Abstract
In this issue of JEVT, Nordon and colleagues 1 present their findings from examination of data on fenestrated endograft morphology used to treat short-necked abdominal aortic aneurysms. This well thought-out study gives us a deep insight into the variation of endografts and examines whether an “off-the-shelf” device is a viable prospect for the near future.
Fenestrated and branched stent-grafting of aneurysmal short-necked and juxtarenal aortic segments using custom-made stent-grafts has attracted great attention, given the high morbidity associated with these conditions. The procedure is especially useful in patients unable to withstand the traditional surgical approach due to significant comorbidities. We have demonstrated, with others, encouraging short- and intermediate-term outcomes, 2—4 and as a result, all of our patients are assessed for fenestrated or branched stent-graft suitability. Much of our elective aortic work is now completed using fenestrated graft technology.
However, one of the primary concerns with this technology is the extended production time that is mandatory when utilizing custom-made grafts. This means that ruptured and urgent cases are automatically excluded even if the anatomical pattern is suitable for endovascular repair. Our referral pattern at Imperial College means that a great number of patients are urgent or emergency cases that may well have the most to gain from a totally endovascular approach to aneurysm treatment. Open surgical repair is the only realistic alternative in this scenario at present, with a well-established track record of high mortality and morbidity rates compared to endovascular approaches. With this in mind, the idea of a stocked range of fenestrated grafts in each major vascular center available for immediate use is a very attractive prospect.
After examination of the available data by Nordon et al., 1 it seems that too large a number of grafts would need to be stocked to treat a population similar to these patients who underwent elective fenestrated endografting. Over 150 grafts would need to be available to treat two thirds of a population that required configurations involving two fenestrations and one scallop. The concept of an “off-the-shelf” graft design is seemingly not practical as things stand.
Having said this, the trend in recurrent patterns of morphology is encouraging, and the prospect of a repository allowing “next day” graft delivery is of great interest if a large graft stock could be produced. What is needed, however, are further innovative approaches to graft design to reduce the number of devices required to treat the majority of patients.
The determination of how many grafts are needed for an “off-the-shelf” approach to fenestrated grafting is largely governed by the “tolerance” to fenestration/target vessel misalignment. The authors have applied a tolerance to differences in stent-graft measurements based on the opinion of endovascular surgeons experienced in fenestrated stent-grafting. These tolerance levels appear sensible, given the limitations of fenestration and scallop design.
Although a fewer number of grafts would be required when a greater tolerance to misalignment is applied, the task of cannulating target vessels becomes more difficult, and the strain placed on the target vessel stent after deployment is unpredictable, presumably depending on the degree of misalignment and flexibility of the target vessel orifice. In designing a strategy for “off-the shelf” stent-grafting, this must be of great importance. It is useless to design a system that allows immediate graft placement, only to find that many target vessels are simply inaccessible or that the anchoring stent is not durable due to kinking or fracture. Conversely, if a greater tolerance for differences in anatomy can be applied, the number of grafts required in a repository would be dramatically smaller.
Advances in graft and catheter technology may well improve the ability of operators to overcome many of the problems of target vessel cannulation. Already introduced into the Nordon article is the concept of double diameter-reducing ties. This allows much greater tolerance for fenestration/target vessel misalignment, and consequently, the number of stock grafts required would be considerably lower. Modifications in design of the scallop in particular could also be considered. Given that the aorta at the level of the superior mesenteric artery (SMA) is normal in this population, a deeper, wider scallop could be considered to negate the variance in the SMA origin location, as well as increase the tolerance for renal vessel position.
Alternatively, advances in catheter technology may be able to overcome the problem of low tolerance for misalignment. Our work at Imperial College London has shown definite advantages for cannulation of vessels using a remotely steerable robotic catheter system. 6
As our experience has grown, we have been able to shorten the cannulation time, reduce the number of movements, and improve overall operator performance. This system, and other advances in catheter technology, may allow easier target vessel cannulation and permit a greater tolerance for fenestration/target vessel misalignment.
There may also be opportunities that make the prospect of an “off-the-shelf” device viable and circumnavigate many of the above difficulties. It is possible to fashion fenestrations in a standard aortic endograft and place it accurately so that the target vessels may be accessed and stented (personal communication: Mark Farber, University of North Carolina, Chapel Hill, NC, USA). This method can deal with one or two visceral vessels and reduce the need for a full retrograde visceral revascularization in thoracoabdominal aortic aneurysms. It may be that advances in graft design and methods to construct fenestrations can allow a safe and durable method of fashioning standard fenestrations in the acute setting to fit a standard endograft to variable anatomy.
Equally, a method of in situ fenestration (fashioning fenestrations with the aortic graft in place at the level of the target vessels and subsequent stenting) may be a future alternative to pre-made fenestrations. It is certainly possible, as many authors have demonstrated, using a retrograde or antegrade approach.7—10 We have found an antegrade approach using the stability of an endovascular robotic system to be feasible in an animal model. 7 This approach to emergent aortic stent-grafting will require significant developments in aortic graft and branch stent design and three-dimensional methods to accurately visualize the arterial tree, as well as refinements to improve the long-term durability of these multi-modular devices.
If one of the above methods of fenestration construction could be combined with a standard device having one fenestration and perhaps a larger scallop for the SMA, recalculation of the number of grafts required may well produce encouraging results. As the current technology stands, however, it seems likely that only a small proportion of patients would benefit from a large number of endografts kept as stock in a vascular center or even a central repository that could provide “next day” delivery. The prospect of such a device, on the other hand, is so appealing that this should not prevent a drive to further investigate methods to reduce the required number of stocked grafts. There are many technological advances that could take place to allow a full “off-the-shelf” fenestrated stent-graft program to become a viable prospect.