Immediate Change in Suprarenal Neck Angulation After Endovascular Aneurysm Repair

Introduction

The vast majority of commercially available aortic stent-grafts today use continuous outward radial force from a nitinol self-expanding stent to fix the device proximally to the aortic neck and secure the graft material to the wall to provide a blood-tight seal. In contrast, the Ovation Abdominal Stent Graft System (TriVascular, Santa Rosa, CA, USA) uncouples the fixation and sealing mechanisms. This trimodular endoprosthesis, recently introduced for the endovascular repair (EVAR) of abdominal aortic aneurysms (AAA),^1–3 has a sealing mechanism based on 2 polymer-filled inflatable rings below the bare 35-mm nitinol stent that provides suprarenal fixation. The compliant, inflatable sealing O-rings are filled with a low-viscosity radiopaque polymer during stent-graft deployment, tightly conforming to the luminal surface of the aortic neck to provide an effective, gasket-like seal (Figure 1). ⁴ Since the presence of radial force has been implicated in aortic neck expansion and degeneration after stent-graft implantation, leading to loss of seal, type Ia endoleak, and device migration, it has been postulated that Ovation’s sealing mechanism avoids significant mechanical load and stress since the O-rings exert a minor pressure of only 1 atm.^5–7

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Figure 1.

Illustration of the structural differences between the Endurant nitinol-based stent-graft (A) and the Ovation sealing-ring mode (B). While the nitinol skeleton of the conventional stent-graft in A exerts a continuous radial force over the infrarenal neck, the polymer-filled sealing rings of the Ovation device in B provide a gasket-like effect without radial force to ensure sealing. The vertical dotted line in B depicts the area of interest, which is better described by the suprarenal angle, as explained in the text.

It has been shown that the neck angulation decreases immediately after EVAR and continues to decrease slowly thereafter.^8,9 van Keulen et al ⁸ investigated the alteration of aortic neck angulation immediately after EVAR using nitinol self-expanding endografts with either suprarenal (Talent; Medtronic CardioVascular, Santa Rosa, CA, USA) or infrarenal fixation (Excluder; W.L. Gore & Associates, Flagstaff, AZ, USA). The authors showed a statistically significant change in the mean suprarenal and infrarenal angles by a mean difference of 5°±1° and 8°±2°, respectively (p<0.01). Similarly, Lee et al ⁹ observed an immediate (1 month) decrease in the supra- and infrarenal angles after EVAR (32.1°±21.6° to 27.0°±17.8° and 43.4°±25.0° to 30.4°±17.6°, respectively). Interestingly, a recent bioengineering study by De Bock et al ¹⁰ demonstrated that the suprarenal angle can more adversely influence neck geometry as it was found to correlate with greater asymmetry in the forces distributed over the aortic neck, which is a relative measure of how these forces act to straighten the proximal neck. To our best knowledge, these mechanical properties of the Ovation Abdominal Stent Graft System have neither been directly examined nor compared with other fixation-sealing mechanisms.

Hoshina et al ¹¹ suggested that straightening of the aortic neck can be device specific. Since nitinol self-expanding endografts can change neck angulation immediately postoperatively, this study compared the immediate postoperative suprarenal neck angulation between an endograft employing a conventional sealing mode and the Ovation’s sealing ring mechanism.

Methods

Measurement Technique

The widest angle between the proximal infrarenal neck beginning just caudally to the lower renal artery ostium and the suprarenal aortic axis as described by current reporting standards^12,13 was calculated by an examiner in each center. To minimize any potential influence of the angulation itself on the measurements, the angulation of the central lumen line was calculated perpendicular to the aortic lumen in the middle of the flexure (Figure 2). For simplicity, the angle values are reported as the acute equivalent (180° minus the measured wide angle; see Figure 2).

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Figure 2.

Measurement of the suprarenal angles in patients with infrarenal abdominal aortic aneurysm prior to (A) and after endovascular aortic repair with the Endurant stent-graft (B) and another patient with the Ovation (C) endoprosthesis. The suprarenal angle is the acute angle derived by subtracting the measured wide angle (shown here) from 180°.

To determine intraobserver variability, one examiner in each center measured all aortic angles (preoperative and postoperative) twice, with an interval of 2 weeks. The intraobserver variability for the angle measurement technique was calculated using the intraclass correlation coefficient (ICC) for each examiner and for the total of all measurements (preoperative and postoperative).

Results

The measurements of the AAA geometric characteristics in both groups are presented in Table 1. The mean preoperative neck angulation in the Ovation cohort (group O) was 23.2°±18.0° (0°–70°) compared to the 23.8°±22.9° (0°–80°) in the Endurant patients (group E; p=0.91). The neck length was 29.2±14.6 mm (8–58) and 23.2±11.0 mm (5–47) in groups O and E, respectively (p=0.1). Similarly, the minimum and maximum neck diameters were 22.4±2.6 mm (17–30) and 25.0±3.5 mm (19–32) in group O vs 23.3±3.6 mm (19–31) and 27.0±5.7 mm (21–31) in group E (p=0.3 and 0.12, respectively). The maximum transverse diameters of the AAA in the groups were similar: 57.0±9.0 mm (50–85) in group O vs 53.2±11.1 mm (50–70) in group E (p=0.17).

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Table 1.

Anatomic Characteristics of the Abdominal Aortic Aneurysms in the Study.

	Ovation a	Endurant a	p
Preoperative angle, deg	23.2±18.0	23.8±22.9	0.88
Postoperative angle, deg	10.3±9.4	18±23	0.1
Angle change, deg	13.2±16.1	6.1±5.9	0.04
Neck length, mm	29.2±14.6	23.2±11.0	0.10
Maximum neck diameter, mm	25.0±3.5	27±5.7	0.12
Minimum neck diameter, mm	22.4±2.6	23.3±3.6	0.30
Aneurysm maximum diameter	57.0±9.0	53.2±11.1	0.17

a

Data are presented as the means ± standard deviation.

Interestingly, the Ovation stent-graft caused significantly greater decrease in the AAA neck angulation postoperatively compared to the Endurant device (13.2°±16.1° vs 6.1°±5.9°, p=0.04). Multiple regression analysis (Table 2) revealed significant influence both of the preoperative neck angulation (β coefficient 0.37, p<0.001) and the type of endograft (β coefficient −7.91, p=0.01) on postoperative neck angulation change.

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Table 2.

The Relative Contribution of Study Parameters to the Change in Suprarenal Neck Angulation.

	β Coefficient	Standard Error	p
Intercept	4.803	15.402	0.756
Preoperative neck angulation	0.376	0.075	<0.001
Neck length	−0.082	0.116	0.483
Maximum neck diameter	0.191	0.331	0.566
Type of endograft	−7.913	3.100	0.013
Maximum aneurysm diameter	0.080	0.525	0.601

The ICC ranged from 0.951 to 0.990 and from 0.911 to 0.999 for the preoperative and postoperative measurements of individual examiners, respectively. Moreover, the ICC for the sum of all patients was 0.980 for the preoperative and 0.976 for the postoperative measurements, respectively.

Discussion

In order to achieve an ultralow-profile (14-F outer diameter), the Ovation device lacks the conventional stent-graft sealing mode. On the contrary, the sealing of the Ovation depends on the first inflatable ring, provided that the neck diameter at this point (ie, 13 mm below the lowest renal artery) is not larger than 30 mm, irrespective of the central aortic neck diameter or any diameter discrepancy along the neck length.

The lack of a nitinol endoskeleton against the infrarenal aortic neck in this endograft, with consequent decrease of radial force, may to some degree theoretically alter the resulting mechanical loading and stresses acting on the aortic neck.^10,14,15 The aforementioned factors are reflected in the post-EVAR geometric neck changes caused by insertion of an endograft. In other words, the angulation change comprises a conformational alteration created by significant mechanical stretching ¹⁰ and is an ongoing process even after 3 years, as clearly depicted by van Keulen et al. ⁸ However, since the post-EVAR aneurysm remodeling and therefore the endograft configuration are affected in the long term (shrinkage, endoleak, kinking, and endotension),^16,17 it would be logical to assume that any change in aortic neck angulation immediately postoperatively (as documented specifically in the first postoperative CTA within the first 30 days) will reflect the effect of different device mechanics between various types of endografts.

Admittedly, both endografts included in the current study employ a nitinol stent extending into the suprarenal segment to ensure fixation, with the stent of the Ovation device being longer and more rigid. Since the suprarenal angulation measured and analyzed here refers to the angle between the proximal aorta and the aortic neck and taking into account that the longer/stiffer suprarenal stent and the inflatable O-rings of the Ovation endograft are located inside these aortic segments (ie, the suprarenal aorta and the infrarenal aortic neck, respectively), these 2 factors may result in the observed increased post-EVAR angulation change with the Ovation device. Since it is not possible to determine the degree to which the suprarenal stent and/or the nitinol-free sealing mechanism contribute to this outcome, the key point remains that the increased postoperative straightening among patients treated with the Ovation endograft may have significant hemodynamic implications, as described previously.

Although the infrarenal angulation has been associated with the magnitude of displacement forces leading to migration and potential type Ia endoleak,^14,18–20 it is the suprarenal angle that defines better those AAA segments that sustain the forces and consequent stresses predisposing to future neck dilatation, that is, forces acting vertically to the aortic neck’s surface in contrast to the displacement forces acting caudally or anterolateraly onto the surface of the endograft and leading to migration of the latter. ¹⁰ In this respect, the rationale of our study was to examine the immediate geometric alterations of this segment, that is, the suprarenal angle, which mirrors the influence of the Ovation’s longer suprarenal stent and its rings-based sealing mechanism immediately caudally to the renal arteries.

Our study was designed to report only the preoperative and the first month postoperative AAA suprarenal angulation changes in order to compare the Ovation’s unique features with that of the nitinol-based continuous radial force. As can be seen in Table 1, the basic anatomic neck characteristics were equal between the groups; therefore, any suprarenal neck angulation change could be attributed to the forces associated with the different modes between the 2 endografts, ie, the self-expanding radial force (Endurant) vs the gasket-like inflatable rings along with the longer and rigid suprarenal stent (Ovation). The preoperative neck angulation was aligned with the IFU of both endografts to avoid any discrepancy between the two, although satisfactory results have been reported even with severe neck angulations with the Endurant device.^21,22 The suprarenal angulation measurement was based on standard guidelines.^12,13

The results of our study show a greater immediate postoperative reduction of the neck angulation with the Ovation device, possibly attributed to its structural changes and modified stiffness.^4,15 As presented in Table 2, the Ovation device may confer an additional decrease of almost 8° in the postoperative suprarenal angulation change when compared with the Endurant stent-graft. One should bear in mind that additional straightening may mirror greater stresses acting on the aortic neck surface and predispose to neck enlargement, so the reduced angles observed with the Ovation device may offer some advantage regarding better average apposition and area reduction of the endograft. ¹⁰ These, however, need to be proven in the long-term performance of the Ovation device. Of note, the long-term outcomes with the Endurant stent-graft showed a very low incidence of type Ia endoleaks (1%) and no proximal migration. ²³ Thus, focused clinical studies are mandatory to examine (or even validate) the clinical significance of our findings and a potential predictive role. It is imperative to say that our preliminary results cannot directly be associated with the outcome after EVAR; more likely, our study focuses the role of aortic neck angulation change as a means of detecting and describing the effects between different structural patterns and properties of AAA endografts and providing a more comprehensive idea about their interaction with the aneurysm.

Although most of the AAAs of our study had rather small angles, our initial findings suggest a variant effect of different endografts’ mechanics. According to the results of multiple regression analysis, the preoperative suprarenal neck angulation was found to significantly influence the degree of postoperative angulation change. An increase of preoperative neck angle by 1° causes an additional decrease in postoperative angle by almost 0.34° or, more practically speaking, a preoperative angulation increase of 30° may lead to a further postoperative angulation decrease of 10°. Therefore, it would be of great interest to expand further the comparison between the different mechanics of various devices in aneurysms with greater angles and/or shorter necks, that is, in really challenging AAA neck anatomies, where geometric differences evoked by the different modes may translate into different clinical adverse effects.

The post-EVAR changes of infrarenal neck angulation were not in the scope of our study, since we intended to focus on suprarenal angulation to better mirror the exertion of forces and stresses onto the AAA neck surface. However, the role of post-EVAR infrarenal angulation change should not be underestimated, since it affects the flow recirculation and separation in the cephalad segment of the endograft, influences strongly the displacement forces acting on the endograft, and has been associated with the risk of migration and type Ia endoleak.^9,17–19

Interestingly, Lee et al ⁹ reported a strong association between the preoperative suprarenal and infrarenal angulation in their study (r=0.72, p<0.001), which included stent-grafts with nitinol-based radial force involving a suprarenal fixation stent (Zenith, Endurant) or not (Excluder). According to their study, the suprarenal aortic angle decreased immediately after EVAR and continued to decrease during the first month, unlike the infrarenal aortic angle that continued to decrease slowly thereafter. In contrast, van Keulen et al ⁸ reported a continuous significant decrease of both the suprarenal and infrarenal angles both for Talent and Excluder after EVAR extending up to 3 years. Admittedly, the literature lacks sufficient evidence to delineate the post-EVAR changes of the supra- and infrarenal neck angles in the long term, the interconnection between them, and their potential influence on the conformational alterations of the distal neck and the iliac angulation. Moreover, further research is warranted to identify whether the aforementioned changes are connected with a device-specific effect; this may have possible implications in the evaluation of various fixation and/or sealing mechanisms and the development of future endografts.

Conclusion

The longer and stiffer suprarenal stent and the nitinol-free sealing mechanism of the Ovation stent-graft system present a totally new perspective in the sealing-fixation philosophy. Our results show that the innovative design of this device may affect directly the post-EVAR suprarenal angulation, causing a greater straightening than the conventional stent-graft sealing mode, at least in cases with small preoperative angulations. Further clinical estimation of the changes in the supra- and infrarenal aorta seems justified in order to evaluate the advantages or disadvantages between the different sealing patterns.