The introduction of endovascular grafts for treatment of abdominal aortic aneurysms represents the major determinant for the endovascular revolution of the last 25 years. This technology has been immediately adopted by the innovators and gained increasing acceptance even by the more conservative open surgeons with the evolution of the technique, writes Fabio Verzini, Perugia, Italy.
Multiple models have been rapidly introduced and most of the earlier generation devices have been briskly withdrawn and substituted for newer ones, once the mechanisms of failure have been revealed after their clinical use. This rapid technological evolution may be positive for ameliorating the future outcomes of endovascular aneurysm repair (EVAR), but it also affects the present evaluation of long-term results for the performance of outdated grafts no longer in clinical use. In previous research by our group, we were able to demonstrate better clinical outcomes of EVAR with newer generation endografts compared with those in use before 2004 in terms of risk of reintervention, conversion or aneurysm growth.
First-generation bifurcated, unibody endografts clearly showed that unsupported prosthesis, like the old EVT (Guidant, discontinued in 2003), were prone to kinking and limb occlusion, secondary to aortoiliac tortuosity and remodelling after aneurysm exclusion. As a result, all the endografts available today employ a full metallic skeleton to support the fabric; however, it took a while for the graft refinements to reach this conversion. The Anaconda (Vascutek Terumo) endograft was the last device to be updated with a fully supported body in the “One-Lok” version, which has been available in Italy since 2011.
Amongst the most used so-called second-generation endografts, AneuRx and Talent (Medtronic) presented favourable mid-term results but with longer follow-up they failed to show stable fixation at the level of the proximal neck. Those endografts presented a passive fixation method relying only on graft oversizing that could not counteract the displacement forces exerted, particularly on graft bifurcation, in cases of neck enlargements. All the manufacturers promptly reacted by designing bifurcated graft models with active fixation, and today, only the AFX (Endologix) offers a passive proximal fixation—but in an aortic tubular segment imbricated with a bifurcated distal segment with the trombone technique to avoid graft migration. Unexpectedly, the newest endograft models with a completely different design, such as Nellix (Endologix), are now in use again with no active fixation. It will be interesting to follow the clinical results of this new concept to understand if aneurysm sac occupation with polymer-filled endobags may be less prone to migration, especially in short necks.
Increasing the fixation strength at the proximal level has been demonstrated to pose a risk of failure at other junction points. Docking sites of modular endografts are at risk of developing type III endoleaks in the long term in patients with stable proximal fixation, especially in large aneurysms with an increased risk of graft movements secondary to remodelling. The obvious solution has been to increase the attrition between segments, incrementing the oversizing of the proximal end of the distal segments, or even adding lock barbs to fix the iliac limb of the grafts in the aortic stumps, as in the case of Treo (Bolton Medical).
Unfortunately, the distraction forces will not expire with these refinements, but are transmitted distally, where no solution to increase fixation has yet been proposed for graft oversizing. Therefore, in cases of short distal necks, especially in dilated iliac arteries, risk of late distal type Ib endoleak is still high. One solution is clearly to land in healthy, long necks that can be found sometimes only at the level of the external iliacs, occluding the hypogastric artery. An alternative solution in such cases has been offered by the introduction of iliac branched devices that afford stable fixation both into the external and the internal iliac arteries preserving pelvic flow.
In the struggle to miniaturise devices for easier arterial navigation and a less invasive arterial approach, inevitably all the newest endografts have a very similar stent design, especially at the level of proximal suprarenal stents. Most suprarenal stents today present fewer long rhomboid or diamond shape crowns with downward-facing hooks. These fixation stents surely will work well but in the case of complications (ie. infection, neck dilation or dissection) they may move the disease upwards, from a juxtarenal position to a more difficult-to-treat thoracoabdominal level. New solutions may be expected only if we start thinking out of the box with some active fixation methods that might work also at an infrarenal level, even in tortuous necks. In this direction, tools like the Endoanchors (Medtronic) may be of use.
Material innovations are also needed to reduce the risk of fatigue. Thinner fabrics and metal wires, always with the same basic composition (polyester, nitinol) as the first-generation endografts, are being used today to decrease device profile. Their long-term performance in unusual anatomical settings, in the presence of calcified plaques or aortoiliac tortuosity, have not been sufficiently tested in vivo to accept new device designs without profound scrutiny. New materials or different concepts, like support rings inflated with polymer as in the Ovation graft (Endologix), might decrease the risk of long-term fatigue.
Lastly, type II endoleak should not be ignored if leading to aneurysm growth; the latter is still a determinant of failure and may be associated with a risk of impending type I endoleak when involving the neck. To date, no device other than Nellix has addressed this issue, while EVAR with preventive sac embolisation techniques such as coils or glues seem to reduce the risk at least in selected cases and small-scale experience. Innovations might involve systems that could couple the features of current endografts, with reliable fixation, with that of sac filling to prevent endoleaks and aneurysm growth. Unfortunately, at the present time we do not have reliable injectable materials with no risk of embolisation into the lumbar or mesenteric arteries, but research will surely propose innovations in this direction soon.
In conclusion, current abdominal aortic endografts are incorporating all innovations suggested by early generation failures and today afford reliable fixation and durability in standard anatomical settings, at least in the mid-term. Unmet needs remain as to how to address difficult anatomies, prevent type II endoleaks and adapt to time-dependent anatomical changes. The newest endograft models are promising, however, an open mind and critical thinking are necessary to evaluate long-term outcomes and determine their role in future developments.
Fabio Verzini is a Professor of vascular surgery at the University of Perugia, Perugia, Italy