Keywords
Ovation Alto - abdominal aortic aneurysm - endovascular aneurysm repair - wide neck
- juxtarenal aneurysm
Introduction
Endovascular aneurysm repair (EVAR) was introduced in the early 1990s as an alternative
method to open surgery for the treatment of abdominal aortic aneurysm (AAA).[1] Nowadays, EVAR comprises the first-choice treatment for the majority of patients
and accounts for >75% of AAA repairs.[1] Multiple randomized trials have documented the lower postoperative morbidity and
mortality, faster discharge, and fewer complications of EVAR compared with open surgical
AAA repair in the early period, enabling repair in an increasing number of patients
with comorbidities as well as application in the emergency setting of ruptured AAA.
Despite the documented advantages of EVAR, there is concern regarding the higher late
reintervention rates and the need for lifelong monitoring due to potential complications.
The preservation of a successful seal between the aortic endograft and the infrarenal
aortic neck determines the long-term durability of EVAR.
Some aortic neck characteristics contribute to the definition of “hostile neck,” particularly
length ≤10 to 15 mm, large diameter >32 mm, tapered or reverse tapered anatomy, mural
thrombus, circumferential calcification, and angulation.[2] Hostile neck morphology is generally associated with higher rates of endoleak, stent
migration, and reintervention.[3] The off-label use of standard EVAR is sometimes applied for patients who are not
eligible for open surgery. Up to 44% of EVAR cases are performed using stent-grafts
outside their Instruction-For-Use (IFU; off-label).[4] In such cases, higher rates of Type I endoleaks may be expected, mainly in short
necks; yet for patients with severe angulation or high thrombus load in the proximal
neck, results of outside IFU EVAR seem comparable to the results of inside IFU.[5]
The Ovation Alto: Application to Short Necks
The Ovation (Endologix Inc. Irvine, CA) aortic endograft combines active suprarenal
fixation via a long nitinol stent and a pair of polymer-filled inflatable rings to
accommodate effectively even a wide range of infrarenal neck irregularities, including
conical morphology, circumferential calcification, and significant thrombus onto the
neck surface. The recently FDA approved Ovation Alto design lacks the nitinol-containing
fabric of the previous Ovation iX platform, repositioning the maximum diameter of
the proximal sealing ring 7 mm below the lowermost renal artery.[6] The key-point in the Alto design is the inflation of a compliant balloon integrated
in the delivery system to accomplish the deployment of the sealing rings and their
optimal apposition immediately after the polymer filling. The accommodation of the
sealing rings immediately below the renal arteries provides the shortest sealing zone
for EVAR, thus broadening the indications to include more complex AAA.
There is only scarce data in the current literature focusing on the applicability
and technical success of the new Alto design. The recently published data by Holden
and Lyden report excellent technical success, yet in AAA with an adequate length to
the infrarenal neck of 11 to 35 mm.[6] Additionally, de Donato et al[7] documented satisfying technical results with the use of the new Ovation Alto platform
in 11 AAA cases, mainly with short necks ≤10 mm. The aim of this manuscript is to
analyze the unique technical features of this innovative endograft design and discuss
their applicability to some AAA anatomies whose treatment presents a surgical challenge.
Materials and Methods
We demonstrate four representative examples of challenging AAA where the use of Ovation
Alto offers a promising solution confronting anatomical issues not amenable to standard
Nitinol-based endografts. The treated AAA were chosen at the authors' discretion,
representing cases of challenging anatomies reaching or falling outside the IFU of
other endografts.
Informed consent has been obtained from all patients for publication of the cases
and the accompanying images. This study was conducted in accordance with the ethical
standards of the institutional and national research committees and in accordance
with the 1964 Helsinki Declaration. The internal scientific review committee approved
the aim and methodology (retrospective review of surgical records).
Case Presentation
Case 1
A 63-year-old male patient presented with a 52-mm AAA ([Fig. 1A]) with a 30-mm wide neck at 7 and 10 mm below the lowermost renal artery ([Fig. 1B]) with an infrarenal angulation of 54 degrees. The patient was treated with a 34 mm
Alto endograft with a distal diameter of the right and left iliac limbs of 18 and
14 mm, respectively. The 1-month follow-up computed tomography angiography (CTA) documented
completely adequate postoperative sealing ([Fig. 1C]) and absence of endoleaks ([Fig. 1D]).
Fig. 1 (A) Case 1 aneurysm with wide neck. (B)The proximally angulated infrarenal neck at 10mm below the renal level. (C) Efficient sealing relies on the adjusted apposition on the conical neck irregularities.
(D)The postoperative computed tomography angiography at one month shows efficient apposition
of the inflatable rings at the sealing level, with no endoleaks.
Case 2
A 71-year old female presented with a juxtarenal AAA of 55 mm. The infrarenal neck
was aneurysmatic starting immediately below the renal arteries. The external diameter
(outer-to-outer) of the neck at the level 7 mm below the lowermost renal artery was
3.3 to 3.7 mm. Interestingly, due to the heavy intraluminal thrombus burden, the true
lumen dimensions were estimated 1.9 to 2.5 mm. It is crucial to delineate that the
sizing philosophy of the Ovation sealing rings takes into account the lumen diameter
plus half the thrombus thickness on both sides but to a maximum value of 2 mm. Hence
the patient was treated with a 26-mm endograft, achieving excellent intraoperative
sealing at completion angiography, as well as in the 1-month postoperative CTA ([Fig. 2]).
Fig. 2 The juxtarenal aortic aneurysm of case 2. The arrowhead shows the pair of sealing
rings accommodated against the thrombus surface in this computed tomography with IV
contrast.
Case 3
A 66-year-old male patient was admitted with a 53-mm AAA of angulated and short neck
of 10-mm length and 26-mm diameter([Fig. 3A]).The patient was treated with a 29 mm Ovation Alto. The radiopaque inflatable rings
showed excellent conformability on the short angulated infrarenal neck ([Fig. 3B–D]) and proper sealing in the 1-month postoperative CTA.
Fig. 3 (A) The short-necked aneurysm of case 3. Note the infrarenal angulation of the neck.
(B) Coronal plane of the 1-month no-contrast postoperative follow up, showing the accurate
apposition of the sealing rings immediately infrarenally. (C) The lack of any fabric zone between the proximal bare stent and the sealing rings
does not restrict the flexibility of the endograft to adjust efficiently in this short
and angulated neck. (D)The endograft adapts efficiently to the curvature between the supra- and infrarenal
angulation despite the long proximal stent and the short main body.
Case 4
A 66-year-old male patient was admitted with a 53mm AAA of conical (reverse tapered)
neck with neck diameter increasing from 20 mm immediately infrarenally to 23 at 10 mm
below the lowermost renal artery ([Fig. 4A]). The patient was treated with a 26-mm Ovation Alto. The radiopaque inflatable rings
showed excellent conformability on the irregular neck surface and efficient sealing
in the 1-month postoperative CTA ([Fig. 4B]).
Fig. 4 (A)The aortic aneurysm of the fourth case with reverse tapered neck. (B)The pair of sealing rings in the 1-month postoperative follow-up accommodating perfectly
in the conical neck configuration (dashed lines).
Discussion
While certain anatomical parameters, that is, neck angulation, short length, and wide
diameter are well documented to limit the successful sealing and/or its long-term
preservation, the term “hostile neck” is still ill-defined, since other parameters,
such as the morphology of infrarenal neck, the percentage of calcification, and the
thickness/circumference of intraluminal thrombus have not been adequately delineated.
The role of neck thrombus is quite ambiguous, although a multivariate analysis in
a study group from the ANCHOR registry identified a potential risk for neck thrombus
based on its thickness and circumference >11 degrees.[8] While a combination of the aforementioned parameters can be associated with intraoperative
challenges and adverse effects, the Alto unique sealing philosophy surpasses these
reservations, challenging the aforementioned traditional definitions.
While the initial indication for Alto refers to short necks, a careful appreciation
of its sealing mechanism and the perception of the exact apposition sites of the inflatable
rings may further prove beneficial for other anatomical constraints. The ability of
gradually inflated rings that accommodate perfectly onto noncylindrical lumen appears
ideal for conical-shaped necks, an anatomical feature associated with proximal failure
of other conventional endografts.
The unique Alto design may also prove beneficial in cases of wide necks ≥30 mm, which
comprise an independent risk factor for neck-related adverse events even in midterm
follow-up, with endoleak Type 1a being predominant. Since the causes of post-EVAR
aortic neck dilatation have not been recognized, factors such as the nitinol-based
continuous radial force have been implicated along with the degree of graft oversizing
and the ongoing degenerative aneurismal process. While none of these has been identified
to be a dominant and independent factor, it seems yet that the wide necks are affected
to a greater degree from the aforementioned and have a greater rate of dilatation
over time. Notably, although there has been no strong association with this regard
in the literature, a short neck correlates more likely with wider neck diameter. Therefore,
a new concept of more proximal sealing with minimal radial force provided by Alto
seems appealing also for wide necks. Most importantly, the unique concept and advances
of the Ovation Alto can be an alternative to the more technically demanding solutions
of fenestrated EVAR and parallel grafts for certain juxta or pararenal AAA.
Typically, the new Alto design is currently the only endograft approved for AAA with
necks ≥7 mm. Notably, in conical neck configurations, as defined by >10% difference
in neck diameter from the inferior renal artery to the level 7 mm caudally or a gradual
neck dilation >2 mm (reverse taper anatomy), or “barrel-shaped” neck shapes, the neck
diameter measurements may not apply to a unique endograft size; for example, a neck
diameter discrepancy from 22 mm at the renal level to 24.5 mm at 7 mm caudally applies
to the 26- and 29-mm endograft size, respectively. Hence, it is imperative that the
physician studies thoroughly the apposition sites of the rings to choose the optimal
endograft size.
Technically, since the use of the Alto platform is applied in very challenging necks
with, by definition, short necks and irregular shapes, it is imperative to ensure
that the apposition and inflation of the sealing rings will be conducted in the exact
proper position, leaving practically no place for the slightest misplacement. Normally,
the configuration of infrarenal neck can be straightened due to presence of super-stiff
guidewires, leading to polymer infusion and sealing in a neck shape that will be altered
after removal of the guidewires with consequent loss of any marginal sealing. Therefore,
it is preferred to withdraw the super-stiff guidewire supporting the endograft until
its soft proximal end approaches the nose cone of the delivery system. This maneuver
facilitates the ring's inflation in a “neutral” infrarenal neck, avoiding any postoperative
tension and strain on the neck surface.
Caution should be paid when using Ovation Alto in AAA with significant suprarenal
and infrarenal angulation due to the short length of the endograft's main body, especially
if the caudal end of the unsupported fabric segment and the origin of the iliac limbs
gates are to be located at the verge of significant angulation, predisposing to kinking
([Fig. 5]). Therefore, additional caution may be needed when treating AAA of short and angulated
necks with Ovation Alto.
Fig. 5 Representation of deployment of Ovation Alto in an abdominal aortic aneurysm with
significant infrarenal neck angulation. The narrow distance between the unsupported
main body and the limb gates can lead to positioning of the latter at a site of steep
angle, predisposing to kinking.
Certain points uniquely related to the Ovation platform should be taken into consideration:
while the outer-to-outer wall diameter determines the size in Nitinol-based endografts,
the Ovation graft is also influenced by the lumen diameter and the thrombus thickness
as well (i.e., the lumen diameter plus half the thrombus thickness on both sides but
not exceeding a maximum value of 2 mm). Given that the thrombus layers are not of
equal density or mechanical properties, the Ovation size achieves optimal apposition
of the rings against a partially compressed thrombus. These tactics, along with the
quite immediate origin of sealing rings below the suprarenal stent, enable the application
of Ovation Alto to para- or juxtarenal AAA, as in case 2, where the sealing relies
exclusively on the positioning onto the thrombus surface ([Fig. 2]), since the typical infrarenal sealing zone can be absent, short, and/or wide and
carrying a significant burden of intraluminal thrombus. In such off-label cases, the
use of Ovation Alto is very ambitious and challenging, especially when taking into
consideration that the current guidelines of the European Society for Vascular Surgery
dictate the fenestrated approach for juxtarenal or pararenal AAA, leaving as a bailout
solution the option of parallel grafts (periscope/chimney technique), as long as these
are limited to a maximum of two parallel grafts. Long-term results on the durability
of this Ovation Alto sealing concept are mandatory to confirm whether this management
of complex AAA comprises a fair alternative.
As previously described in the literature, a great concern associated with the use
of Ovation in EVAR remains the limitation and applicability of solutions to manage
central endoleaks, since the stiffening of the sealing rings precludes the effective
use of central cuffs or stents due to the polymer solidification. The sealing agents
most effectively used are glue and coil embolization for Type-Ia endoleaks. Therefore,
especially with the new Ovation platform, the best way to manage such a complication
is avoiding creating it, by careful selection of patients and very meticulous preoperative
planning with a thorough identification of the sealing sites.
Conclusion
The Ovation Alto design is the evolution of previous Ovation platforms relocating
the unique polymer-filled sealing rings more proximally and omitting the nitinol-containing
zone in-between, enabling treatment of AAA with short or conical-shared necks. While
the initial FDA-approved indication is strictly limited to AAA with proximal parallel
necks of length ≥7 mm with angulation ≤60 degrees, other neck irregularities or challenging
anatomies such as wide diameter ≥28 mm or juxtarenal morphologies could be hypothetically
addressed with the new design in carefully selected patients by experienced physicians.
