TEVAR - the devices part 1

http://dqsendovascular.blogspot.kr/2012/10/tevar-devices-summary.html

Thoracic endovascular aortic repair (TEVAR) is becoming increasingly popular due to reduced perioperative morbidity and mortality compared with open surgical repair . Although TEVAR was developed for the treatment of degenerative aneurysmal disease, other pathologies can be treated using these devices including aortic dissection, aortic transection, intramural hematoma and penetrating aortic ulcer. Endovascular aortic repair requires that specific anatomic criteria be fulfilled, and, for those with appropriate anatomy, this technique allows the treatment of patients who might not otherwise be candidates for aortic repair. So It is still warrant sthat this procedurest is always an alternative way to manage AAA, not replacing the gold standart of open repair due to rate of complication which going to discussed elsewhere

Advanced endovascular aortic devices that allow flow into specific aortic branches depending upon the level of repair (eg, innominate, left subclavian, renal artery, internal iliac artery) are available to treat more complex anatomy.
The placement of aortic endografts is associated with device-related complications that can include component disconnection, stent-graft buckling and migration over time often due to disease progression (eg, aortic dissection). In the thoracic aorta, secondary intervention is needed in 10 to 60 percent of patients, more commonly in patients undergoing endovascular repair of thoracic dissection and more complicated hybrid repairs . As such, these devices require lifelong surveillance; the long-term outcomes continue to be studied.
The specific devices available for endovascular repair of the thoracic aorta will be reviewed here. The indications for, placement of, and complications of these devices are discussed elsewhere.
The aorta is the major arterial conduit conveying blood from the heart to the systemic circulation. It originates immediately beyond the aortic valve ascending initially. Then it curves forming the aortic arch, and descends caudally adjacent the spine. The ascending thoracic aorta gives off the coronary arteries. The aortic arch branches are typically the brachiocephalic trunk (branches to the right common carotid and right subclavian arteries), left common carotid and left subclavian arteries; however, aortic arch anatomy is variable.
The descending thoracic aorta provides paired thoracic arteries (T1-T12) and continues through the hiatus of the diaphragm to become the abdominal aorta which extends retroperitoneally to its bifurcation into the common iliac arteries at the level of the fourth lumbar vertebra. The abdominal aorta lies slightly left of the midline to accommodate the inferior vena cava which is in close apposition. The branches of the aorta (superior to inferior) include the left and right inferior phrenic arteries, left and right middle suprarenal arteries, the celiac axis, superior mesenteric artery, left and right renal arteries, left and right internal spermatic arteries, inferior mesenteric artery, left and right common iliac artery, middle sacral artery and the paired lumbar arteries (L1-L4).
The common iliac artery bifurcates into the external iliac and internal iliac arteries at the pelvic inlet . The internal iliac artery gives off branches to the pelvic viscera and also supplies the musculature of the pelvis. The external iliac artery passes beneath the inguinal ligament to become the common femoral artery


 Endovascular aneurysm repair refers to the insertion of endovascular graft components, usually via a femoral approach. A thoracic endograft is constructed by the delivery and deployment of these components in an established order in vivo. Upon deployment, the thoracic endograft self-expands, contacting the aortic wall proximally and distally excluding the native aortic wall from aortic blood flow and pressure.
Although there are significant variations in endovascular graft design, three types of components are common: the delivery system, the main device, and device extensions.

Delivery system — Thoracic endografts are typically delivered through the femoral artery, percutaneously or by direct surgical cutdown. The size of the delivery system varies depending upon the diameter of the device that is needed to provide proper endograft fixation.
If the femoral or iliac artery is too small to accommodate the delivery system, access can be obtained by direct puncture of the iliac artery or aorta through a retroperitoneal incision or by suturing a synthetic graft to the iliac artery (ie, iliac conduit). An alternative method creates an endovascular conduit by first lining the iliac artery with a covered stent and then allowing a contained rupture of the diseased iliac segment as the endograft is passed through.
Main device — The main thoracic endograft device is straight or tapered, and two distinct device components are often used (eg, straight graft proximally, tapered graft distally). Endovascular grafts rely primarily upon tension in the proximal graft to maintain the positioning of the graft. Fixation systems may include barbs or uncovered proximal stents.

Extensions — One or more extension devices (proximal or distal) may be needed to provide a complete seal. Following the deployment of the main device, an aortogram is performed to assess for endoleak. If additional ballooning of the device does not firmly appose it to the aortic wall and eliminate a type I endoleak , placement of additional proximal or distal aortic extensions may be needed.

THORACIC DEVICES — The indications for endovascular treatment of thoracic aortic pathologies are broadening. Although thoracic endovascular aortic repair was developed for the treatment of degenerative aneurysmal disease, other pathologies can be treated using these devices including aortic dissection, aortic transection, intramural hematoma and penetrating aortic ulcer
Endovascular aortic repair requires that specific anatomic criteria be fulfilled, and, for those with appropriate anatomy, this technique allows the treatment of patients who might not otherwise be candidates for thoracic aortic repair. Advances in endograft design have allowed the treatment of patients with increasingly complex anatomy, but the ideal graft suited to all the possible combinations of pathology and anatomy does not exist. Thus, it is not surprising that a number of thoracic stent-graft devices are commercially available and the design of the available grafts is ever-evolving to address problems that are frequently encountered with the delivery, deployment, and fixation of thoracic endografts
Controlled comparisons of the various thoracic endografts have not been performed [8]. Data supporting the use of these devices are primarily from clinical trials performed to seek approval from the United States Food and Drug Administration (FDA) or Conformité Européenne (CE) mark. Devices that are currently approved or under investigation for treatment of descending thoracic aneurysms include the Gore TAG™, Cook Zenith® Tx2®, Medtronic Talent Thoracic Stent-Graft System, Medtronic Valiant Thoracic Stent-Graft System, Bolton Relay™, TAArget™, and E-Vita® grafts. The Conformable TAG® graft (CTAG) has also been approved in the United States for the treatment of blunt aortic injury (see 'TAG™' below) [1]. A number of investigational devices are in early phases.

The essential features of the various thoracic endografts and clinical data supporting their use are discussed below.
TAG™ — The TAG™ device (W.L. Gore & Associates), is a self-expanding tube-shaped stent-graft supported by a nitinol exoskeleton lined with expanded polytetrafluoroethylene (ePTFE) and covered with an additional reinforcing sleeve of ePTFE and fluorinated ethylene propylene (FEP), which reduces the potential for type IV endoleak . The proximal and distal ends of the graft have scalloped flares that are also lined with graft material and an ePTFE sealing cuff at the base of each flare. A radiopaque band is located at the base of each flared end to facilitate placement under fluoroscopy. The flared sealing regions are designed to improve conformity and better apposition of the graft to the aortic wall.
The TAG™ device is available in diameters ranging from 26 to 45 mm in 10, 15, and 20 cm lengths. The delivery sheath ranges from 20 to 24 French (6.67 to 8 mm) in diameter [10]. The device is constrained within a deployment line. Once the device is positioned, the deployment line unlaces from the middle of the device toward both ends, thus avoiding a “windsock” effect. Once deployed, the device is ballooned open with a trilobed balloon that allows aortic flow during inflation.
The TAG™ pivotal trial compared open surgical repair with endovascular repair in patients with descending thoracic aortic aneurysm, but was interrupted due to device failure . After modification of the device, a multicenter study compared 51 patients treated with the TAG™ device with the surgical group from the pivotal study, and an additional 80 patients were enrolled as part of a confirmatory study . Significantly improved aneurysm-related survival (96 versus 88 percent) was found at five years (original pivotal) and three years (confirmatory study) for patients treated with thoracic endovascular repair and a reduced incidence of major adverse events (71 versus 21 percent at 30 days, 37 versus 21 percent long-term) compared with open surgery. No device fractures were reported since the device modification consisting of removal of a longitudinal spine.
A next generation device, the Conformable TAG® graft (CTAG), has received approval for use and is available in a wider range of diameters (21 to 45 mm) with a sheathless delivery system [14]. The design is intended to conform to smaller, more tortuous and/or tapered thoracic aortic anatomy and to provide treatment of pathologies that can affect nonaneurysmal (normal diameter) aortas such as blunt aortic injury, penetrating aortic ulcer, aortic dissection. A nonrandomized study evaluated the safety and efficacy of the CTAG device for the repair of traumatic aortic transection in 51 patients [15]. Technical success was 100 percent with no operative mortality. No major device events have been reported. Perioperative (30-day) mortality was 8 percent, which was not felt to be device- or procedure-related. No conversions to open repair have been reported to date. Endovascular repair for the treatment of blunt aortic injury is discussed in detail elsewhere.
Zenith® TX2® — The Zenith® TX2® (Cook Medical) endograft is a two-piece system. Both components are constructed from modified Gianturco Z-stents (stainless steel) sutured to woven polyester graft material (ie, Dacron®). The graft material lines the graft and is also external to the stent structure at the proximal and distal ends of the graft to allow apposition of the graft to the aortic wall. Active fixation with external barbs at the proximal and distal aspects of the device is present for each component. Straight and tapered proximal components are available. A bare metal stent is attached to the distal graft and is similar to the suprarenal stent component of the Zenith® device for endovascular repair of abdominal aortic aneurysm. The extension allows for active fixation of the device over the origins of the visceral vessels. A suture modification of the device (Pro-form) is intended to improve conformability of the graft in the aortic arch during very proximal descending thoracic aortic deployments to minimize the risk of graft infolding and collapse .
The proximal and distal Zenith® TX2® device components are available in diameters ranging from 28 to 42 mm [17]. The components range in length from 12 to 21.6 cm. The devices are delivered through a 20 or 22 French (6.67, 7.33 mm) sheath depending upon the diameter of the device. The device is constrained with several trigger wires which are released sequentially once the device is positioned. After deployment, the device is self-expanding; subsequent ballooning is an option.

The safety and effectiveness of the Zenith® TX2® was evaluated in a multicenter study of 230 patients with descending thoracic aortic aneurysms or thoracic aortic ulcers who were treated with endovascular repair using the TX2® (n = 160) or open surgery (n = 70) [18]. Perioperative (30-day) mortality was not significantly different between the groups; however, perioperative morbidity was lower for endovascular repair (composite index 1.3 versus 2.9). Although endovascular repair was associated with fewer cardiovascular, pulmonary, and vascular adverse events, the incidence of neurologic events was not significantly different. At 12-months follow-up, aneurysm growth was identified in 7.1 percent of endovascular patients, endoleak in 3.9 percent, and migration >10 mm in 2.8 percent.

In another study of patients considered at high risk for open surgery, the TX2® graft was used to treat primarily thoracic aneurysms in 100 patients [19]. The overall mortality was 17 percent and aneurysm-related mortality was 14 percent. Secondary interventions were needed in 15 percent of patients. Stroke and paraplegia rates were 3 and 6 percent, respectively. Stent migration occurred in 6 percent of patients at one-year follow-up.

The Zenith® TX2® dissection system is designed to treat acute complicated type B aortic dissection. It is a two-component system composed of a proximal TX2 graft combined with a distal bare stainless steel Z-stent. The proximal component covers the entry tear and the bare stents are intended to expand the true lumen while allowing flow to branch vessels. The bare stent can be deployed to the aortic bifurcation, if needed. The distal stent is available in 8.2, 12.3 and 16.4 cm lengths and a single 46 mm diameter that accommodates aortic diameters from 24 to 38 mm.

The safety and effectiveness of the Zenith® TX2® dissection system was evaluated in a multicenter study of 40 patients in which the indications for enrollment included branch vessel malperfusion, impending rupture, aortic diameter ≥40 mm, rapid aortic expansion, and persistent pain or hypertension despite maximum medical therapy [20]. Most patients (31/40) were treated for impending aortic rupture or branch vessel malperfusion. The mean time from symptom onset to treatment was 20 days (range, 0 to 78 days). Seven different combinations of stent-grafts and dissection stents were used. The perioperative (30-day) mortality rate was 5 percent and one-year survival rate was 90 percent. Perioperative morbidity included stroke (7.5 percent), transient ischemic attack (2.5 percent), paraplegia (2.5 percent), retrograde progression of dissection (5 percent), and renal failure (12.5 percent). None of the patients with renal failure became dialysis-dependent. Favorable aortic remodeling was observed during the course of follow-up, completely thrombosed thoracic false lumen observed in 31 percent of patients. [21,22]