Hostile Neck in Abdominal Aortic Aneurysms: Does it Still Exist?

Abstract

Since its introduction in 1991, the endovascular repair of abdominal aortic aneurysms (AAAs) has been widely accepted by physicians and patients due to its minimally invasive nature and its lower perioperative mortality and morbidity. However, certain anatomical limitations in the geometry of the proximal aortic neck and the iliac arteries preclude the safe and efficient use of endovascular aneurysm repair (EVAR), that is, short infrarenal length 60 , neck diameter >28 mm, conical or tapered morphology as well as significant calcification and thrombus lining of >50% of the neck circumference. These factors are associated with higher rates of proximal type I endoleaks, reintervention, and aneurysm-related mortality. The Human Aortic Anatomy Project comprises a thorough, detailed analysis of the EVAR-suitability based on a study of computed tomography scans of 1063 patients with AAA. According to this report, only 32% of men and 12% of women fulfill all 3 instructions for use (IFU) for the infrarenal neck (length, angulation, diameter) for AAA of 5 to 5.5 cm, whereas these percentages decrease significantly for AAA greater than 6 to 6.5 cm. The neck length was identified as the most determinative criterion for EVAR eligibility. However, as Schanzer et al showed, a great percentage of physicians in real-world practice overcome those restrictions by performing EVAR outside the IFU. Interestingly, it has been estimated that a potential reduction in the IFU neck-length criterion to 7 mm would further enhance the EVAR eligibility to 70% and 45% from 46% and 25% for men and women, respectively, provided that other IFU criteria are kept constant. Therefore, the need to develop enhanced or new device designs that would downregulate the minimum anatomic requirements for EVAR was eminent. Many endograft designs have been developed or improved to address efficiently the proximal fixation and sealing problems. The Aorfix (Lombard Medical, United Kingdom) can be used in infrarenal angulation up to 90 in neck lengths >20 mm. The Ovation Prime abdominal stent graft (Trivascular, Inc, Santa Rosa, California) is a trimodular device with suprarenal fixation achieved by a proximal stent and anchors. Its main characteristic is the dissociation between the fixation and the sealing mechanism which is accomplished by a pair of compliant, inflatable rings that are filled with a low-viscosity radiopaque polymer. The conformable paired O-rings guarantee a precise accommodation to neck surfaces with excessive calcification or significant amount and/or eccentric distribution of thrombus. Interestingly, the Ovation stent graft does not depend on the length of the neck (ie, a straight, cylindrical segment whose diameter discrepancies do not exceed 10% along its length) as long as the first ring seals 13 mm below the inferior renal artery in a diameter less than 30 mm. Furthermore, the endovascular aneurysm sealing (EVAS) philosophy introduced with the Nellix system (Endologix, Irvine, California) offers an alternative approach to endovascular treatment of AAA. This platform consists of dual balloon-expandable endoframes, each surrounded by a polymer-filled endobag, to achieve anatomical fixation in the aneurysm sac, dissociating the sealing efficiency from the shape and contour of the proximal aortic neck. The endobags fill the aneurysm sac obstructing the side branches, and, therefore, EVAS has the potential to reduce the incidence of type II endoleaks. The Nellix stent graft is particularly suitable for patients with conical necks as the endobags apply on the entire length of the conical neck elongating the sealing zone. The use of nitinol-based conventional endografts in challenging neck anatomy is generally associated with inferior results of endoleakand reintervention-free intervals. On the other hand, the off-label use of certain third-generation endografts with suprarenal fixation has been broadened to include challenging neck anatomies, such as the Endurant stent graft (Medtronic, Santa Rosa, CA, USA), yielding greater—but acceptable—endoleak type I results compared with the use according to IFU. An alternative, more advanced approach to treatment of challenging neck anatomies are the use of fenestrated EVAR