Continuous aortic enlargement at the level of infrarenal aortic neck has been reported after endovascular repair (EVAR) of abdominal aortic aneurysm. Current concepts regarding the reasons for this phenomenon remain poorly understood, although it is known that the amount of proximal device oversizing with self-expanding stent grafts influences neck progression. Once deployed, self-expanding grafts continue to expand until the nominal diameter is reached, unless tissue resistance limits its expansion. It has been reported that when aortic neck dilatation occurs it is related to adverse mid-term outcomes.
The beginning of the EVAR era was characterised by endografts that essentially consisted of a tubular fabric graft attached at both ends by a large balloon-expandable stent graft, such as the home-made “Parodi endograft” and the “Montefiore endograft system”. Not surprisingly, because of the lack of a chronic outward force at the level of proximal neck, these devices were not associated with neck dilatation over time, even though stent migration and endograft collapse into the aneurysmal sac were possible complications. With the advent of self-expanding grafts, balloon-expandable grafts have been rapidly disappearing from the market for several years.
Traditional self-expanding stent grafts require an infrarenal non-aneurysmal segment—the so-called aortic neck—to adequately seal the aneurysm sac from chronic circulatory pressures. Sealing is obtained by oversizing the stent (from 10–30%), expecting that the chronic radial force exerted longitudinally against the aortic wall will circumferentially prevent any leakage.
For a long time, the only available technology for sealing had been based on the application of self-expanding forces to a portion of healthy proximal aortic neck. The recent appearance on the market of the Ovation endograft (Endologix) represents a valid alternative to chronic outward forces for an appropriate sealing.
The Ovation endograft, with its new concept of sealing by non-expansive circumferential apposition of polymer-filled rings to aortic wall, creates no chronic outward force at infrarenal aortic level.
The technology is based on a new sealing concept that redefines the idea of aortic neck length. The polymer-filled sealing ring provides uniform, non-expansive, continuous wall apposition that aims to isolate the aortic neck from circulatory pressures. Sealing is not longitudinal and related to neck length, but circumferential and based on the apposition of the polymer-filled ring to the aortic wall 13mm below the lower renal artery. This makes Ovation the only stent graft approved by the FDA for EVAR that is not restricted by the conventional measurement of aortic neck length. Actually, the FDA criteria for the use of the endograft is the presence of an inner wall diameter ≥16mm and ≤30mm at 13mm below the inferior renal artery to allow correct apposition of rings to the aortic wall.
This sealing mechanism, which is completely different from that obtained by self-expanding grafts, promises to isolate the aortic neck from blood pressure, thus preventing aortic neck evolution over time.
We reported, for the first time in the literature, mid-term clinical outcomes from a series of patients treated with this new sealing technology, describing really encouraging data (two-year assisted primary clinical success rate of 100%, three-year estimated freedom from type I endoleak of 98%).
With a core lab analysis of morphological changes, our research evaluated aortic neck evolution at two years after EVAR by endograft with no chronic outward force, revealing no aortic neck dilatation. This may suggests that aortic neck evolution is not associated with EVAR at mid-term follow-up when an endograft with no chronic outward radial force is implanted.
The aortic neck can definitely maintain its diameter when the radial force applied by the stent graft is lower than the recoil force of the elastic aortic wall. Nowadays—with balloon-expandable stents removed from the market—the sealing technology of Ovation, which does not apply chronic outward force, may guarantee a similar phenomenon as suggested by the absence of neck dilatation and migration in our study.
In a recent subanalysis of our registry we have further investigated the behaviour of Ovation in the absence of a suitable neck length of 7mm, revealing that it was not associated with poor outcomes in the midterm period. These data showed that the use of Ovation for the treatment of infrarenal abdominal aortic aneurysms is not restricted by the conventional measurement of neck length, as confirmed by the recent clarified FDA indication statement. Our data confirm that the Ovation sealing ring creates uniform continuous wall apposition, and provides circumferential non-expansive seal at midterm follow-up, independent of neck length. This means that the Ovation system may be used in all aneurysmal necks when the inner wall diameter is the range of compatibility with the polymer-filled rings (16–30mm).
Carlo Setacci is the chief of the Vascular and Endovascular Surgery Unit at the University of Siena, Siena, Italy