Multibranched Endovascular Repair of Thoracoabdominal Aortic Aneurysm

Timothy A.M. Chuter and Linda M. Reilly

Endovascular aneurysm repair, which requires neither direct aortic exposure nor temporary interruption of aortic flow, is particularly appealing when treating thoracoabdominal aortic aneurysms (TAAAs). Yet more than a decade has passed since a multibranched stent graft was first used to treat a TAAA, and the endovascular approach has yet to displace open surgery from its role as the mainstay of TAAA treatment, mainly because there is no simple way to exclude the thoracoabdominal aorta from the circulation while preserving flow to the visceral branches.

Multibranched Thoracoabdominal Stent Grafts

The necessary stent graft has to have as many branches as the TAAA, and multibranched stent grafts are complicated to plan, complicated to make, and complicated to insert. In theory, a branched stent graft may be inserted whole (unibody design), or assembled in situ (modular design). In practice, the unibody approach, pioneered by Inoue and colleagues, becomes more complex—and more likely to fail—with every additional branch. The modular approach is simpler, easier to manufacture, and easier to insert and therefore is more widely used. The only real disadvantage of a modular stent graft is the potential for component separation, which varies according to the type of attachment site and construction of its branches.

There are two basic types of branch attachment site: fenestrations and cuffs. A fenestration is merely a reinforced hole in the wall of the trunk (Figure 1A), whereas a cuff is a short branch of the stent graft, extending up, down, or around the trunk (Figure 1B). Cuff-based and fenestration-based stent grafts have several fundamental similarities. For example, both involve inserting a bridging catheter through the attachment site and across the aneurysm cavity into the target artery, followed by inserting a bridging covered stent. There are also fundamental differences. For example, most fenestration-based branches are inserted transfemorally, whereas most cuff-based branches are inserted transbrachially; most fenestration-based branches are balloon expanded, whereas most cuff-based branches are self-expanding; and most fenestration-based branches are inserted while the stent graft is held (by a constraining wire) in a partially expanded state, whereas most cuff-based branches are inserted once the stent graft is fully deployed. The deployment of a fenestration-based branch has much in common with the deployment of a typical fenestration, which is a procedure familiar to many outside the United States, where fenestrated stent grafts have long been commercially available.

FIGURE 1 A, The transaxially oriented, balloon-expanded covered stent of a fenestration-based branch perpendicular to the trunk of the stent graft. B, The axially oriented, self-expanding covered stent of a cuff-based branch.

The first multibranched TAAA repair employed a tapered polyester graft with four caudally directed cuffs and an exoskeleton of Z-stents, with self-expanding covered stents for branches. This combination of an axially oriented (up-going, or down-going) cuff and a self-expanding covered stent has proved to be versatile, forgiving, and stable. Each cuff-based branch runs up or down the outside of the stent graft for a variable distance, turning to the right or left, on its way from the cuff to the target artery. The stiff little branches that radiate downward and outward from a typical cuffed stent graft help to resist the destabilizing effects of aortic flow, pressure, and pulse.

The same cannot be said of fenestration-based branches, which tend to be less versatile, less forgiving, and less stable than cuff-based branches. Unlike an axially oriented cuff-based branch, a fenestration-based branch radiates in a transaxial plane straight out from the fenestration into the target artery. For this to work, the fenestration and the target artery have to line up precisely. There is little margin for error in stent graft design or placement. Moreover, a stiff, radially oriented, fenestration-based branch exerts enormous leverage on the already tenuous intercomponent connection at its proximal end, especially when the aorta is wide and the branch is long.

However, the fenestrated approach does have advantages, especially when the aorta is narrow (nondilated) at the level of the branch. Under these circumstances, fenestrated branches are easier to insert than cuff-based branches, and the short distance traversed by the fenestration-based stent graft when the aorta is narrow results in less leverage on the attachment point. Moreover, fenestrations add no bulk to the stent graft, and fenestrated stent grafts of all kinds have long been commercially available outside the United States, so many surgeons are familiar with their use.

Most published series of endovascular TAAAs include cuff-based branches, fenestration-based branches, and simple fenestrations. More than half of the cases worldwide were performed at the Cleveland Clinic using helical cuffs for branches to the celiac and superior mesenteric arteries and fenestrations for branches to the renal arteries. The resulting stent graft requires a 24-Fr sheath, mainly because helical cuffs are bulky objects. Cuffs of a less bulky design have allowed us at the University of California at San Francisco to transition to an all-cuff-based approach. Currently we rarely use fenestrations, fenestration-based branches, or up-going (cranially directed) cuffs. Indeed, most repairs employ the same premade, off-the-shelf thoracoabdominal component with four caudally directed cuffs.

Standardization

The modular approach to endovascular TAAA repair allows an otherwise complex operation to be reduced to a series of relatively simple maneuvers using a narrow range of standard techniques and standard technology. However, standardization does have its limits. Every patient is unique, and the anatomy of the diseased thoracoabdominal aorta varies widely. By the time a patient presents for TAAA repair, atherosclerosis has often narrowed the access arteries, aneurysm formation has often lengthened the route of device insertion, and the dilatation of the thoracoabdominal aorta has often disturbed the relative positions of the visceral arteries.

Some patients lack the anatomic substrate for endovascular repair, some only become treatable once the unfavorable anatomy has been corrected by surgical or endovascular means, some require modifications in the technique of stent graft implantation, and almost all require some degree of customization. The anatomy of the stent graft has to approximate the anatomy of the aorta. This does not mean that all stent grafts should be custom made. One can customize a modular stent graft at the time of implantation by combining various standard stent graft components and branches of varying length, width, and orientation.

Standard Stent Graft

The standard premade thoracoabdominal stent graft (Figure 2) is combined with a variety of premade proximal and distal extensions, some specific to TAAA repair, others commercially available for the treatment of thoracic aortic aneurysms (TAA) and dissection. Minor disparities between distribution of cuffs on the stent graft and the distribution of branches on the aorta can be accommodated by variation in branch morphology. The exceptions are usually cases of type II and type III TAAA, in which the dilating visceral aorta has also lengthened, taking its branches in different directions. The original off-the-shelf TAAA component has a 22-Fr delivery sheath; the current low-profile version has an 18-Fr delivery sheath. This, combined with low-profile versions of TX-2 thoracic stent grafts, has greatly reduced the need for iliofemoral bypass, especially in women.