Secure hemostatic proximal stent-graft implantation is an absolute requirement for successful EVAR.1-3
Under ideal circumstances, the proximal end of the stent-graft conforms to the shape of the neck, resulting in a long zone of overlap, secure hemostatic implantation, and durable aneurysm exclusion.9
If, on the other hand, the neck is short and more than half of the proximal stent resides within the aneurysm, the two may not assume the same shape, and the overlap will be neither stable nor hemostatic. In the case of the Zenith stent-graft, the sole device in this series, the proximal stent is 15 mm long. Therefore any neck shorter than 10 mm will likely jeopardize infrarenal implantation. Other stent-grafts have shorter proximal stents and some have been used to treat short necks, but these stent-grafts have problems such as migration.10-12
The other approach is to change the aortic side of the equation by implanting the stent-graft at a more proximal level and simultaneously maintain flow through, over, or around the stent-graft using renal stents. Fenestration has the best track record of durable success, but the necessary stent-grafts are not available in the United States. 5-6
In addition, fenestrated stent-grafts take a long time to prepare and cannot be used in symptomatic patients for whom long manufacturing delays may be fatal.
The technique referred to here as “encroachment” was first used as a way to rescue inadvertent renal artery coverage. The barbs of the Zenith stent-graft permit very little caudal movement once the proximal stent has been deployed. Traction on a balloon or cross-femoral wire is ineffective and potentially dangerous. When the degree of encroachment is small, it is rarely difficult to obtain access to the renal artery, using either a transfemoral or transbrachial approach, as evidenced by our high success rate. The most difficult aspect may be making the diagnosis, since the partially occlusive graft wall may create no filling defect or apparent delay in renal perfusion.
More extensive renal coverage does impede renal catheterization. Planned coverage of the entire renal artery calls for pre-emptive renal access through a brachial approach, as in the “snorkel” technique.7
We have not performed bilateral “snorkel” procedures. We consider the proximity of the aneurysm to both
renal arteries a relative contra-indication to the procedure, as we are hesitant to risk the perfusion to both kidneys. Our limited experience with this approach has yielded a high rate of success under difficult circumstances. Others have reported similar technical success with this approach, also known as the “chimney” technique, in the pararenal aorta and the aortic arch.8, 13-14
However, the long-term durability of this technique has yet to be determined. The ideal characteristics of the renal artery stent (self-expanding versus balloon-expanded, covered versus uncovered) for use in the “snorkel” procedure have also not yet been clarified. We have used all of the above in our small series of cases. We have only used one covered stent in our series because these tend to be bulkier, stiffer, and more difficult to insert than uncovered stents. We are not entirely convinced that the covered nature of the stent confers any advantage in renal patency or any protection against a type I endoleak. Our current preference is to use a balloon-expandable uncovered stent in the renal artery during “snorkel” procedures, as these stents have good trackability, deploy precisely, and provide excellent radial force.
Since our primary indication for the elective combination of renal stents and aortic stent-grafts was the presence of a short proximal aortic neck, our data on neck length warrants scrutiny (). Inadvertent renal coverage was the result of technical error. Some of the necks were short, but most were quite long, certainly long enough to meet the standard selection criteria for EVAR. The same cannot be said of planned encroachment, where the mean proximal neck length was <10mm. However, the neck lengths for the cases of planned encroachment were not as short as the necks treated using the “snorkel” technique. The need to obtain access to the already covered renal artery limited the degree of encroachment, which limited the additional neck length to be gained. In the “snorkel” technique, access to the lower renal artery was obtained before stent-graft implantation. The upper margin of the stent-graft was deployed at the inferior margin of the contralateral renal artery. The resultant increase in the length of the implantation site allowed the inclusion of shorter necks, some as short as 3 mm.
Our technical success rate and overall small number of patients in this series deprived us of the data we need to determine the minimum neck length required for each technique. In our series, there were no cases in which too much renal artery was covered for subsequent catheterization and stenting. In addition, there were very few cases of type I endoleak, and none of persistent type I endoleak.
We recognize that while adjunctive renal stenting may permit successful EVAR in the presence of a short neck, it will not work when there is no neck. Some neck is necessary to create a seal below the renal arteries, because the presence of a renal stent disrupts contact between the pararenal stent-graft and the pararenal aorta. In some cases the infrarenal seal needs a little help in the form of a Palmaz stent. This can be a problem when the inflation of a balloon within the aorta threatens the patency of an adjacent renal stent. Our current approach avoids the issue by deploying the renal stent at the same time as the routine planned deployment of an aortic Palmaz stent in cases where the “snorkel” technique is utilized.
Long-term data will be needed to assess whether a hyperplastic response to the presence of the renal stent will ultimately threaten renal perfusion. This does not appear to be a common outcome in the short to medium-term. The sole case of intraoperative renal artery injury was the result of a technical error: the balloon was inflated beyond the orifice of the stent. Only one patient in this series had a late-occurring renal artery occlusion. This observation is consistent with data from reports on stent-graft fenestration, which resemble our own series in that most stented renal arteries were free of intrinsic disease.5-6
The Zenith stent-graft has two particular advantages in cases of short proximal aortic neck. First, the two stage deployment allows precise positioning, especially when one accounts for the downward displacement of the renal arteries that sometimes accompanies sheath withdrawal in patients with iliac artery disease. Second, the uncovered proximal stent provides secure suprarenal fixation, which is especially important because the short neck provides little infrarenal fixation, and even small amounts of migration would cause type I endoleak, aneurysm pressurization and rupture. Although the uncovered proximal stent has the advantage of additional fixation, it has the potential disadvantage of complicating renal catheterization in cases of encroachment. Nevertheless, the gaps between stent apices are generally wide enough to accommodate a catheter and stent.
While the renal encroachment technique does not require any unique variation in the sizing or deployment of the aortic stent-graft, the “snorkel” procedure does. When we utilize the “snorkel” technique, we oversize the main body component by an extra 2-4mm, in order to create a gutter for the renal stent. For example, for a 28mm neck, we would use a 36mm device instead of a 32mm device. The additional oversizing allows the fabric of the main body to fold around the renal stent ().
Figure 5 Fly-through view from postoperative CT scan in a patient who underwent placement of renal artery “snorkel.” White arrow: renal artery “snorkel” stent; block yellow arrow: aortic stent graft with Palmaz stent; thin yellow (more ...)