Infrarenal Aortic Devices: Failure Modes and Unmet Needs

Infrarenal Aortic Devices: Failure Modes and Unmet Needs

Infrarenal Aortic Devices: Failure Modes and Unmet Needs W. Anthony Lee, MD Endovascular repair of abdominal aortic aneurysms has become part of the s...

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Infrarenal Aortic Devices: Failure Modes and Unmet Needs W. Anthony Lee, MD Endovascular repair of abdominal aortic aneurysms has become part of the standard of care for those patients with appropriate anatomy. Since its initial reporting in 1991, numerous devices have been manufactured and undergone various stages of clinical trials and subsequent postmarket use. Currently, there are four commercially available devices. Without exception, all of the devices are subject to late failure and complications, and, therefore, diligent postoperative surveillance is mandatory. Some of the failure modes apply to the therapy itself and some are device-specific. These failure modes shed light into the unmet needs of the current technology and directions for further improvement. Semin Vasc Surg 20:75-80 © 2007 Elsevier Inc. All rights reserved.

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NDOVASCULAR DEVICES FOR treatment of aortic disease have been used in clinical practice now for over 15 years. Although significant improvements in the endografts and their delivery systems have been made since some of the early designs,1 their fundamental construction remains essentially unchanged and the failure modes that lead to late complications and the need for lifelong follow-up are similar. In this monograph, early and late failure modes and the unmet needs of currently commercially available devices used to treat infrarenal aortic aneurysms will be discussed.

Device Construction To understand the failure modes of aortic endografts, a brief overview of their materials and construction is necessary. Figure 1 shows all four devices that are commercially available at the time of this writing. All endografts are basically constructed of a metallic scaffold (“stent”) that is covered by a semi-permeable cover (“graft”). The stent component could be made of steel (eg, Zenith; Cook, Bloomington, IN) or another alloy, most commonly nitinol (nickel-titanium) (eg, Excluder; W. L. Gore, Flagstaff, AZ), and can take on a variety of shapes, including Z-type, sinusoid, or close-cell designs. The type of metal can impact the type of postimplant imaging that can be safely performed, such as incompatibility of stainless steel and magnetic resonance angiography. Second-order design elements include peak-to-peak distances and peak-toDivision of Vascular Surgery and Endovascular Therapy, University of Florida, Gainesville, FL. Address correspondences to W. Anthony Lee, Division of Vascular Surgery and Endovascular Therapy, 1600 SW Archer Road, Suite NG-45, PO Box 100286, Gainesville, FL 32610-0286. E-mail: [email protected]

0895-7967/07/$-see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1053/j.semvascsurg.2007.04.009

valley height of the stent. The graft component is typically made of either polyethylene terephthalate or polytetrafluoroethylene material. Important design considerations involve thickness, porosity, and pliability, which have direct implications on clinical performance, such as puncture resistance and persistent endotension. In general, what is gained in one property results in a loss of another, and the optimal material depends on the balance of these factors. The basic stent design is stacked serially along the length of the graft material to give the endograft its structural rigidity. The distribution of the stents can also vary widely from placement of them just at the proximal and distal ends of the graft (“unsupported”) (eg, Ancure; Guidant, Menlo Park, CA) or evenly spaced along the entire graft (“supported”) (eg, AneuRx; Medtronic, Santa Rosa, CA). The density of this distribution pattern imparts axial flexibility or columnar rigidity to the endograft. Although not yet commercially available, novel biopolymers that cure in vivo have been used as a substitute for conventional metal stents in some of the newer endograft designs (eg, TriVascular; Boston Scientific, Natick, MA). A variety of methods of joining the stent and the graft have been devised. These include simple suture fixation and layered “sandwich” methods. The stent-graft can be constructed as an endoskeleton (stent inside, graft outside) or exoskeleton (stent outside, graft inside), depending on which can theoretically result in either a better seal or a friction fit. In some devices, both designs are used within a single endograft depending on whether a particular section is meant to appose the native artery (exo-) or another endograft (endo-). One of the most fundamental mechanical principles underlying endovascular repair of aneurysms is that of fixation 75

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Figure 1 Currently commercially available abdominal aortic endografts. From left to right, AneuRx (Medtronic, Santa Rosa, CA), Excluder (W. L. Gore, Flagstaff, AZ), Zenith (Cook, Bloomington, IN), and Powerlink (Endologix, Irvine, CA).

and seal, and they must occur and be maintained at the three attachment sites of a typical bifurcated endovascular aneurysm repair (Fig 2). These two functions are separate but related, and loss of one usually, but not always, results in a loss of the other. The importance of these two functions in the early and late success of an endovascular repair is self-evident. Mechanisms of fixation can be broadly divided into “active” and “passive” types. Passive fixation relies on the simple friction fit imparted by the radial strength of the stent, which is oversized relative to the nominal diameter of the artery. In most instances, this force is not uniform due to the highly variable diameters and eccentricities along the contact surface of the landing zone. Typically, passive fixation alone is not sufficient to achieve long-term stability and these devices must be “fully-supported” with tightly stacked series of rigid stents to yield the necessary columnar rigidity to oppose distal migrational forces. Active fixation relies both on a friction fit and some type of hook or anchoring mechanism integral to the stent such that it securely engages the arterial wall. Some investigational designs include use of endo-staples as another form active fixation (Aptus; Aptus Endosystems, Sunnyvale, CA). Active fixation can be utilized in either the proximal attachment of the aortic neck, where the caudal displacement forces are the greatest, or also at the distal iliac attachment sites. Experimental data exist that show significantly higher pull-out forces required to dislodge active fixation devices as compared to those with passive fixation alone.2 The functions of fixation and seal can be incorporated within the same component, or the fixation separated from the seal by taking advantage of an adjacent segment of the artery that may be less prone to late degenerative changes. In these designs, there are bare stents above and/or below covered section of the endograft whose primary function is fix-

ation (eg, Zenith). However, certain redundancy in the fixation and seal functions of the different device components or sections is desirable in the event that one fails.3 The final design consideration is that of modular versus unibody construction (Powerlink; Endologix, Irvine, CA).

Figure 2 The fixation-seal triangle. These two essential endograft functions must occur at the three points of a bifurcated repair: (1) proximal neck, (2) right iliac artery, and (3) left iliac artery.

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Figure 3 Caudal migration of the proximal attachment in an AneuRx graft. Note further that the top stent has separated from the underlying graft from suture breaks possibly contributing to the loss of fixation.

Most commercially available devices today are modular, where two or more components are assembled in situ to achieve the final repair. Operationally, modular designs allow the greatest ability for customization to individual anatomy with an economy of inventory, profile of the main body device, and deployment steps. In reality, even for so-called unibody devices the sheer diversity of the aortoiliac anatomy and the practical limitations in precase planning and sizing makes a single piece repair uncommon without compromising fixation and seal at one or more of the three attachment sites. Clearly, regardless of how well an endograft is designed, if it cannot be delivered to its target precisely, reliably, and consistently it cannot be expected to perform as intended. The delivery systems vary from the very simple single-stage mechanisms to more complicated multistage mechanisms. There is a trade-off between accuracy and simplicity. On balance, however, while the perception of complexity can be overcome with practice and experience, simplicity gained at the expense of accuracy can never be overcome. Most devices are constrained in a sheath that is retracted to allow the selfexpanding stent to deploy. This process typically can be performed either slowly or in stages that allow accurate position-

ing of the device at the proximal and distal landing zones. The degree with which the device can be rotated, moved, or even reconstrained during the course of deployment is the control that one gains at the expense of complexity. As the anatomy becomes more complex or the device is used under adverse conditions at the margins of the instructions for use, the ability to deploy the device with submillimeter accuracy is critical for success.

Failure Modes Migration This represents a failure of the fixation mechanism, which may be due to device, anatomic, and/or deployment-related issues. Regardless of the root cause, the end result is the loss of seal and repressurization of the aneurysm.4 Migration is typically a late event and can occur at the proximal neck (caudal migration)5 (Fig 3) or at the iliac attachments (cranial migration, “limb retraction”) (Fig 4). Device-related causes include passive fixation (versus active fixation), separation of the stent from the graft due to suture breaks, hook fractures, stent fractures. The latter two mechanisms have been a direct

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perfect technical execution at the time of the original implantation. This is facilitated (or hampered) by the delivery system and the control and reliability it affords the operator. Although far less common, aneurysmal dilation of the iliac arteries can also occur with the resulting loss of fixation and seal. More frequently the causes of iliac fixation include (1) inadequate fixation at the time of original repair, which is why the iliac limbs should be routinely extended to the hypogastric arteries thereby using the full length of the iliac arteries, (2) deployment of the endograft limb in a preaneurysmal or aneurysmal artery, which would make late degeneration more likely, and again (3) undersizing the endograft relative to the native artery.

Stent Fracture and Fabric Tear

Figure 4 Cranial migration (retraction) of the right iliac limb of a Vanguard (Boston Scientific, Natick, MA) endograft.

Stent-related fractures can occur at the proximal and distal attachment sites or in the body of the endograft, and can involve both the actual stent struts and the hooks attached to the stents. Most mid-body stent fractures do not seem to result in any clinical sequelae but, clearly, the sharp fractured ends can puncture the fabric. Improvements in metal composition, stent design, and surface treatment, such as electropolishing, may have decreased this risk. Suture breaks leading to separation of the stent from the graft can lead to abrasion and eventual tear of the adjacent fabric due to repetitive motion.10 Rarely, dense sharp calcifications within the aorta can also cause a fabric tear (Fig 5). Regardless of the cause, the end result is a type III endoleak that typically does not seal on its own and requires urgent revision.4 Fortunately, the repair is fairly straightforward requiring relining the site of the tear with another device.11

Porosity result of underestimation of the axial loads under physiologic and real-life conditions involving angulated necks, chronic morphologic changes and remodeling, and repetitive asymmetric motions associated with the aortoiliac segment. In some investigational devices, computational methods using finite element analysis have helped predict areas of maximal strain leading to metal fatigue and introduce design changes to decrease this risk of stent fractures. Anatomic mechanisms include progressive degeneration of the landing zones, severe neck angulation,6 conical neck shapes, and short necks. Interestingly, mural thrombus and (circumferential) calcifications have not been associated with migrations. The proximal neck has been known to enlarge in a subset of patients after endograft repair.7,8 This is analogous to late aneurysmal dilation of the proximal neck in surgical repairs if the anastomosis is performed too low in the infrarenal aorta. However, while the seal and fixation of the surgical graft are maintained by physical sutures, the endograft relies on the integrity of the aortic wall. This may be one scenario where endostaples as a method of active fixation hold some attraction in that it most closely duplicates the transmural fixation of surgical sutures. Technical issues include low-deployment with or without inadequate proximal apposition and undersizing of the devices.9 Of all the causes cited, deployment is the only operator controllable variable. Long-term success is dependent on

This is uniquely a design failure of the graft material. In the immediate postimplantation period increased porosity can lead to type IV endoleak, which, in most instances, seal spontaneously after reversal of anticoagulation and clinically in-

Figure 5 Large type III endoleak from fabric erosion near the flow divider in an Excluder bifurcated endograft.

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consequential. On the other hand, if this porosity is due to suture needle holes, these may not seal. While not necessarily resulting in a visible contrast extravasation on a computed tomography angiogram, it can become a potential cause of endotension. Porosity can also result in late failure due to ongoing transudation of fluid and progressive aneurysm enlargement.12-14

Component Separation This failure mode has been typically associated with modular devices, because unibody devices, by definition, have a single component. Component separation is mainly due to inadequate overlap junctional overlap between endografts. While in earlier-generation devices this was a common failure mode, it is relatively rare today due to the collective experience of operators and improved device designs. Diligent surveillance can detect this impending failure before it occurs, usually on plain x-ray or maximal intensity projection imaging. The repair of either impending failure or actual junctional separation resulting in a type III endoleak mandates a revision with a bridging endograft. Occasionally, if the separation leads to misalignment of the two separated ends with minimal gap in between, passage of a through-andthrough guide wire can be difficult and may require advanced catheter-guide wire techniques and, rarely, aortouni-iliac conversion.

Limb Thrombosis Limb thrombosis is related to device-related factors and anatomic factors.15 Unsupported devices have been particularly prone to this complication due to kinking and/or twisting of the limbs that were not recognized during the original procedure when stiff guide wires were in place, extrinsic compression from stenotic lesions, or which occurred due to conformational changes of the aortoiliac segment as the aneurysm shrunk in size. This was managed with adjunctive bare-metal stenting of the limbs, thereby, essentially converting an unsupported device to a supported one. Anatomic issues mainly involved outflow from the iliac limb. Distal iliac occlusive disease, dissection, or hypogastric occlusion can increase the risk of limb thrombosis. The hypogastric artery provides an important outflow similar to the role that the profunda femoris serves in maintaining patency to an aortofemoral bypass limb. Compromised outflow (relative to the flow capacity of the endograft limb) is suggested by development of circumferential intraluminal thrombus within the limb (Fig 6). Although this does not always predict eventual limb thrombosis, closer examination of the outflow vessels is warranted.

Acute or Primary Type I Endoleak Primary Type I endoleak represents a failure of the sealing function and can lead to a serious failure of the entire therapy (ie, rupture and death). At the macroscopic level, aortic neck angulations greater than the maximum radius of curvature tolerated by the device, which is a function of the stent design and device flexibility, can lead to this failure mode. At the

Figure 6 Thrombus lining the luminal surface of the left iliac limb in this Zenith bifurcated endograft 1 month postprocedure. Follow-up examination showed diminished left femoral pulse and a focal left mid-external iliac artery dissection, which was treated with a conventional self-expanding peripheral stent.

microscopic level, surface irregularities, which prevent the graft material from adequately apposing the lumen, can also be a cause. Fortunately, many primary type I endoleaks seal spontaneously after reversal of anticoagulation, but those do not ultimately mandate some adjunctive secondary revision due to persistent pressurization of the aneurysm sac.

Unmet Needs No single device can be expected to perform perfectly in every anatomy, but as the boundaries of the original instructions for use are pushed, the limitations of each device are unexpectedly (and sometimes painfully) realized. Migration would obviously not be an issue if the fixation were perfect. The benchmark to beat or match is a surgically created anastomosis. Devices should be more conformable to angulations and tortuosity of native anatomy. The desire is for the endograft to fit the anatomy instead of the anatomy to the endograft, both proximally and distally. Proximally, it is the exception rather than the rule that aortic necks are tubular with parallel walls. By far, most take on some form of a barrel, cone, funnel, or hourglass shape. Regardless of how the device is oversized, with current designs conformation to these irregular shapes is inadequate, and some portions of the aortic neck is not apposed and “wasted” in terms of providing a seal. Designs that allow improved conformation to this type of local diameter change while providing uniform radial force against the aortic wall is desirable. Distally, improved flexibility of the iliac limbs while maintaining sufficient structural rigidity to resist extrinsic compression can avoid kinking and limb thrombosis. Deliverability is another area where there exist several unmet needs. While significant occlusive disease is fortunately uncommon in most patients with aortic aneurysms, the need for iliac conduits or alternate access sites due to small and/or

80 diseased iliac arteries remain a source of significant complications and prolongation of procedure time.16 Smaller profile, increased lubricity, and trackability of delivery catheters would decrease the need for these adjunctive access techniques. On the other hand, although lower profile is desirable, it should not be achieved at the expense of durability. No device can be expected to function if not deployed accurately. In this regard, more control of the device at the expense of complexity is clearly more desirable than less control for the sake of simplicity. One ultimate form of control would be complete reconstrainability, such that if the device is (mostly) deployed and the result unsatisfactory, it can be totally repositioned proximally, distally, or rotationally.

Conclusion Late failure is an “equal opportunity hazard.” Every device is susceptible given enough time and it has as much to do with the device as patient selection. While in an ideal world every device should function perfectly in every anatomy, in reality, root-cause analysis will reveal that failure after endovascular repair represents a combination of device, anatomy, and technique-related factors. Durability as a global concept remains an important but elusive goal, although clear improvements are being made. In this sense, durability pertains not only to the integrity of the device and its materials, but equally important, to the repair itself. Although chronic type II endoleaks and endotension will continue to mandate lifelong surveillance after endovascular repair, the overall burden of postoperative follow-up may be decreased if the durability can be further improved.

References 1. Torella F: Effect of improved endograft design on outcome of endovascular aneurysm repair. J Vasc Surg 40:216-221, 2004 2. Resch T, Malina M, Lindblad B, Malina J, Brunkwall J, Ivancev K: The impact of stent design on proximal stent-graft fixation in the abdominal aorta: an experimental study. Eur J Vasc Endovasc Surg 20:190-195, 2000

W.A. Lee 3. Ghanim K, Mwipatayi BP, Abbas M, Sieunarine K: Late stent-graft migration secondary to separation of the uncovered segment from the main body of a Zenith endoluminal graft. J Endovasc Ther 13:346-349, 2006 4. Fransen GA, Vallabhaneni SR Sr, van Marrewijk CJ, et al: Rupture of infra-renal aortic aneurysm after endovascular repair: a series from EUROSTAR registry. Eur J Vasc Endovasc Surg 26:487-493, 2003 5. Greenberg RK, Turc A, Haulon S, et al: Stent-graft migration: a reappraisal of analysis methods and proposed revised definition. J Endovasc Ther 11:353-363, 2004 6. Hobo R, Kievit J, Leurs LJ, Buth J, EUROSTAR Collaborators: Influence of severe infrarenal aortic neck angulation on complications at the proximal neck following endovascular AAA repair: a EUROSTAR study. J Endovasc Ther 14:1-11, 2007 7. Litwinski RA, Donayre CE, Chow SL, et al: The role of aortic neck dilation and elongation in the etiology of stent graft migration after endovascular abdominal aortic aneurysm repair with a passive fixation device. J Vasc Surg 44:1176-1181, 2006 8. Sampaio SM, Panneton JM, Mozes G, et al: Aortic neck dilation after endovascular abdominal aortic aneurysm repair: should oversizing be blamed? Ann Vasc Surg 20:338-345, 2006 9. Zarins CK, Bloch DA, Crabtree T, Matsumoto AH, White RA, Fogarty TJ: Stent graft migration after endovascular aneurysm repair: importance of proximal fixation. J Vasc Surg 38:1264-1272, 2003 10. Zarins CK, Arko FR, Crabtree T, et al: Explant analysis of AneuRx stent grafts: relationship between structural findings and clinical outcome. J Vasc Surg 40:1-11, 2004 11. Lee WA, Huber TS, Seeger JM: Late type III endoleak from graft erosion of an Excluder stent graft: a case report. J Vasc Surg 44:183-185, 2006 12. Trocciola SM, Dayal R, Chaer RA, et al: The development of endotension is associated with increased transmission of pressure and serous components in porous expanded polytetrafluoroethylene stent-grafts: characterization using a canine model. J Vasc Surg 43:109-116, 2006 13. Haider SE, Najjar SF, Cho JS, et al: Sac behavior after aneurysm treatment with the Gore Excluder low-permeability aortic endoprosthesis: 12-month comparison to the original Excluder device. J Vasc Surg 44:694-700, 2006 14. Tanski W 3rd, Fillinger M: Outcomes of original and low-permeability Gore Excluder endoprosthesis for endovascular abdominal aortic aneurysm repair. J Vasc Surg 45:243-249, 2007 15. Sivamurthy N, Schneider DB, Reilly LM, Rapp JH, Skovobogatyy H, Chuter TA: Adjunctive primary stenting of Zenith endograft limbs during endovascular abdominal aortic aneurysm repair: implications for limb patency. J Vasc Surg 43:662-670, 2006 16. Lee WA, Berceli SA, Huber TS, Ozaki CK, Flynn TC, Seeger JM: Morbidity with retroperitoneal procedures during endovascular abdominal aortic aneurysm repair. J Vasc Surg 38:459-463, 2003