Current and future developments in the endovascular repair of aortic aneurysms

Richard G McWilliams
Royal Liverpool University Hospital, Liverpool, UK

Address for correspondence:
Dr Richard G McWilliams
Consultant Vascular Radiologist
Royal Liverpool University Hospital
Liverpool, UK.
Tel: +44-151-706-2744 Fax: +44-151-706-5822
Email: Richard.McWilliams@rlbuht.nhs.uk

Abstract
The purpose of this article is to document and assess the current developments in endovascular aneurysm repair.

Introduction
A large evidence base now exists to support the continued use of endovascular grafts in the treatment of infra-renal abdominal aortic aneurysms. The 4-year data from the UK-EVAR 1 trial document a reduction in aneurysm-related mortality in patients treated with EVAR compared to those treated with open repair [1]. In response to this, the UK National Institute of Clinical Excellence (NICE) issued guidance in March 2006 stating that the current evidence appears adequate to support the use of stent-graft placement in abdominal aortic aneurysms [2]. NICE have also issued guidance indicating that there is sufficient evidence to support the continued use of endografts as a suitable alternative to surgery in appropriately selected patients with thoracic aortic aneurysms and dissections [3].

Technological developments have now widened the range of aneurysms that are treatable by endovascular repair. Fenestrated endografts, branched iliac endografts and branched thoraco-abdominal endografts now permit the endovascular treatment of juxta-renal and para-renal aneurysms, common iliac aneurysms with preservation of the hypogastric artery and thoracoabdominal aneurysms.

The purpose of this article is to describe the current status of these new endografts and to discuss the evidence in support of their use.

Fenestrated endografts
Most manufacturers recommend that there should be a minimum of 15 mm of infra-renal neck to allow the safe use of standard infra-renal endografts. If standard endografts are used in short-necked aneurysms then there is a significant risk of primary failure due to proximal endoleak [4].

Fenestrated grafts were developed to allow the endovascular sealing zone to move higher into the normally narrower and more stable peri-renal aorta thus facilitating the treatment of juxta-renal and shortnecked aneurysms. There are many new challenges involved in graft planning, deployment and surveillance generated by the decision to include the renal and mesenteric arteries in the seal zone.

Precise planning is needed and this is performed at the CT workstation using a dataset from a multidetector CT scanner. The planner must anticipate the position of the endograft once released and ensure that the fenestrations for the relevant target vessels are of the correct diameter and at the correct position on the circumference of the graft (Figure 1). The majority of grafts have three fenestrations, one for each renal artery and another for the superior mesenteric artery, and the correct vertical distance between each fenestration must be achieved.

Graft deployment
Graft deployment is more challenging as the graft must be deployed at the correct height and in the correct orientation. Small, tortuous and calcified arteries are the enemy of all physicians involved in endografting and this particularly applies to fenestrated grafts. Rotational control of the graft is required after introduction to allow the correct position of the fenestrations and this is greatly assisted by the orientation markers on the graft. The additional manipulation required for final positioning increases the risk of atheroembolisation and extensive atheroma/thrombus in the aortic neck is a relative contraindication to fenestrated endografting.

The majority of renal fenestrations are primarily stented at the time of the procedure to avoid early or late shuttering leading to target vessel thrombosis (Figure 2). The renal arteries must be approached from within the graft from the femoral direction and there is no option for a transbrachial approach in acutely angled renal arteries. The use of angled introducer sheaths, such as the Flexor^® (Cook Medical, Bloomington, IN, USA), with its good tracking properties, usually simplifies this geometrical challenge. The choice of renal stent is of critical importance. The renal stent must be able to withstand shear stresses at the junction of the renal artery and the graft fenestration. The cardiac and respiratory cycles will generate repetitive stresses at this interface and a weak stent may crush. Examples of crushed stents are documented and the preferred stent has a closed cell design with good connectivity between the stent components. There are potential penalties from using a stent that is too stiff. Repeated stresses are best borne by a structure that has some flexibility. In this way, there is a trade-off between the risk of stent fracture and stent crushing. Fortunately, reports of renal stent damage associated with fenestrated grafts are rare. The ideal stent is likely to incorporate a stronger segment that retains some flexibility at the level of the graft fenestration and a more flexible segment distally at the interface with the normal branch artery.

Figure 2. (a) Intra-operative image showing partial deployment of the proximal component of a fenestrated graft. Sheaths and stents are in place in the superior mesenteric artery and both renal arteries. (b) Reconstructed sagittal CT image showing stents in the right renal artery and superior mesenteric artery.

Surveillance
Graft surveillance is mandatory following fenestrated endograft repair and involves ultrasound, CT scanning and plain abdominal radiographs. There is increased modularity in a fenestrated graft and involvement of visceral arteries. The specific features to screen for, in addition to those after standard EVAR, are modular separation, branch stent deformation and visceral artery stenosis.

Evidence
The largest published series is from the Cleveland Clinic [5]. In their series of 119 cases from 2001-2005, there was no acute visceral artery loss and a total of 302 visceral arteries were incorporated in the seal zone. At mean follow-up of 19 months, there were no ruptures or conversions to open repair. Renal stent occlusion occurred in 10 of 231 stented renal arteries and four of these patients required temporary or permanent dialysis. Review of other published series and our own presented series of 44 patients allows analysis of 259 patients [5-7]. This total series records 1.9% 30-day mortality, 4.8% incidence of target vessel loss and 1.5% incidence of dialysis. These figures compare favourably to the results of open repair for juxta-renal aneurysms. A recent series of patients treated with open juxta-renal aneurysm repair recorded 5.1% mortality and 28.3% transient renal insufficiency with 5.8% of patients needing dialysis [8]. Randomised trials in this area are now being planned.

Bifurcated iliac grafts
Preservation of at least one internal iliac artery is strongly desirable during the endovascular repair of aorto-iliac aneurysms. There are well-documented risks of colonic and neurological sequelae if both internal iliac arteries are occluded, in addition to erectile dysfunction and potentially disabling buttock claudication. The options for preservation of the internal iliac artery during aorto-iliac aneurysm repair include surgical bypass from the external to the internal iliac which creates a sealing zone in the external iliac above this anastamosis, and branched iliac grafts.

The branched iliac graft (Figure 3) incorporates a short branch which is placed just above the ostium of the internal iliac artery and a bridging stent is tracked from the opposite femoral or the left brachial artery into this. Typically, this is part of the endovascular repair of an aorto-iliac aneurysm and the procedure is completed with the deployment of a bifurcated aortic stent-graft (Figure 4). If bilateral common iliac artery aneurysms exist and both internal iliac arteries are patent, our policy is to preserve one internal iliac artery with a branched graft and accept the loss of the opposite internal iliac artery with coil embolisation of this.

The bridging stents used in branched grafting are either balloon-expandable or self-expanding covered stents. These were not specifically designed for this application and dedicated stents are required which should provide a stable seal at the level of the branch and which are flexible to avoiding kinking in tortuous anatomy and yet are not too stiff to avoid intimal injury at the stent/ arterial junction.

The feasibility of branched iliac grafts has been established and the encouraging early results of these grafts have been published [9,10]. The increased modularity of branched grafts increases the complexity of endograft deployment and adds extra junctions which may be the site of endoleaks in follow up. The Malmo series records both intra-operative and late junctional, or type 3, endoleaks due to this increased modularity [9]. Improvements in the stability of these junctional overlaps are needed and longer term studies are needed to prove the durability of iliac branched grafts.

Branched thoraco-abdominal grafts The high morbidity and mortality of open surgical repair of thoraco-abdominal aneurysms has engendered interest in alternative strategies for repair of these more complex pathologies. An intermediate treatment between open surgical and branched endovascular repair is the hybrid operation which involves open surgical bypass to the mesenteric and renal arteries before relining of the aorta with an endovascular graft. The peri-operative morbidity and mortality of these hybrid procedures remains high - 13% for elective/urgent repairs in the St Mary’s series - and there remains a strong interest in a less invasive and purely endovascular approach [11,12].

A large degree of experience has now been gained with the incorporation of the visceral arterial segment in fenestrated endovascular repair of juxta-renal aneurysms. The bifurcated iliac graft has allowed for the development of experience with branched grafting. This combination has led to the use of multi-branched stent-grafts for treating aneurysms that involve the visceral arterial segment (Figure 5).

Because the fabric of the stent-graft does not abut the aortic wall in thoraco-abdominal aneurysms, standard fenestrated grafts are not the ideal endovascular solution. The bridging covered stents make only point contact with the reinforced fabric edge of a fenestrated stent-graft and this is a potential pivot point as the branch artery moves during the cardiac and respiratory cycles. The endovascular repair of thoraco-abdominal aneurysms is performed with devices which incorporate caudally orientated branches for the mesenteric and renal arteries. The branches provide a longer overlap zone where the bridging stent joins the main body of the stent-graft. This longer overlap reduces the risks of junctional endoleak in these multi-modular devices.

Figure 4. (a) Reconstructed CT image showing the luminal details of an aorto-bi-iliac aneurysm; (b) Completion angiographic image showing exclusion of the aorto-iliac aneurysms and preservation of flow to the right internal iliac artery due to the branched iliac graft.

The main component is inserted from the femoral artery and the graft branches are placed one or two centimetres above the target visceral arteries. The bridging stent-grafts are tracked into the visceral arteries from the brachial artery.

Branched grafts may be more forgiving with regard to the exact position of the branch during planning, manufacture and deployment as the branches are placed above the target arteries and it is still possible to catheterize the target artery even if there is some misalignment. This is in contrast to fenestrated grafts where the fenestration will lie against the aortic wall at the level of the target artery thus requiring greater precision in planning and deployment.

Figure 5. (a) Complex endograft comprising branches and fenestrations for the mesenteric and renal arteries; (b) Bench top demonstration of a covered stent inserted into an external branch of an endograft; (c) CT reconstruction of a complex aneurysm which involves the visceral segment of the abdominal aorta; (d) Composite image from completion angiograms after deployment of a branched graft showing normal flow into all the visceral arteries.

The early results of branched grafting for thoracoabdominal aneurysms have been encouraging [10,13]. Currently, branched grafting for thoraco-abdominal aneurysms has been limited to a few centres including our own. The deployment techniques have now been refined and new centres are taking up this procedure. It is possible that future devices may not need intensive customisation and the branches may be placed at predetermined locations of best fit. This would reduce the delay in obtaining current devices for which there is a considerable time needed for planning and manufacture.

In-situ fenestration
An alternative concept for preservation of important aortic branches is to intentionally cover these vessels with an endograft and then re-establish flow by fenestrating the fabric in vivo [14]. This technique of in-situ graft fenestration has been performed successfully to preserve flow in the left subclavian artery during repair of a thoracic aortic aneurysm [15]. The technique requires downstream access to the target artery and from this direction the fabric is punctured, the fabric hole then enlarged and a branch stent placed to re-establish antegrade flow.

The technique remains experimental; however, there is ongoing interest in developing ways of antegrade access from the aortic lumen into branch arteries that may involve intravascular ultrasound and improved devices for creating stable holes in surgical fabric.

Conclusions
There has been progressive evolution in the endovascular management of aorto-iliac aneurysms. Standard endografts are now established in the treatment of isolated thoracic and infra-renal aortic aneurysms. The advent of fenestrated and branched grafts now allows successful endovascular repair of juxta-renal, thoracoabdominal and aorto-iliac aneurysms with preservation of vital visceral arterial branches. There is level 1 evidence to support the use of standard endografts for the treatment of infra-renal aneurysms. The evidence base is lesser for the younger procedures of fenestrated and branched endovascular repair. These currently are reserved for patients deemed at increased risk from open surgical repair of juxta-renal and thoraco-abdominal aneurysms. Randomised trials comparing fenestrated grafts and open repair of juxta-renal aneurysms are expected.

Key Learning

• Fenestrated EVAR (f-EVAR) requires precise planning, accurate deployment and careful surveillance
• The early results of f-EVAR are encouraging. Durability data are awaited
• Branched grafts to preserve the hypogastric artery are now available
• Branched grafts for treatment of thoraco-abdominal aneurysms are evolving rapidly
• Complex endovascular aneurysm repair is reserved for patients deemed to be at increased risk from open surgery

References

1. EVAR trial participants. Endovascular aneurysm repair versus open repair in patients with abdominal aortic aneurysm (EVAR trial 1): randomized controlled trial. Lancet 2005;365:2179-86.
2. National Institute for Clinical Excellence. Interventional Procedure Guidance 163: Stent-graft placement in abdominal aortic aneurysm. Issue date: March 2006.
3. National Institute for Clinical Excellence. Interventional Procedure Guidance 127: Endovascular stent-graft placement in thoracic aortic aneurysms and dissections. Issue date: June 2005.
4. Stanley BM, et al. Evaluation of patient selection guidelines for endoluminal AAA repair with the Zenith Stent-Graft: the Australasian experience. J Endovasc Ther 2001;8:457-64.
5. O’Neill S, et al. A prospective analysis of fenestrated endovascular grafting: intermediate-term outcomes. Eur J Vasc Endovasc Surg 2006;32:115-23.
6. Muhs BE, et al. Mid-term results of endovascular repair with branched and fenestrated endografts. J Vasc Surg 2006;44:9-15.
7. Semmens JB, et al. Outcomes of fenestrated endografts in the treatment of abdominal aortic aneurysm in Western Australia (1997-2004). J Endovasc Ther 2006;13:320-9.
8. Sarac TP, et al. Contemporary results of juxtarenal aneurysm repair. J Vasc Surg 2002;36:1104-11.
9. Malina M, et al. Feasibility of a branched stent-graft in common iliac artery aneurysms. J Endovasc Ther 2006;13:496-500.
10. Greenberg RK, et al. Beyond the aortic bifurcation: branched endovascular grafts for thoracoabdominal and aortoiliac aneurysms. J Vasc Surg 2006;43:879-86.
11. Resch TA, et al. Combined staged procedures for the treatment of thoracoabdominal aneurysms. J Endovasc Ther 2006;13:481-489.
12. Black SA, et al. Complex thoracoabdominal aortic aneurysms: endovascular exclusion with visceral revascularization. J Vasc Surg 2006;43:1081-9.
13. Chuter TA, et al. Endovascular treatment of thoracoabdominal aortic aneurysms. J Cardiovasc Surg 2006;47:619-28.
14. McWilliams RG, et al. In situ stent-graft fenestration to preserve the left subclavian artery. J Endovasc Ther 2004;11:170-4.
15. McWilliams RG, et al. Retrograde fenestration of endoluminal grafts from target vessels: feasibility, technique, and potential usage. J Endovasc Ther 2003;10:946-52.

02-2007 BUY1154818/JB2548/MB002444/SC 13th edition

 

Source: C2I2, Volume V, Issue 1, 2007

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