoleaks in patients with the Nellix EVAS system include stent graft extension of both limbs with stent grafts and filling of the endoleak space either with coils or liquid embolization material. In our study 5 out of 6 patients could sufficiently be treated endovascularly, at least in the short term. In one case endovascular repair failed (progressive limb separation with persistent type 1a endoleak) and therefore open surgical aortic repair was performed as a salvage therapy.
Our study is limited by size and partial non-adherence to IFU in a third of our patients but nevertheless displays a weakness of EVAS. Even if only patients with neck lengths according to IFU were considered, the complication rate would still amount to 33%. We do not yet have short- or mid-term results of secondary intervention outcomes in all patients.