Commentary: Understanding the Role of EndoAnchors in Infrarenal Endovascular Aneurysm Repair

Superior short-term outcomes related to endovascular abdominal aortic aneurysm (AAA) repair (EVAR) compared with open AAA repair (OAR) have been clearly demonstrated in randomized controlled studies.^1,2 Long-term analyses of these randomized trials, however, as well as some literature syntheses have raised concerns regarding the longevity of EVAR; the early mortality and morbidity advantage over OAR may be eroded by the need for reintervention, especially after the second postoperative year.^1–6 Reinterventions following EVAR can be secondary to aortic dilatation in the sealing zones, use of devices outside the instructions for use (IFU; eg, inadequate seal zone length), inaccurate placement, or secondary to device-related complications such as migration, especially in older-generation endografts.^7,8

Circumferential seal in the proximal aortic fixation zone is a key factor in preventing type Ia endoleaks, which repressurize the aneurysm sac and can in turn lead to rupture. ⁷ Despite key advances in endograft design in recent years, such as the incorporation of proximal anchoring pins, barbs and hooks, suprarenal fixation modalities, and active sealing mechanisms (based on polymers or the fabric of the graft), type Ia endoleaks are still not an uncommon complication after infrarenal EVAR.^7,9–13 The addition of the aforementioned fixation mechanisms can increase the force required to displace the proximal element of an infrarenal aortic endograft; however, a hand-sewn anastomosis still outperforms these modern devices in terms of fixation strength.^9,14 Spanos et al ¹¹ reported a series that demonstrated an almost 9% incidence of proximal migration after infrarenal EVAR over a 3-year period, even with new generation devices; 22% of these migrations were accompanied by a type Ia endoleak. In a meta-analysis of 16,974 individuals undergoing infrarenal EVAR, Antoniou et al ¹³ also documented that nearly 1% of patients developed an AAA rupture over a 37-month period, mostly secondary to a type I endoleak (66% had a type I endoleak at the time of sac rupture).

It is well established that, despite fixation, use of infrarenal stent-grafts outside the IFU is associated with an exceedingly high rate of endoleak and should therefore be avoided. ¹⁵ One of the most common reasons for use of endografts outside the IFU is a suboptimal proximal landing zone due to short length, calcification, thrombus load, and/or neck shape/geometry (ie, conical, barrel, or tapered neck), leading to short- and long-term seal failure.^10,11,16 Fenestrated EVAR, branched EVAR, or OAR are considered effective alternatives in this scenario.

The Heli-FX EndoAnchor System (Medtronic Vascular, Santa Rosa, CA, USA) mimics a hand-sewn anastomosis; helically shaped anchors are deployed intraluminally and through the endograft material, affixing the graft onto the adjacent aortic wall.^14,17,18 The system was initially developed to augment the sealing of the Aptus endograft but has since been licensed for use with other devices. There are two potential theoretical advantages when using EndoAnchors in the proximal neck as part of infrarenal EVAR: (1) augmenting the fixation of the endograft to the aortic neck and therefore preventing migration of the device and (2) improving apposition of the graft material on the aortic wall, thereby reducing the chance of type Ia endoleak due to gutters or future aneurysmal dilatation of the neck.^17,18

We previously analyzed the effect of displacement force on infrarenal endografts in a cadaveric model after circumferentially deploying 4 or 6 EndoAnchors. ¹⁴ In that model, the addition of EndoAnchors significantly increased the force necessary to migrate the endografts caudally. Interestingly, the addition of a total of 6 EndoAnchors approximated the strength of a surgical hand-sewn anastomosis. ¹⁴ Since then, EndoAnchors have been increasingly adopted in clinical practice, both as a prophylactic measure aimed to increase proximal fixation during the index procedure or as a therapeutic measure for type Ia endoleaks.^17–21 The Aortic Securement System Global Registry (ANCHOR) prospective registry has also reported favorable short-term outcomes. ¹⁷ In a subcohort of ANCHOR patients though, it was recently reported that almost 30% of the EndoAnchor implants had maldeployed. ²⁰

The current EndoAnchor IFU recommend deploying at least 4 or 6 EndoAnchors for proximal AAA neck diameters ≤29 or >29 mm, respectively. The IFU also suggest a circumferential deployment when planning the use of EndoAnchors as a primary adjunct. In practice, this may not always be possible, even after careful planning, due to the presence of calcified regions, thrombus, or severe aortic neck angulation. In the subanalysis of the ANCHOR registry, some EndoAnchors were in fact not deployed where originally intended. Furthermore, when using EndoAnchors as a therapeutic measure, the anchor is meant, at least in principle, to be deployed at the exact anatomical site of the type I endoleak. This may again be prohibited by the presence of calcium or thrombus at that site. Consequently, the circumferential deployment of 6 EndoAnchors, as advised by the IFU, may not always be possible in clinical practice. It is therefore important to understand the impact of different EndoAnchor deployment configurations, as well as various distances between the EndoAnchors, on the endograft’s seal force.

In the October 2019 issue of the JEVT, Goudeketting et al ²² used an in vitro model to study the effect of different EndoAnchor configurations on aortic endograft displacement. The model was a silicone tube that simulated the aortic wall; 5 different EndoAnchor configurations were tested. The primary outcome of interest was the displacement force necessary to dislodge the deployed device by >3 mm. The circumferential distribution of 6 EndoAnchors was the optimal configuration, which is in agreement with previous in vitro findings. A new finding of the study is of particular interest; notably, reducing the distance between the EndoAnchors over the circumference of the endograft increases the displacement resistance considerably.

Accepting that there are limitations when applying these in vitro findings to clinical practice, the results of the study have two main implications. First, this study further supports the notion that a circumferential distribution of at least 6 EndoAnchors is the best possible configuration in terms of increasing proximal endograft support. Clinicians should therefore plan accordingly all anchoring procedures, either in a prophylactic or therapeutic setting. One must be cognizant, however, that despite optimal planning the actual deployment achieved may differ in practice. The second message from this preclinical study is that if the deployment of 6 circumferential EndoAnchors is not possible (eg, due to thrombus load or calcification) and the use of this device is still deemed appropriate, one should aim to decrease the distance between adjacent anchors to maximize the potential benefit.

There are some important limitations to this study. The authors have essentially focused on the type of EndoAnchor configuration that can increase the proximal fixation of an endograft to avoid migration due to caudal forces. The in vitro model used was based on a silicone tube, which does not behave in the same manner as the aorta/proximal sealing neck. Most important, the presence of calcium, thrombus, or the unique anatomical characteristics of a proximal AAA neck (ie, conical, tapered, or barrel shape) cannot be simulated using a silicone tube. Furthermore, in this in vitro model, the bare stent and anchoring pins of the tested endograft were removed so that the endograft could be reused. This may have impacted considerably on the performance of the device and in particular on the interaction between the inherent endograft fixation and the additional EndoAnchor fixation.

A common reason for using the EndoAnchors is poor fabric/graft apposition to the aortic neck, but the in vitro model used in the present study was not designed to address how the EndoAnchors should be optimally placed in this context. Future research may use refined in vivo models or even in silico simulations that not only address the optimal configuration of EndoAnchors but also help identify the infrarenal neck morphology in which use of EndoAnchors would be of greatest benefit.