Anatomic suitability for endovascular repair of abdominal aortic aneurysms and possible benefits of low profile delivery systems

Abstract

The aim of the study was to evaluate the anatomic suitability for endovascular abdominal aneurysm repair (EVAR) according to instructions for use (IFUs) of three commercially available bifurcated stent graft devices and explore the possible benefits of low-profile delivery systems. Computed tomography scans of 241 patients with abdominal aortic aneurysm (AAA) were evaluated for suitability of Zenith Flex^®, Gore Excluder^® and Endurant^® bifurcated stent graft systems according to their IFUs. The most common exclusion criteria and possible benefits of smaller diameter delivery systems were analyzed. When choosing the most suitable graft model for each patient, the overall suitability was 49.4%. By brand, the suitability was 28.6% for Zenith^®, 25.7% for Gore Excluder^® and 48.1% for Endurant^®. By step wise accepting iliac diameters of ≥6 mm, ≥5 mm and ≥4 mm the overall suitability increased to 56.7, 58.9 and 60.2%, respectively (P < 0.001). Diameters below 4 mm had no additional effect on suitability as combinations of other anatomical features, with or without narrow iliacs, accounted for the remaining excluding factors. In conclusion, Less than half of patients with AAAs are suitable for EVAR according to current IFUs. Low-profile delivery systems may allow for endovascular treatment in up to 60% of patients.

Keywords

endovascular aortic aneurysm repair suitability low-profile delivery systems instructions for use

Introduction

Since the first report of successful endovascular aneurysm repair (EVAR) in 1991,¹ EVAR has become the first-line of treatment in most specialized centers. Evidence suggests that liberal use outside the instruction for use (IFU) definitions is common, compromising the long-term durability of the repair.² Previous reports have shown variable anatomical suitability for EVAR but few are based on the most conservative definitions making comparison difficult between patient cohorts. During the past years, endograft technology has been constantly improving with each new generation of devices being superior to their precursors allowing more patients to be treated. In hope of improving suitability further, a number of EVAR devices with low-profile delivery systems are now being tested in the clinical arena and awaiting regulatory approval. The aim of this study was to assess the current suitability of three widely used infrarenal EVAR devices and explore the possible benefits of low-profile delivery systems on suitability in an unselected cohort of patients with abdominal aneurysms.

Methods

All patients diagnosed with abdominal aortic aneurysms (AAA) at our clinic in 2006 and 2007 were prospectively enrolled in a computerized database. Evaluation of EVAR suitability was analyzed retrospectively in all patients where preoperative computed tomography (CT) scans were available. CT scans were performed on a 16- or 64-slice multidetector spiral CT scanner (Siemens, Erlanger, Germany) and reconstructed with 0.75–5 mm axial slices.

Proximal aortic aneurysm neck was defined as the aortic segment extending from the most caudal main renal artery to the onset of the aneurysm. When no clear transition between aortic neck and aneurysm sac could be defined, the caudal aspect of the neck was considered the point where the outer diameter reached 32 mm. Neck diameter was measured from axial cuts as outer wall-to-wall distance perpendicular to the maximal ellipse on the first transverse CT slice directly distal to the lowermost renal artery. Suprarenal angle (α) was defined as the angle between the flow axis of the suprarenal aorta and the infrarenal neck, and infrarenal angle (β) as the angle between the flow axis of the infrarenal neck and the body of the aneurysm. Angle measurements were done with digital calipers and recorded as the higher value on either anteroposterior or lateral view reconstructions. Aneurysm diameter was defined as the aortic diameter perpendicular to the maximal ellipse at the widest point on axial CT scans. Iliac attachment sites were measured as the outer wall-to-wall diameters in the common iliac arteries directly proximal to the internal iliac artery take-off and smallest iliac diameters as the narrowest inner wall-to-wall lumen from the inferior epigastric artery to the aortic bifurcation. Stent graft IFUs used for analysis of suitability were those of Zenith Flex^® (www.cookmedical.com), Gore Excluder^® (www.goremedical.com) and Endurant^® (www.medtronic.com) (Table 1). To evaluate potential benefits of low-profile delivery systems, we reassessed the suitability after accepting smaller iliac diameters step wise, not changing any other limiting anatomical factors for that particular stent graft model.

Table 1

Anatomical requirements for EVAR according to IFUs

	Neckdiameter(mm)	Necklength(mm)	Suprarenalangle(degrees)	Infrarenalangle(degrees)	Iliacattachment(mm)	IntroducerOD (French)	IntroducerID (French)	Smallest iliacdiameter (mm)
Excluder^®	19–26	≥15	–	≤60	8–18.5	–	18	6.8
	27–29	≥15	–	≤60	8–18.5	–	20	7.6
Endurant^®	19–22	≥15	≤60	≤75	8–25	18	–	6
	23–32	≥15	≤60	≤75	8–25	20	–	6.8
	19–22	≥10	≤45	≤60	8–25	18	–	6
	23–32	≥10	≤45	≤60	8–25	20	–	6.8
Zenith ^®	18–22	≥15	≤45	≤60	7.5–20	–	18	7.1
	23–28	≥15	≤45	≤60	7.5–20	–	20	7.7
	29–32	≥15	≤45	≤60	7.5–20	–	22	8.5

OD, outer diameter; ID, inner diameter; EVAR, endovascular aneurysm repair; IFU, instruction for use

Note that wide necks require wider iliac artery lumen for delivery of the bifurcated component

Statistics

One-way analysis of variance was used for analysis of length, diameter and angulation measurements. Chi-square test was used for gender and suitability comparisons in relation to iliac artery diameter. All statistical analysis was done in SPSS version 17.0 (SPSS Inc, Chicago, IL, USA, www.spss.com). Measurements are presented as mean ± standard deviation (SD). P < 0.05 was considered significant.

Results

A total of 255 patients were diagnosed with AAA at our clinic from January 2006 to December 2007. Of those, 241 had preoperative CT scans available and were included in the study. Of the included patients, 83% were men. Details of aneurysm morphology and morphological differences between genders are given in Table 2. When choosing the most suitable graft model for each patient, the overall EVAR suitability was 49.4%. By brand, the suitability was 28.6% for Zenith^®, 25.7% for Excluder^® and 48.1% for Endurant^®, with 55, 53 and 28 different combinations of exclusions criteria in the patient cohort for each system, respectively. Suitability was significantly less for women (26.8 versus 54.0%, P = 0.002). Narrow iliacs, short necks and combinations of these were the most common reasons for non-suitability (Table 3). By accepting iliac diameter of ≥6 mm the suitability increased by 36.4% for Zenith Flex^®, by 24.5% for Gore Excluder^® and by 13.1% for Endurant^®, making suitability increase by 45.5% in women and by 11.7% in men to an overall suitability of 56.7% (P < 0.001) with no significant difference between genders. Reducing the accepted iliac diameter to ≥5 and ≥4 mm was even more beneficial, making overall suitability 58.9 and 60.2%, respectively (P < 0.001). Diameters below 4 mm had no additional effects on suitability as combinations of other anatomical features, with or without narrow iliacs, accounted for the remaining excluding factors. Details for suitability are given in Table 4.

Table 2

Aneurysm morphology and morphological differences between genders

	Mean	Men	Women
Aeurysm diameter (mm)	63.8 ± 13.9	64.9 ± 14.1	58.6 ± 11.7
Neck diameter (mm)*	26.9 ± 6.6	27.0 ± 6.2	26.8 ± 8.0
Neck length (mm)	21.8 ± 14.5	22.8 ± 14.3	16.9 ± 14.7
Suprarenal angulation(degrees)*	26.2 ± 24.0	25.3 ± 22.8	30.8 ± 4.7
Infrarenal angulation(degrees)*	46.8 ± 25.1	46.3 ± 24.9	49.2 ± 4.3
Right attachmentsite (mm)	15.4 ± 3.9	15.8 ± 3.9	13.4 ± 3.2
Left attachmentsite (mm)	15.5 ± 3.5	15.9 ± 3.5	13.5 ± 2.9
Smallest iliac diameterright (mm)	7.9 ± 1.9	8.2 ± 1.9	6.4 ± 1.2
Smallest iliac diameterleft (mm)	8.0 ± 1.8 mm	8.4 ± 1.7	6.4 ± 1.1

*Non-significant differences

Table 3

Most common exclusion criteria in the patient cohort

	Zenith^®(%)	Excluder^®(%)	Endurant^®(%)
Short neck	27	27	22
Wide neck	13	25	13
Big infrarenal angle	22	23	15
Big suprarenal angle	15	–	8
Attachment too wide right	11	15	0.4
Attachment too wide left	8	16	0.4
Narrow iliac right	33	27	17
Narrow iliac left	33	27	16

There were 55 different combinations of exclusion criteria for Zenith^®, 53 for Excluder^® and 28 for Endurant^®

Table 4

Current differences in suitability between different stent grafts and when reducing the acceptable diameter of the iliac arteries step wise

Suitability	Zenith^®	Excluder^®	Endurant^®	Most suitable
Current	28.6%	25.7%	48.1%	49.4%
OD 6 mm (18F)	39.0% (36.4%)	32.0% (24.5%)	54.4% (13.1%)	56.7% (14.8%)
OD 5 mm (15F)	41.1% (43.7%)	33.2% (29.2%)	56.0% (16.4%)	58.9% (19.2%)
OD 4 mm (12F)	41.9% (46.5%)	43.0% (67.3%)	57.3% (19.1%)	60.2% (21.8%)

OD, outer diameter of delivery system

The relative increase from current suitability is given in parentheses

Discussion

Randomized trials have shown EVAR to be a valid treatment option in patients with AAAs. This minimally invasive approach offers patients a treatment with less morbidity and mortality than conventional open aneurysm surgery.^3–5 The access route is most commonly through the iliac arteries where the delivery of stent grafts through tortuous, calcified or narrow vessels can lead to endothelial damage, dissection or arterial rupture. Significant access related complications occur in 5–17% of cases and poor access is reported as the most common exclusion criteria for EVAR and the leading cause of conversion into open repair.⁶ When used on-label and in adherence to the manufacturers IFUs, currently approved and commercially available devices seem to perform remarkably well but unsatisfactory outcomes continue to occur. It was recently demonstrated that approximately half of patients have been treated outside the approved indications, resulting in aneurysm expansion during follow-up with risk of aneurysm rupture occurring later in life.²

During the past few years, the field of endovascular treatment has rapidly expanded with substantial technological improvements, allowing for treatment of more complex aneurysms. Shorter, wider and more angulated necks are accepted and the delivery systems are hydrophilic and more flexible.⁷ Despite this, a number of unmet needs are unresolved and have appropriately become the primary drivers for ongoing development. Among those are low-profile delivery systems, which are meant to obviate access issues and improve deliverability, as well as to facilitate and strengthen the evolving shift to percutaneous EVAR. In addition to the Endurant^® device, which has an 18-F delivery system for necks up to 22 mm in diameter, a number of devices are waiting US approval with delivery systems between 14F and 18F in outer diameter (Ovation^®, TriVascular, Inc, Santa Rosa, CA, USA; Incraft^®, Cordis Corporation, Bridgewater, NJ, USA; Fortevo^®, Aptus Endosystems, Sunnyvale, CA, USA; Zenith LP^®, Cook Medical, Bloomington, IN, USA). As reports on anatomical suitability for EVAR are inconsistent, varying between 25 and 66%^8–14 and most often referring to the most liberal anatomical restrictions for each device, we assessed the suitability of three commercially available EVAR devices according to the most conservative IFU definitions and explored the benefits of smaller diameter delivery systems on suitability. Our results demonstrate that even with continuous progress in stent graft design, majority of patients are still unsuitable for EVAR. Suitability is highest for Endurant^® (48%) which is the newest device included in this study (US Food and Drug Administration approval in December 2010). This is achieved by smaller diameter delivery system and by accepting more hostile neck morphology and wider iliac attachment zones than the other two models. It is important to emphasize that not only outer diameter but even mechanical properties (flexibility, hydrophilic coating, etc.) are of great importance affecting not only advancement into the aorta but also the rotational movement needed for accurate deployment of the graft. Preoperative or intraoperative adjunctive maneuvers (i.e. angioplasty or the use of internal and external conduits) can also increase the number of patients treated without compromising the durability of the repair. Reducing diameters of delivery systems will increase overall suitability of EVAR devices by 10% at the most if no other factors in stent graft design are modified. This relates to the fact that non-suitability depends in equal amount on aneurysm neck characteristics excluding patients from infrarenal repair. In order to offer patients with hostile neck anatomy the positive benefits of EVAR, more complex devices are now available that allow for the sealing zone of the graft to be moved more proximal, either to the level of the visceral arteries or even further to the supracoeliac region. In these devices, the arterial blood flow to the vital organs is secured through fenestrations (fenestrated EVAR) or branches (branched EVAR), respectively. So far, those advanced systems require even larger delivery systems due to the more complicated graft design and the simultaneous need for multiple catheters and wires for successful deployment. When smaller deliver systems can also be applied to these more complex endografts, many of the anatomical restrictions inhibiting EVAR can be overcome.

Conclusion

Despite rapid device development during the past few years, most patients remain anatomically unsuitable for infrarenal EVAR according to the most conservative IFU definitions. Step-wise reduction in delivery system diameter can increase suitability by 10% at the most as hostile neck anatomy accounts for the remaining exclusion criteria.

Footnotes

Ethical approval: The study was approved by the institutional review board.

Guarantor: TK.

Contibutorship: TK, BS, ND, MM and TR.

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