Many factors are responsible for endograft migration (Box 1). Disadvantaged aortic neck anatomy is likely the single greatest risk factor for later migration. Zarins and other investigators have conclusively demonstrated that the risk of migration increases as the aortic neck fixation length decreases. Severe aortic angulation and a conical neck have also been associated with increased migration. Patients with multiple anatomic limitations of the aortic neck are at particularly high risk for subsequent endograft migration. Adhering to the instructions for use (IFU) recommendations for acceptable aortic neck anatomy lessens the late migration risk. However, large population-based studies suggest that 30% to 50% of EVARs performed in the United States are performed on patients who do not have IFU-suitable anatomy. Pushing the anatomic envelope should be done on a case-by-case basis after weighing the relative early and long-term risks of endovascular repair, open repair, and continued observation.
Excessive endograft oversizing of more than 30% has been demonstrated by Sternbergh and colleagues to be associated with an increased risk of endograft migration in patients treated with Zenith or AneuRx endografts. Such excessive oversizing can cause pleating of the endograft fabric, thus impairing graft apposition to the aortic wall. Bench studies by Kratzberg and colleagues have subsequently demonstrated that barb penetration is also adversely affected by more than 30% oversizing, further explaining the increased risk of migration. Endografts should be oversized 10% to 20% in relation to the minor axis of the aortic neck.
Late aortic neck dilatation is an intuitive risk factor for late migration. If the aortic neck dilates beyond the size of the endograft, there may be loss of seal and/or fixation. More controversial are the risk factors for such late aortic neck dilatation. Short, conical necks are more likely to dilate over time than long and straight necks. Larger-diameter aortic necks (>25–28 mm) have been associated with an increased risk of late dilatation in some studies. Excessive oversizing can contribute to subsequent neck dilatation, although the literature is conflicting in this regard.
The relative risk of endograft migration is device dependent. Although all endografts have the potential to migrate, those without active barb or hook fixation or anatomic fixation at the iliac bifurcation have been demonstrated in multiple studies to have a higher incidence of late migration. This increased risk is particularly heightened in patients with disadvantaged aortic necks. The increase in late mortality in patients treated with such a device (AneuRx) prompted the FDA to issue a clinical warning in 2008 to ensure that the device is implanted only in patients with IFU-acceptable anatomy.
Finally, the relative length of distal iliac fixation in devices without active proximal fixation has been postulated to modulate the risk of late migration. Because these devices (AneuRx, Talent) also rely partially on longitudinal columnar support, it has been suggested that routinely extending the iliac limbs to the common iliac artery bifurcation reduces late migration. There are no data suggesting a similar phenomenon in devices with active fixation. In that the manufacturer of these devices (Medtronic) has now essentially replaced them with a new endograft incorporating active proximal fixation (Endurant), the issue of optimizing iliac limb length is largely moot.
