Physicians discuss stent graft sizing and selection, as well as their hopes for future technology.
When sizing an endovascular stent graft, do you prefer
measuring inner wall to inner wall or outer wall
to outer wall? Why?
Dr. Cheng: I do not use a strict rule but take a measurement based on what I think would best suit the aorta. Most of the time this is a value between the inner wall to outer wall. I do not think this is particularly important, because I make final adjustments to the required graft size based on the pathology and anatomy of the neck; for favorable, long necks I tend to oversize the graft less, whereas for difficult necks I oversize more. This decision would make the relatively small difference between inner and outer wall irrelevant.
Dr. Illig: I guess it’s not really what I prefer but rather what the grafts are designed for. I always measure outer wall to outer wall, simply because that is how the grafts are engineered. The sizing algorithms are designed for this, and when using this technique (along with good patient selection), we essentially never see proximal type I endoleaks.
Dr. Verhagen: Definitely outer wall to outer wall. The adventitia is responsible for the majority of the strength of the wall—a vascular graft is never sewn to the intima in open surgery. Besides that, thrombus is not a firm structure to secure a graft in. Furthermore, the worst thing that can happen is undersizing a graft, especially when hooks or barbs are present because that cannot be corrected endovascularly.
What unmet clinical needs do you have that can be
met with future technology?
Dr. Cheng: Referring to endografts, a lower-profile, larger-diameter thoracic device with a more accurate deployment. A practical device for the aortic arch with preservation of the branches is also very much called for.
Dr. Illig: The obvious need is for a graft (or graft system) that allows safe and straightforward treatment of branches. The obvious application at this point is to be able to deal with juxtarenal and paravisceral aneurysms and aneurysms that involve the hypogastric bifurcation. I would estimate that about 75% of all aneurysms can be treated with endovascular devices right now; these two improvements (both of which are in clinical trials) may increase this percentage to 90%. The next step is being able to treat true thoracoabdominal aneurysms, and the final frontier will be the ability to treat arch and even valve/coronary/ ascending aorta pathology.
Dr. Verhagen: Stent grafts with smaller introduction sheaths, especially for thoracic pathology.
The whole concept of fenestrated and branched endografts is beautiful, but it's still far too complex, time consuming, and expensive.
Thoracic endovascular aneurysm repair (TEVAR) for acute and chronic dissections is still in its infancy.
Future technology should enable us to change the follow-up scheme for EVAR/TEVAR patients back to the original follow-up after open repair.
What is your current method of treatment for
type B aortic dissection?
Dr. Cheng: For uncomplicated acute type B dissection, the majority of patients are treated medically. For complicated dissection, thoracic endografting to cover the primary tear with appropriate branch revascularization. We rarely use distal bare stents.
Dr. Illig: We are fairly aggressive in considering endografting for acute dissection. Although prospective randomized data do not show benefit at 2 years for TEVAR in this setting, the risk of future thoracoabdominal aneurysm degeneration and the mortality of their repair are formidable and must be kept in mind. The Investigation of Stent Grafts in Aortic Dissection (INSTEAD) trial (prospective randomized) and numerous other series consistently show an improved rate of false lumen thrombosis and reduced overall aortic size after endovascular treatment of acute dissection, which in turn strongly suggests a reduced rate of thoracoabdominal aneurysm degeneration. Exactly analogous to chronic venous disease after deep vein thrombosis, vascular surgeons see the late effects of medically treated dissection. After a few 8-hour operations with poor outcomes, the prospect of doing something at the time of the original insult in order to reduce the need for eventual thoracoabdominal aneurysm repair several years down the line becomes very appealing.
Dr. Verhagen: Acute: conservative treatment (optimal medical therapy) unless complicated. Then TEVAR for covering the entry tear, possibly followed by stenting of compromised side branches
Chronic: open repair in case of aneurysmal dilatation of the aorta as no one has ever shown me any proof that stent grafting of chronic dissections actually solves the clinical problem. Other complications in chronic dissections are so rare that a customized solution should be followed.
What device features do you consider most important
when choosing an endovascular stent graft?
Dr. Cheng: For the abdominal aorta, I consider a wide choice of lengths (and diameters) and the ability of fine adjustments during deployment (both of the top and of the iliac extensions) to be of paramount importance, because this will enable very accurate landing of the graft in patients with short necks and short iliacs. For the thoracic aorta, a low-profile device with a flexible graft to negotiate complex arches and a stable delivery mechanism.
Dr. Illig: I very strongly believe that the three most critical things needed for successful stent grafting of aortic aneurysms are active proximal fixation, controlled release, and modularity. Active fixation ensures that the graft will stay where it is put, essentially forever. We have never seen a graft with this feature migrate and have fixed many, many grafts without this fixture that have migrated. Controlled release, along with active proximal fixation, allows us to treat aneurysms with very short or disadvantaged necks; we can put the upper lip of fabric within a millimeter or so of where we want it (even safely overlapping a slight bit of the renal artery, if needed) and then “hang” the graft right there. Finally, modularity offers the key to the future. At present, its benefit is allowing “off-the-shelf” components to be used to treat exactly what is needed on both sides (rather than settling for the closest available premade device on one [or both] sides), but in the future this seems to us the best bet for being able to treat branches.
Another important feature, structurally, would seem to be a graft that functions in tension rather than compression (ie, one that “hangs” from the top rather than one that needs to support itself from the bottom). As these grafts are made of fabric and need a degree of flexibility to cope with conformational changes in the aorta, they will not function well in compression. In turn, grafts that are stiff enough to function in compression may not behave well because the shape of the aorta changes over time.
Dr. Verhagen: Accurate deployment, conformability of the graft, size of introduction system, durability.
How do long-term follow-up data affect your decision-
making when selecting your endovascular stent
graft of choice?
Dr. Cheng: Not much, as most grafts in our market are matured third-generation devices with good track records. I would rank graft performance during implantation over long-term results because the latter are determined by patient selection and operator skills.
Dr. Illig: Long-term behavior is obviously the whole point. Financial issues are important to keep in mind, but ultimately the need to treat a patient with the device that has the best chance of long-term success overwhelms any other factor. Factors applicable only at the point of implantation (eg, ease of delivery, number of steps required, and time needed for the procedure) should play no role in graft selection unless long-term success can be ensured.
Dr. Verhagen: Of course, durability is extremely important, possibly the most important characteristic of an endograft. The ones with real long-term data are either not available anymore, or have (sometimes theoretical) disadvantages over newer grafts. Due to the rapid changes in technology, no long-term data are available for most current endografts. Therefore, unfortunately, long-term data can only affect us minimally in our decision-making when selecting an endograft of choice.
Dr. Stephen Cheng is Professor of Surgery, Chief of Vascular Surgery, The University of Hong Kong, Queen Mary Hospital in Hong Kong.
Dr. Karl A. Illig is Vascular Surgeon, University of Rochester; Professor, Department of Surgery, University of Rochester; and Professor, Department of Neurosurgery, University of Rochester in Rochester, New York.
Dr. Hence J.M. Verhagen is Professor and Chief of Vascular Surgery, University Medical Center, Erasmus Medical Center, in Rotterdam, The Netherlands.
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