Some form of anatomical fixation during endovascular aneurysm repair (EVAR) is advantageous and should be considered necessary in 2012. There is a surfeit of experiential and scientific data indicating the benefits of anatomic fixation in general and suprarenal fixation in particular.

THE PROBLEM: STENT GRAFT MIGRATION

Stent graft migration is defined as distal movement > 10 mm or movement ≤ 10 mm when resulting in secondary intervention, according to the Society for Vascular Surgery reporting standards for EVAR.1 It is not surprising that migration is an Achilles' heel of EVAR. Using cadaveric aorta, Resch et al showed that the tractional force required to dislodge any stent graft was much less (by a factor of 6) than that required to disrupt a sutured anastomosis.2 Stent graft migration is associated with type I proximal endoleak and sac pressurization; therefore, reintervention is required to avoid aneurysm growth and potential rupture. Consequently, meticulous attention to stent graft sizing, device selection, and deployment should be undertaken to minimize the risk of migration. With regard to preventing migration and proximal endoleak, two adverse outcomes that are intimately related, authorities in the field have previously advocated for transrenal baremetal stents in aortas with short- or large-diameter proximal necks.3

DEVICES WITH SUPRARENAL FIXATION: EXCELLENT RESULTS

The Zenith endograft (Cook Medical, Bloomington, IN) (Figure 1A) was the first device approved by the US Food and Drug Administration (FDA) that has suprarenal fixation. The 5-year results of its pivotal trial demonstrated very low migration rates.4 Owing to its pararenal bare-metal stents, attachment hooks arising from those stents, and the availability of a 36-mm proximal sealing stent (still among the largest proximal devices available and approved for EVAR), this device is reputed to be able to treat unfavorable necks.5 In one single-institution study comparing the clinical results of various endografts, the Zenith device had no incidence of migration.6

Two Medtronic, Inc. (Minneapolis, MN) EVAR devices incorporate suprarenal stents. The Talent device incorporates suprarenal bare-metal stents without hooks or barbs and is FDA approved for use in short (10 mm) aortic necks, one of only two devices with such approval. The Endurant device (Figure 1B), meanwhile, has both suprarenal stents, as well as hooks on those stents, and is also approved for use in short (10 mm) aortic necks. There were no migrations, ruptures, conversions, or type I endoleaks during the first year of follow-up in the United States pivotal trial of the Endurant graft.7 Remarkably, in a small study examining patients with hostile anatomy (including proximal neck lengths between 5 and 10 mm and highly angulated proximal necks) treated with the Endurant device, there were no migrations and no type I endoleaks on early follow-up.8

The AFX device (Endologix, Inc., Irvine, CA), the successor to the Powerlink stent graft (Endologix, Inc.), is designed to rest on the aortic bifurcation.9 The AFX device incorporates optional pararenal stents (Figure 1C) without fixation hooks or barbs.

EXPERIMENTAL MODELING TO PREDICT MIGRATION FORCES

Using electrocardiogram-gated computed tomography scans, the Utrecht group showed that patients with stent graft migration had more aortic distensibility than those who did not, although none of the patients had grafts with suprarenal fixation.10 Similarly, the UCSF group has demonstrated, using computational fluid dynamics data derived from patient-specific computed tomography data, that hydrostatic pressure results in larger forces on the stent graft than do sheer stresses from pulsatile flow.11

In an elegant study using fluid structure interaction modeling—a combination of finite element analysis of solid structures and computational fluid dynamics of blood flow—Molony et al demonstrated that anteroposterior neck angulation increased drag forces, and presumably migration risk, in a variety of stent grafts in 10 patient-specific geometries.12

Using a mathematical construct, the University of Liverpool group studied factors expected to increase stent graft migration and identified—in addition to hypertension and aneurysm sac features—graft tapering from proximal neck to iliac limbs as a factor expected to increase migration forces.13 Similarly, Morris et al demonstrated increased drag forces with larger-diameter infrarenal sealing stents using a computational fluid dynamics model and idealized endograft geometry.14

These experimental studies suggest that migration forces are increased in patients with hypertension, angulated necks, or large-diameter necks. Unfortunately, the anatomy of an individual patient's proximal aortic neck is not modifiable. Therefore, choosing the most appropriate endograft to counteract tendencies to migrate in these clinical scenarios is crucial.

In demonstrating that the force required to dislodge an EVAR device was almost an order of magnitude smaller than that needed to disrupt a proximal aortic open surgical anastomosis, Resch et al also provided evidence that balloon-expandable stents and hooks or barbs significantly increase the proximal fixation strength of endografts.2

SUPRARENAL FIXATION IN CHALLENGING INFRARENAL NECK ANATOMIES

Case 1

A 56-year-old man with an asymptomatic 6-cm abdominal aortic aneurysm (AAA) with a short trapezoidal aortic neck (Figure 2) underwent elective aneurysm repair using a 28-mm main body Endurant device. One month after surgery, his aneurysm was stable in diameter without endoleak.

Case 2

A 69-year-old man with a symptomatic 6-cm AAA with a short neck was admitted and underwent EVAR urgently. A 28-mm Zenith main body was implanted (Figure 3). On completion arteriography, flow into the right renal artery seemed sluggish; therefore, the right renal artery was stented using a 6-mm Herculink stent (Abbott Vascular, Santa Clara, CA). Two years later, there was no evidence of endoleak or migration.

WHY NOT USE SUPRARENAL FIXATION?

There is no convincing evidence of renal function compromise with suprarenal stents. A meta-analysis comparing EVAR devices with and without suprarenal fixation did not detect a clear medium-term decrement in renal function in patients receiving the former.15 Similarly, one of the larger single studies included in that analysis compared Powerlink grafts with and without suprarenal bare-metal stents and demonstrated no significant difference in postoperative creatinine clearance at 12 months.16

In the current era of more frequent snorkel or chimney procedures, it is remarkable that bare-metal stents above the proximal seal zone of the aortic graft do not interfere with cannulation or stenting of the aortic branches. For example, Figure 4 shows an AFX device with pararenal bare-metal stents that was used with iCast covered stents (Atrium Medical Corporation, Hudson, NH) in the renal and superior mesenteric arteries to accomplish endovascular repair of a suprarenal AAA.

Pararenal stents do not seem to adversely affect the ability of devices to conform to angulated neck anatomy. For instance, Robbins et al demonstrated that the Talent (with suprarenal bare-metal stents) did not exhibit more frequent migration or endoleak in angulated as compared to straight proximal infrarenal necks.17

There are clinical data to indicate that the suprarenal and visceral segments of the aorta dilate less frequently and rapidly than does the infrarenal neck after EVAR.18 This implies that fixation in the suprarenal aorta might be more durable than fixation in the infrarenal neck alone.

Finally, one cautionary note regarding the implantation of EVAR devices with suprarenal uncovered stents, especially with hooks or barbs. If it ever becomes necessary to explant the device, supraceliac clamping is generally required, and excision of the proximal portion of the stent graft can be difficult and require sterile wire cutters.19

CONCLUSION

Given that there is no evident disadvantage to pararenal bare-metal stents and accompanying suprarenal fixation, the fact that anatomic fixation is advantageous in EVAR, and migration rates are generally extremely low with devices featuring suprarenal fixation, it seems prudent to recommend the routine use of grafts with suprarenal fixation in treating AAA with EVAR.

Benjamin M. Jackson, MD, is with the Division of Vascular Surgery and Endovascular Therapy, Hospital of the University of Pennsylvania in Philadelphia. He has disclosed that he has no financial interests related to this article. Dr. Jackson may be reached at (215) 614-0176; benjamin.jackson@uphs.upenn.edu.

Ronald M. Fairman, MD, is with the Division of Vascular Surgery and Endovascular Therapy, Hospital of the University of Pennsylvania in Philadelphia. He has disclosed that he has no financial interests related to this article.

  1. Chaikof EL, Blankensteijn JD, Harris PL, et al. Reporting standards for endovascular aortic aneurysm repair. J Vasc Surg. 2002;35:1048-1060.
  2. Resch T, Malina M, Lindblad B, et al. The impact of stent design on proximal stent-graft fixation in the abdominal aorta: an experimental study. Eur J Vasc Endovasc Surg. 2000;20:190-195.
  3. Thomas B, Sanchez L. Proximal migration and endoleak: impact of endograft design and deployment techniques. Semin Vasc Surg. 2009;22:201-206.
  4. Greenberg RK, Chuter TA, Cambria RP, et al. Zenith abdominal aortic aneurysm endovascular graft. J Vasc Surg. 2008;48:1-9.
  5. Zayed HA, Bell RE, Clough RE, et al. Results of endovascular repair of abdominal aortic aneurysms with an unfavorable proximal neck using large stent-grafts. Cardiovasc Intervent Radiol. 2009;32:1161-1164.
  6. Abbruzzese TA, Kwolek CJ, Brewster DC, et al. Outcomes following endovascular abdominal aortic aneurysm repair (EVAR): an anatomic and device-specific analysis. J Vasc Surg. 2008;48:19-28.
  7. Makaroun MS, Tuchek M, Massop D, et al. One year outcomes of the United States regulatory trial of the Endurant Stent Graft System. J Vasc Surg. 2011;54:601-608.
  8. Georgiadis GS, Trellopoulos G, Antoniou GA, et al. Early results of the Endurant endograft system in patients with friendly and hostile infrarenal abdominal aortic aneurysm anatomy. J Vasc Surg. 2011;54:616-627.
  9. Wang GJ, Carpenter JP. The Powerlink system for endovascular abdominal aortic aneurysm repair: six-year results. J Vasc Surg. 2008;48:535-545.
  10. van Keulen JW, Moll FL, Barwegen GK, et al. Pulsatile distension of the proximal aneurysm neck is larger in patients with stent graft migration. Eur J Vasc Endovasc Surg. 2010;40:326-331.
  11. Howell BA, Kim T, Cheer A, et al. Computational fluid dynamics within bifurcated abdominal aortic stent-grafts. J Endovasc Ther. 2007;14:138-143.
  12. Molony DS, Kavanagh EG, Madhavan P, et al. A computational study of the magnitude and direction of migration forces in patient-specific abdominal aortic aneurysm stent-grafts. Eur J Vasc Endovasc Surg. 2010;40:332-339.
  13. Mohan IV, Harris PL, Van Marrewijk CJ, et al. Factors and forces influencing stent-graft migration after endovascular aortic aneurysm repair. J Endovasc Ther. 2002;9:748-755.
  14. Morris L, Delassus P, Walsh M, McGloughlin T. A mathematical model to predict the in vivo pulsatile drag forces acting on bifurcated stent grafts used in endovascular treatment of abdominal aortic aneurysms. J Biomech. 2004;37:1087-1095.
  15. Walsh SR, Boyle JR, Lynch AG, et al. Suprarenal endograft fixation and medium-term renal function: systematic review and meta-analysis. J Vasc Surg. 2008;47:1364-1370.
  16. Parmer SS, Carpenter JP; Endologix Investigators. Endovascular aneurysm repair with suprarenal vs infrarenal fixation: a study of renal effects. J Vasc Surg. 2006;43:19-25.
  17. Robbins M, Kritpracha B, Beebe HG, et al. Suprarenal endograft fixation avoids adverse outcomes associated with aortic neck angulation. Ann Vasc Surg. 2005;19:172-177.
  18. Sonesson B, Malina M, Ivancev K, et al. Dilation of the infrarenal aneurysm neck after endovascular exclusion of abdominal aortic aneurysm. J Endovasc Surg. 1998;5:195-200.
  19. Brinster CJ, Fairman RM, Woo EY, et al. Late open conversion and explantation of abdominal aortic stent grafts. J Vasc Surg. 2011;54:42-46.