Commentary: Outcomes of Endovascular Aneurysm Sealing According to the Revised Nellix Instructions for Use: The Impact on Complications Is Not Yet Apparent

The concept of endovascular aneurysm sealing (EVAS) was introduced to the vascular surgical community in 2011 and was positioned as a disruptive technology in the endovascular aneurysm repair (EVAR) commercial space. EVAS differs from EVAR in that EVAS uses anatomic fixation and aneurysm exclusion with balloon-expandable stent-grafts surrounded by polymer-filled endobags occupying the sac. ¹ EVAR, on the other hand, achieves stent fixation by applying radial force proximally and distally, with or without suprarenal fixation and attachment barbs. The aneurysm sac is sealed from blood flow by the graft material at the proximal and distal necks.

The Nellix device (Endologix Inc, Irvine, CA, USA) was first used commercially in 2013 and with its wide morphologic applicability appeared to target an unmet need for the treatment of infrarenal abdominal aortic aneurysm (AAA). ² Early results using the Nellix system were extremely promising, with high rates of technical success and low incidences of complications, morbidity, and mortality.^3,4

However, midterm results reported that distal device migration may be a concern in some more adverse morphologies, and a Field Safety Notice was issued in October 2016. ⁵ Subsequent analyses and modeling of treatment failures led to a refinement of the instructions for use (IFU), disseminated in December 2016. Specifically, the updated IFU was more constrained than the earlier version; aneurysms with a thrombus index >1.4 (maximum aneurysm diameter/maximum flow lumen diameter), aortic neck diameter >28 mm, aortic neck conicity >10%, and distal seal zone <10 mm were excluded.

This refinement of the IFU reduced the morphologic applicability of the Nellix device to a level more in line with the IFUs of established EVAR devices. In the August 2018 issue of the JEVT, Zerwes and colleagues ⁶ reported that when the 2016 IFU were applied retrospectively to a cohort of patients treated with EVAS in a single center, the proportion of IFU-compliant aneurysms reduced from 75% to 34%. Zoethout et al, ⁷ in a Dutch multicenter study, documented a reduction from 63% to 18%, and in our own patient cohort, we saw applicability reduce from 45% to 17%. The most common factor that moved a previously IFU-compliant aneurysm to outside the 2016 IFU was the thrombus index, a factor that is not considered in the IFU of bifurcated EVAR devices.

With increased EVAS use and follow-up, complications have been reported, idiosyncratic failure modes have been recognized, and strategies have been developed to address them. It was expected that adherence to the updated IFU would effectively prevent complications, including distal migration. Results from our own institution, along with those published by Drs Zerwes ⁶ and Zoethout, ⁷ suggest that when clinical outcomes from aneurysms inside and outside the 2016 IFU are compared, there are very few statistically significant differences. None of these groups has identified significant differences in mortality or reintervention rates by applying the 2016 IFU. In the series of Dr Zerwes, ⁶ the incidence of migration was not reduced by applying the new IFU retrospectively, despite a large reduction in potential applicability, a finding mirrored by other large case series.

The complications of migration, type Ia endoleak, sac expansion, and potential aortic rupture appear to be occurring beyond 2- to 3-year follow-up. Robust surveillance programs are necessary to define the longer-term outcomes of EVAS and to fully understand the effect of the refined IFU.