The first stent grafts used at Stanford were manufactured using 2.5-cm self-expanding Gianturco Z stents (Cook Co., Bloomington, IN), which were fastened together and then covered with a woven Dacron graft (Meadox-Boston Scientific, Natick, MA; see Fig. 54-1). These stent grafts were oversized approximately 10 to 15% above the cross-sectional diameter ascertained by computed tomography (CT) in an effort to obtain sufficient radial force to achieve an endoseal and prevent stent-graft migration. A minimum of 2 cm of normal aorta was required for adequate fixation, otherwise referred to as the "landing zone," both proximally and distally. The covered stent was loaded into a delivery capsule that required femoral and iliac arteries greater than 8 mm to allow the introduction of a 28-French delivery sheath. This dilator contained a sheath that had been previously placed over a super-stiff guidewire and was positioned proximal to the point of deployment. Once this was achieved, the compressed stent graft was advanced into the sheath and deployed by using a "pusher" rod. Devices were limited to a maximal diameter of 40 mm in that aortas larger than 37 mm in diameter were themselves aneurysmal and unlikely to serve as stable attachment zones. Other anatomical constraints of these early "homemade" or first-generation stent grafts that precluded either delivery or secure fixation included acute angulation at the distal arch, and severe sigmoid-like tortuosity coursing through the diaphragmatic crura, reflecting the relative inflexibility of these early delivery systems.
The advent of this new stent-graft technology required a new terminology for endoleaks that allowed blood to leak around or through the stent graft, thus allowing the aneurysmal sac to remain pressurized. Type I endoleaks occur at the proximal or distal attachment sites, and signify a failure to achieve a hemostatic seal at these implantation sites.14,24,25 Type II endoleaks denote a communication between a branch vessel and the excluded aneurysm sac. These usually occur from a back-bleeding inferior mesenteric artery in the abdomen, or intercostal artery in the chest. Type III endoleaks originate from the middle graft sections, and are usually caused by disruption of graft-to-graft overlaps, or by leakage through the graft itself. Type IV endoleaks are characterized by an increase in size of the aneurysm sac in the absence of an identifiable patent branch vessel, variously referred to as endotension.
Years of experience with endovascular abdominal aorta repair and follow-up of thoracic aortic repairs has yielded important information for improved stent graft technology.11,48,49 Commercially produced second- and third-generation stent grafts are more flexible and have a lower profile, and thereby allow use of a smaller introducer sheath in the femoral vessels. Experience has shown that tapered, flexible, over-the-wire delivery systems that are less than 20 French in diameter rarely fail to traverse tortuous femoral or iliac arteries. Hooks at the proximal end of the stent graft appear to provide the most secure means of attachment, but may be suboptimal for treating patients with acute dissections. It is likely that different grafts may be developed for different pathologies, with devices for dissections being devoid of hooks and proximal uncovered metal components. For traumatic aortic lacerations, smaller device sizes are necessary, as these are usually nonatherosclerotic normal-sized aortas with small access vessels. Ideal device components have been broken down into three categories: delivery system, graft material, and metal frame.50 The delivery system should be of low profile, flexible for maneuverability, rigid enough to resist kinking, and hemostatic during use. The graft material should also be of low profile, strong and durable, and reasonably thin. Ideally, this material could also hold sutures. The graft metal frame should provide high column strength and ductility, be compression and kink resistant from external forces, radiopaque, and corrosion and fatigue resistant. Nitinol is now used for the stent material in the majority of grafts and the graft material is usually polytetrafluoroethylene (PTFE) or polyester.
Currently, the Gore Excluder TAG system (W.L. Gore, Sunnyvale, CA), the Medtronic Talent graft (Medtronic, Sunrise, FL), and the Cook Zenith (Cook Co., Bloomington, IN) are the only Food and Drug Administration (FDA)–approved thoracic grafts (Fig. 54-2). These second- and third-generation endoprostheses are more flexible and have a lower profile with smaller delivery systems that can be inserted easily to treat a number of thoracic aortic pathologies.
Second-generation commercially manufactured thoracic aortic stent graft. The thoracic Excluder TAG system by W.L. Gore contains a thin-walled PTFE graft covered by a nitinol exoskeleton.
Clinical Results Using Endovascular Stents for Thoracic Aortic Disease
Thoracic Aortic Aneurysms
In January 2005 the phase II multicenter trial of the Gore Excluder TAG thoracic endoprosthesis results was reported.51 This multicenter prospective nonrandomized trial was conducted at 17 sites and compared results of stent-graft repair of descending thoracic aortic aneurysms in 140 patients with results of open repair in 94 patients. Strict inclusion and exclusion criteria were defined in an attempt to ensure comparability of both groups. Follow-up CT scans were obtained at 1, 6, 12, and 24 months. For stent-graft patients, operative blood loss, renal failure, paraplegia, and mortality rates were all significantly less than for the open repair group (Fig. 54-3). Interestingly, stroke rates were about equal in both groups. ICU stay and total hospital stay, and time to return to normal activity were 50% shorter for the stent-graft group than for those with open repair. Although stent-graft patients maintained an advantage from aneurysm-related mortality out to 2 years (97 versus 90%), interestingly, all-cause mortality was similar between groups at 2 years, which is similar to results of recent randomized trials in abdominal aortic aneurysm stent-graft trials.52
(A) Angiogram of a descending thoracic aortic aneurysm suitable for stent-graft repair. (B) Angiogram illustrating successful exclusion of the aneurysm sac with a thoracic stent graft.
Ricco and colleagues have most recently reported on an independent nationwide study in France using a variety of endovascular devices to treat descending thoracic aortic aneurysms in the majority of cases.53 The Gore Excluder TAG (Gore) and Talent (Medtronic) devices were used in 84% of patients and an operative mortality of 10% was reported. A complication rate of 21% was reported, which included endoleaks in 16% of patients that were fatal in three patients. The 6-month survival rate was 86% and freedom from other complications (other than endoleak) was only 63% at 6 months. This study, involving 166 stent-graft repairs, performed in 29 centers and using six different types of endoprostheses, demonstrated that stent-graft repair of thoracic aortic disease could be performed with acceptable morbidity and mortality, at short-term follow-up. They do, however, qualify their conclusions by stating that endograft treatment of thoracic aortic aneurysms should continue to be used in an investigative setting.
Perhaps similar to abdominal aortic aneurysms, stent grafts may become the preferred treatment for ruptured thoracic aortic aneurysms. Previously, open surgical repair of ruptured thoracic aortic aneurysms was associated with significant mortality and morbidity. Specialized centers with both expertise and appropriate devices may be capable of endovascular repair in patients with sufficient 2-cm landing zones to allow secure fixation of an appropriately sized endograft. Although thoracic ruptures are far less common than abdominal aneurysm ruptures, the increasing penetrance of this technology will likely allow timely intervention in multiple specialized centers.
Thoracic Aortic Dissection
For dissections originating distal to the origin of the left subclavian artery, referred to as Stanford type B dissections, aggressive antihypertensive therapy has been the mainstay of treatment.40 For complicated dissections, however, defined as those dissections with rupture, impending rupture, intractable pain, rapid expansion, or malperfusion syndromes, surgical intervention is indicated. In these instances, however, surgical mortality has been reported to be as high as 50 to 60% in this high-risk group of patients. Endograft coverage of the primary intimal tear, redirecting flow into the true lumen, appears to be an ideal application of this endograft technology. The Stanford group, with their colleagues in Mie University in Japan, initially reported the use of the first-generation stent grafts in 19 patients with complicated type B aortic dissections. Placement was successful in all patients, and revascularization of ischemic branch vessels occurred in 76% of cases. There were three hospital deaths, two of which resulted from late referral, with irreversible end-organ damage, and one in a patient with Ehlers-Danlos syndrome who was perhaps untreatable by any modality. Although the follow-up was short, there were no incidences of aortic rupture or aneurysm formation, and a single-lumen aorta to the level of the stent graft was achieved in the majority of patients27 (Fig. 54-4).
(A) Intravenous contrast-enhanced CT scan of the upper abdomen demonstrating an aortic dissection with compression of the true lumen. (B) Angiogram of the thoracic aorta demonstrating a type B dissection involving the descending thoracic aorta. (C) CT scan of the abdomen, and (D) angiogram of the descending thoracic aorta after stent-graft implantation into the true lumen in the proximal descending thoracic aorta.
Multiple groups have now reported their results with endograft repair of complicated acute type B aortic dissections. Dynamic malperfusion is typically reversed with stent-graft coverage of the primary intimal tear, restoring flow into the true lumen and expanding collapsed segments. Static mechanisms of malperfusion, with branch vessel occlusion from orifice tears of the intima, can be diagnosed during the same intervention, and frequently reversed with uncovered stents into the true lumen of branch vessels. Mortality rates for this high-risk population have been significantly lowered from the previous surgical figures of 30 to 60%.
Likely because of these good results, interest developed in the utility of stent grafts for uncomplicated acute type B aortic dissections. A randomized trial referred to as the INSTEAD trial (INvestigation of STEnt in patients with type B Aortic Dissection)* investigated all-cause mortality as the primary outcome; secondary outcome variables included conversion to stent and/or surgery, thrombosis of the false lumen, cardiovascular morbidity, aortic expansion, quality of life, and hospital stay. Given the relatively low 1-year mortality for uncomplicated type B dissections, especially if treatment is delayed for at least 2 weeks, it is not surprising that the survival of the stent-graft population was inferior to that achieved with standard medical therapy. Another randomized multicenter trial, the ABSORB trial, will randomize uncomplicated patients during the acute phase, and is currently enrolling patients.54 The early experience suggests that, in experienced hands, stent-graft morbidity is low. Covered portions should not extend below T-6 to T-7 to minimize the incidence of paraplegia. Proximal uncovered stents should not be placed in the curvature of the aortic arch to avoid retrograde extension into a more dangerous type A dissection. More distally, uncovered stents may promote false-lumen thrombosis while allowing continued perfusion of intercostal arteries. False-lumen thrombosis is likely for the extent of stent coverage. Stent grafts specifically designed for dissections may well incorporate these features, ie, flexible stent grafts with low hoop strength, no uncovered elements on the proximal end, approximately 10 cm of covered stent, and a longer segment of uncovered stents distally.
As stated, the utility of stent grafts for the management of chronic aortic dissections is quite problematic. Because of disease complexity and extent, many were hopeful that stent grafts would prove an effective modality for treatment. However, the increasing thickness of the dissection flap with time, the frequent severe narrowing of the true lumen, the variable origin of visceral vessel origins from both true and false lumens, and the presence of multiple fenestrations between true and false lumens have severely limited the utility of stent grafts in the management of chronic aortic dissections. One exception may be the focal enlargement of the false-lumen in the proximal descending thoracic aorta. Coverage of the proximal intimal tear in these patients may allow false-lumen thrombosis for the proximal thoracic aorta. However, fenestrations almost invariably present at the level of the diaphragm and below allows a high- pressure entry into the false lumen, which then frequently propagates proximally. Although some local growth may be limited, further enlargement usually continues distally beyond the extent of the stent graft. More distal coverage in an attempt to promote false-lumen thrombosis may increase the risk for paraplegia. Uncovered stents may stabilize the flap, promote false-lumen thrombosis, and allow continue perfusion of patent intercostals arteries. These strategies will require further investigation.
Penetrating Atherosclerotic Ulcers and Intramural Hematomas
IMH of the aorta is attracting growing interest as a variant of aortic dissection.44 The exact pathophysiology is not well understood. Although by definition, pure IMH likely occurs from hemorrhage into the media from the vasa vasorum, many maintain that an intimal disruption is present in all cases.
Certainly, in the absence of any intimal disruption, there would be no indication for stent-graft repair.44 IMH, however, is often associated with or even precipitated by PAUs of the descending thoracic aorta.42 Therefore, covering the PAU with a stent graft may limit the progression of the IMH and allow healing to occur.19,42 Unfortunately, even with successful stent graft implantation using both first- and second-generation grafts, retrograde aortic dissection, new ulcer formation, and endoleaks have been noted in a significant percentage of patients, emphasizing the diffuse and severe nature of this disease.55–58
The Stanford group has reported their midterm results treating PAU of the descending thoracic aorta, with an average of 51 months of follow-up19 (Fig. 54-5). Using both first- and second-generation commercial devices, 26 patients were treated, 14 of whom were deemed nonoperative candidates. The primary success rate was 92%, with actuarial survival estimates of 85, 76, and 70% at 1, 3, and 5 years, respectively. Perioperative mortality was 12%. Increasing aortic diameter and female gender were determinants of treatment failure. These risk factors reflect the importance of careful patient selection based on anatomical criteria and clinical factors. In addition, long-term follow-up with serial CT angiography is necessary to detect late complications.
Three-dimensional CT scan of a giant penetrating ulcer involving the descending thoracic aorta that is perfectly suited to treatment with a thoracic stent graft.
Aortic injuries secondary to nonpenetrating trauma are lethal lesions, with 80 to 90% of patients dying in the hour after the accident.59 Urgent surgical graft replacement of the aorta has been the standard treatment, but these patients frequently have other major injuries, including closed-head injuries, pulmonary contusions, and other solid-organ injuries, which may limit options for open surgical repair. Several authors have reported the improved results of stent grafts over open repair for these acute injuries.60,61 Although long-term durability may be a concern, there appear to be significant short-term benefits.61,62 The major difficulty at present is the absence of thoracic stent grafts sufficiently small in diameter as to be appropriate for these relatively normal-sized aortas, frequently less than 20 to 22 mm in diameter. Additionally, small iliac and femoral vessels may limit access. Because of these limitations, and perhaps because we rarely see patients within the first few hours of their injury, during which the risk for late rupture is probably greatest, we have embarked on a strategy of permissive hypotension, intervening only for those patients with signs of impending rupture, those with increasing mediastinal hematoma or hemothorax, or persistent pain. For these patients, conventional repair is preferred, using heparin-bonded circuits, unless closed-head injury or pulmonary contusions contradict. Stent-graft repair is used for those patients in whom a conventional repair could not be tolerated. Computed tomography is repeated at 24 hours, and again intervention is elected only if there has been enlargement or progression in the extent of the pseudoaneurysm. Under very careful surveillance, either open or endovascular repair is then performed when the patient has sufficiently recovered from his or her other injuries. It is likely that, as stent grafts with more suitable characteristics become available, they may become the repair of choice.
There are no reliable reports of sufficient numbers to afford real long-term assessment of stent-graft performance. Modern devices appear to have good 5-year durability, although reports of fabric tears associated with sharp angulations or grafts within grafts still surface occasionally. More important, even with complete exclusion of the aneurysm sac, aortic elongation may still occur, allowing the late development of type I endoleaks. Therefore, lifelong monitoring is essential, and currently requires cross-sectional imaging with either computed tomography or magnetic resonance imaging. For these reasons, we have preferred open repair for younger good-risk patients, using endovascular repair for older individuals with favorable anatomy. The Stanford group has reported on their midterm results of stent-graft repair of chronic traumatic aneurysm of the descending thoracic aorta63 (Fig. 54-6). Among 15 patients treated with either first- or second-generation stent grafts, deployment was successful in all patients without need for surgical conversion. No neurologic complications were reported. Actuarial survival estimates at 1 and 6 years were 93 and 85%, respectively. Freedom from reintervention on the descending thoracic aorta was 93 and 70% at 1 and 6 years, respectively. Freedom from treatment failure at 1 and 6 years was 87 and 51%, respectively. They therefore concluded that stent grafts are safe in patients with chronic traumatic aneurysms, and are associated with satisfactory but not optimal midterm durability. They state that younger, low-risk patients should be offered conventional, open surgery and stent-grafting should be reserved for those patients who are at prohibitive operative risk.63
(A) Thoracic angiogram demonstrating a contained rupture of the descending thoracic aorta in a trauma victim. (B) Thoracic angiogram revealing repair of the aortic rupture with a thoracic stent graft.
*Nienaber C, Rousseau H, Eggebrecht H, et al: A randomized comparison of strategies for type B aortic dissection: the Investigation of STEnt grafts in Aortic Dissection (INSTEAD) trial. Circulation 2009; 120:2513-2514.