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Vascular Disease
Transrenal Bare Stents in Endovascular Treatment of Abdominal Aortic Aneurysms
January 2002
Since the first report of successful endovascular stent graft (EVSG) repair of an abdominal aortic aneurysm by Parodi in 1991,1 a revolution in AAA surgery has been occurring. Numerous series of EVSG of AAA have been reported2–4 using physician-made as well as industry-designed devices. In the United States, two infrarenal grafts have been approved for use in AAA patients with appropriate anatomic characteristics. These devices generally require a sufficient length of normal infrarenal aorta (neck) of at least 1.5–2 cm to allow successful fixation and prevent proximal endoleak.2,4 Such a strict criterion eliminates a large percentage of infrarenal AAA patients from consideration for endovascular repair. From the beginning, a bare proximal stent of variable length has been a feature of numerous physician-made as well as experimental devices (Figure 1). The theoretical advantage of a bare stent across the renal segment is that it may allow secure fixation of the device in the suprarenal aorta, thus increasing the stability and length of seal between the device and the aortic wall. While such transrenal fixation may improve sealing and allow aneurysms with shorter infrarenal necks to be successfully excluded, there is obvious concern that crossing the renal ostia with stents may have a deleterious effect on renal blood flow.
Early aortic stent-grafts, such as the Parodi-Palmaz device shown in Figure 1, feature a Palmaz stent sutured to a dilated polytetrafluoroethylene (PTFE) graft. The proximal portion of the stent is uncovered and can traverse the renal ostia. The devices currently approved for commercial use in the United States do not have proximal bare stents and therefore the entire device is placed in the infrarenal position. Of the devices currently being studied, the Gore Excluder™ (W.L. Gore, Flagstaff, Arizona) lacks a proximal bare stent (Figure 2), while the Talent™ (Medtronic AVE, Santa Clara, California), Teramed Ariba™ (Teramed Corporation, Minnesota) and Zenith™ (Cook Incorporated, Bloomington, Indiana) devices feature transrenal stents (Figures 3–5). With these and other devices under development, it remains unclear whether there is a clear benefit to transrenal stent fixation of aortic endografts. The responsibility of the endovascular community is to provide evidence for the safety and utility of both types of stent-grafts in order to assure the most appropriate device designs. This review will address the safety and potential advantages of transrenal bare stents in aortic grafts based on current experience.
Safety of transrenal stent placement. Placement of the proximal bare stent across the renal artery orifices raises a major question regarding the long-term safety of such a practice since renal blood flow must be maintained via stent interstices (Figure 6). It has become apparent that there is no immediate diminution in renal blood flow or renal function as a result of placing the bare stent across both renal artery orifices. A number of animal studies demonstrated the safety of crossing the renal artery origins with bare stents.5,6 In Kichikawa’s series of 18 cases with transrenal stent placement, all renal arteries crossed were patent at a mean of 14 months post-operation and renal function was affected only transiently in one patient.7 Lobato and associates reported 35 patients with infrarenal neck lengths Length of proximal landing zone. The evolution of endovascular devices as well as the acquisition of technical skills have allowed extension of this technology to a larger percentage of lesions than was previously possible. The challenge of obtaining a stable seal between the device and the infrarenal neck becomes more difficult as lesions with shorter necks are being treated. It has been demonstrated that shorter infrarenal necks are associated with a higher incidence of type I endoleaks.15,16 Dias and colleagues found neck length not to be predictive of type I endoleak; however, the mean neck length in both of their groups was greater than 2 cm.17 It appears, therefore, that neck lengths Access to the renal arteries after stent coverage. Access to the renal artery orifices for future interventions is another important consideration when discussing transrenal bare stents. Stent interstices are large enough to allow passage of wires and catheters for renal artery intervention. Successful interrogation, angioplasty and stenting of renal arteries through stent interstices have been reported.7,8 We have been able to access renal arteries without difficulty in patients with transrenal stents in place. In addition, the presence of renal stents has not hindered our ability to place endografts with transrenal stents in 8 attempted cases. It seems reasonable to state that transrenal placement of bare stents in the treatment of aortic aneurysms does not prevent future diagnostic and therapeutic intervention of the renal arteries. In addition, the presence of stents in the renal arteries should not affect the decision to use transrenal bare stents to treat AAA.
Current development. Endovascular specialists continue to try to clarify the role of transrenal stenting in EVSG of AAA. In the meantime, endovascular technology is advancing to new levels. The use of branch vessel technology holds promise for permitting treatment of aneurysms with little or no neck of normal infrarenal aorta. One example of this technology is the Teramed Ariba AAA stent-graft. The basic device shown above has a long transrenal bare stent portion (Figure 4). A cuff with covered stents attached for the renal arteries can be used to exclude the pararenal aorta while maintaining perfusion to both renal arteries. The Teramed cuff device can be rotated to allow custom configuration for the orientation of the renal arteries. Such technology will permit coverage of aneurysmal segments of aorta while maintaining perfusion to vital organs.
Discussion. A number of endovascular devices are being developed that feature bare stents which cross the renal artery ostia. Experience with these devices is growing, and the safety of this technique seems well described. Endografting with transrenal devices is not new and there have not yet been reports describing significant adverse effects on renal function. All published series suggest that this technique is safe in the intermediate term; however, data describing similar safety at further follow-up are still pending.
A decreased rate of proximal endoleak and improved sealing of proximal attachment site with transrenal fixation are not as unanimously supported by the current literature. While it makes intuitive sense that longer contact length between the stent and normal aorta should improve the stability and security of the device, large numbers are not yet available to clearly support this assumption. Our experience suggests that transrenal fixation will improve security of attachment, allowing the use of EVSG for patients with shorter and more tortuous necks. Future developments in branch vessel technology should expand the application of endovascular technology in the treatment of more complex lesions, including aneurysms involving the suprarenal, visceral and thoraco-abdominal aorta.
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