Richard C. Kovach, MD, FACC, FSCAI, FACP Deborah Heart and Lung Center Browns Mills, New Jersey
Open surgical repair for abdominal aortic aneurysm (AM) was first introduced in 1951. The aneurysm was approached via open laparotomy or a retroperitoneal approach. The aorta was then divided ( opened) and a graft was sewn into normal aorta proximally and distally, with the aneurysmal tissue then used to oversew and cover the graft itself.
40 years later in 1991, endovascular aneurysm repair (EVAR) was first introduced via a bilateral femoral approach (open cut-down due to the large French sizes of the early devices). Between 2004 and 2005, endovascular repair surpassed open repair, with 65% of AM repair now being done via an endovascular approach. I The endograft is typically fixed internally
to healthy tissue at the proximal and distal edges of the aneurysmal segment(s), thereby excluding the aneurysmal segment and allowing it to thrombose, sclerose, and hopefully shrink over time, reducing the risk of rupture, acute bleed and death.
Most endografts have employed a modular bifurcated design with unilateral or bilateral extensions into the iliac vessels in order to establish proximal and distal seal zones. Device structure has typically been a self-expanding nitinol frame with a fabric covering. The self-expanding property of the nitinol frame creates a seal at the proximal and distal edges of the graft, while the fabric covering prevents flow between the stent struts into the aneurysmal space. Key to all designs has been apposition to enough healthy aortic tissue proximally to create an adequate seal, while still maintaining unobstructed flow into the renal and mesenteric arteries. Various iterations of this basic design concept are currently available in the US. (Figure 1)
Early graft designs were limited in their ability to address complex (“hostile”) aneurysm anatomy, including short or steeply angulated proximal necks, reverse proximal neck taper, and heavy calcification, as well as patients with small caliber femoral and iliac access vessels. Newer, more flexible stent designs, supra-renal fixation of the proximal graft, fenestrations, and novel techniques such as snorkels and chimneys have allowed for greatly increased success when addressing hostile neck anatomy. Simultaneously, significant reduction in device French sizes has now made the option of endovascular AM repair possible even for patients with small caliber peripheral vessels, especially women.
As French size has decreased, so has femoral access changed, with many procedures now able to be performed fully percutaneously using closure devices (PEVAR). PEVAR allows for the use of conscious sedation with local or regional anesthesia only 2, typically shorter procedures 3, 4, 5, reduction in groin complications 6, 7, and shorter lengths of stay 8, 9.
The following discussion will serve to highlight the latest technologies and techniques now available and employed to address ever more complex AM anatomy.
HOSTILE NECK ANATOMY:
Anatomical features which define the hostile neck include: neck length <15 mm, neck angulation > than 60 degrees, reverse taper;, calcification of > 50% of the circumference, and proximal neck thrombus. (Figure 2) 20% of patients with MA have neck anatomy that is unfavorable for standard stent grafts.10 Unfavorable neck anatomy may be responsible for up to 60% of patients who are excluded from consideration for endovascular MA repair.11 Unfortunately, no randomized controlled studies exist that compare outcomes between patients with hostile vs favorable neck anatomy, nor between various stent designs. Newer stent designs, however, now make possible a secure seal in these hostile proximal necks.
Supra-renal fixation is one basic feature common to most current stent graft designs. (Figure!) The proximal portion of the main body graft containing the fixation barbs is uncovered. This allows graft fixation to occur above the renal arteries while still maintaining flow. The graft is then deployed such that the covered portion of the main body is positioned as close as possible to the lowest renal artery, thereby maximizing the length of the proximal seal. As a result, proximal necks as short as 10mm are able to be addressed. Improvements in nitinol strut design have also resulted in far greater flexibility and conformability of these devices. (Figure 3)
The Trivascular (Enclologix) OvationTM stent graft system employs supra-renal fixation similar to other devices, but rather than a fabric covered nitinol frame, employs a system of polymer filled rings within the fabric of the main graft body and proximal limbs. (Figure 4). This type of graft architecture is advantageous in that the lack of a nitinol frame allows for a much lower graft delivery profile. In addition, rather than the aorta being forced to take the shape of a circular graft to create a seal, the polymer instead conforms to the shape of the aorta to create the seal. Once the polymer hardens, there is no longer a continuous outward force on the aorta as with nitinol. Controversy currently exists as to what may contribute to aneurysm neck enlargement over time leading to late type 1 endo-leaks. More specifically, it is not clear whether late neck enlargement post endograft repair is due to natural progression of the aneurysm, versus enlargement due to the continuous outward force of the oversized nitinol frame, which may also, theoretically, be creating a constant local inflammatory response. The polymer’s ability to conform may also be useful for the snorkeling technique (see below). Currently, the “LUCY Study” is under way to explore the clinical benefits of this platform in women, including eligibility rates in females with small access vessels, access related vascular complications, and mortality.
Snorkeling is a technique whereby the covered portion of the main body graft is positioned above the renal or even mesenteric arteries in order to extend the neck length and effective seal zone. Access to the major side branches is maintained by simultaneously deploying a covered stent(s) into the side branch(es) as the main graft body is being deployed. The side branch stent is typically a balloon expandable stent, as a balloon expandable stent’s radial strength is required to resist the compressive force of the nitinol framework of the endograft. The stent is positioned such that the (size matched) distal end extends well into the side branch, with the proximal encl positioned flush with or slightly above the material portion of the endograft. (Figure 5) As mentioned above, polymer grafts may be able to better conform around the side branch stent and create less “guttering” than a nitinol stent. (Figure 6). A key advantage of the snorkeling technique is that “off the shelf’ devices can be used.
Fenestrated grafts are now available which also lengthen the effective aneurysm neck and seal zone. Similar to the snorkeling technique, the fabric portion of the graft is positioned above the renal or mesenteric arteries. However, in contrast to snorkeling, 30 reconstruction of the patient’s CT angiogram is used to model individually manufactured endografts with fenestrations cut into the graft material that precisely match and align with the patient’s side-branch anatomy. Although the main body fenestrations should align with the side branches, dificulty in positioning the endograft still may be encountered due to aortic and ileo-femoral angulation and tortuosity resisting endograft movement. Covered stenting of the side branches may still required to reduce the possibility of endoleak via the fenestrations. Significant operator experience is needed to take advantage of this device’s unique properties. These grafts are also not routinely used in urgent clinical presentations as individual graft construction may take several weeks to complete. Currently only the Cook fenestrated endograft system is FDA approved in the US. (Figure 7).
Long-term graft migration with late development of type 1 endoleak remains a potential complication for most current endograft systems. Migration may occur as a result
of aneurysm neck enlargement or chronic hemodynamic stress pushing the graft distally. The Endologix Af’X2TM graft system (Figure 8) is specifically designed to resist distal migration. In contrast to most other endograft systems which rely on proximal fixation barbs to prevent graft migration, this graft system is designed such that the upside-<lown “Y” of the main body graft sits on the aortic bifurcation itself, making distal migration of that portion of the graft virtually impossible. The endograft is then built from distal to proximal until the proximal seal zone just below the renal arteries is established. Because a more natural aortic bifurcation can be achieved with this device, contralateral access for distal peripheral interventions is also maintained.
Another novel investigational device to prevent graft migration is the AptusTMHelifXTMEndoAnchorTM system developed by the Medtronic Corporation specifically to address very short necks. Helical anchoring pins are individually deployed at multiple points around the circumference of the aneurysm neck piercing both the endograft and the aorta, thereby joining them both together. (Figure 9)
Endoleak classification describes the various sources of persistent blood flow into the aneurysm. (fable 1) Type 2 endoleaks are the most common and often most persistent and problematic leaks to address, with lumbar branches, the IMA, or collateral branches from the internal iliac arteries typically being the most common source.
Collateral flow from the internal iliac artery or it’s branches can be eliminated by the use of vascular plugs or coil embolization before or after endograft deployment. Because they typically cannot be accessed directly, persistent flow from lumbar branches is much more difficult to interrupt. Novel treatments are now being developed to address this problem as well.
The NellixTM system developed by Endologix employs a polymer filled bag to completely occupy the aneurysm space. Covered stents with endobags are used to create parallel flow lumens form the aortic neck to the non-aneurysmal segments of the iliac arteries. Polymer bags attached to each stent are then filled with saline to confirm required volume to fill the aneurysm space as well as proximal and distal seal, after which the saline is aspirated and replaced with a polymer that hardens and conforms to the aneurysm shape and seal zones. (Figure 10)
One year data from the EVAS-Forward IDE trial was recently reported by Carpenter at the SVS 2016 Annual Vascular Meeting in Washington, DC13: 100% technical success was obtained. This device achieved the lowest overall endoleak (3.1 %) and secondary intervention rate (3.4%) reported at 1 year as compared to all other FDA approved EVAR devices.
The concept of filling the aneurysm space is also being developed by the Medtronic Corporation in partnership with Arsenal Medical LLC. Post standard stent graft deployment, the aneurysm space is penetrated and filled, occluding branch vessels, while again stabilizing proximal and distal seal zones.
Even the latest endograft technologies require careful case planning in order to achieve a successful result. Taking advantage of the latest imaging techniques is therefore absolutely essential.
Dynamic CTA, for example, takes into account the variation in size of the aortic neck between systole and diastole. Aortic neck area increases by 8.4% +/- 4.1 % at the suprarenal level and 5.9% +/- 4.2% at the infrarenal level during systole.14 In one study, comparing static and dynamic and static images resulted in a 30% change in the size of the chosen endograft.15 3D center lumen line (CLL) analysis also provides for much more accurate endograft sizing. In a study comparing 3D CLL vs 2D CLL analysis, similar endograft sizing occurred only 17% of the time.16 Stretched 2D CLL may also result in gross overestimation of the aneurysm neck length, especially in severely angulated necks.17 At our own institution, we have begun to employ true 3D imaging with the ECHO PIXEL™ system. This system processes the CT angiogram at a 3D workstation. While wearing the accompanying 3D glasses, the image is lifted off the screen into virtual space, allowing the aneurysm to be viewed from any direction; the operator can also move in and out through the image in any plane or angle. An attached light pen, which also operates in the virtual space, can be used to make very precise measurements. (Figure 11)
Abdominal aortic aneurysms effect 1.4% of the US population resulting in approximately 15,000 deaths and 57,000 hospital admissions annually. Incidence increases with age and rupture is associated with a >90% mortality. Only about 15% of AAAs are diagnosed.18 Legislation introduced in 2005 now provides coverage for AAA screening for Medicare beneficiaries, including all males with a history of having smoked > 100 cigarettes in their lifetime, and both males and females with a family history of AAA Open surgery for AAA repair has typically been associated with significant morbidity and mortality, as well as prolonged hospital stays. Many elderly patients are too sick or have too many co-morbidities to even consider. Fortunately endovascular repair (EVAR and PEVAR) has been proven to be a safe and viable alternative to open repair, with endovascular repair now far surpassing open repair in volume and as the therapy of choice. Rapidly advancing technologies and novel techniques now allow foe even the most complex AAA anatomies to be addressed via an endovascular approach.
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- Carpenter; SVS 2016 Vascular Annual Meeting, Washington, DC
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