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Moving toward the next generation of thoracic aortic stent grafts
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By Ron Fairman, M.D. University of Pennsylvania, Philadelphia, USA
With the recent completion of follow-up in the three arms of The Valor Trial (Evaluation of the Medtronic Vascular Talent Thoracic Stent Graft System for the Treatment of Thoracic Aortic Aneurysms), Medtronic is preparing to introduce into US clinical trials their next generation thoracic aortic endovascular stent graft, called the Valiant device. This technology has already been introduced in Europe and has become widely accepted. The Talent stent graft system evaluated in The Valor Trial has been used extensively outside the US for many years. In addition physicians have gained considerable experience with the Talent stent graft since the feasibility Phase I high-risk trial was performed in the US in 1998.
The evolution of the Talent stent graft to the Valiant design is a result of accumulated feedback from thousands of implants worldwide. Medtronic engineers have enhanced both the delivery system as well as the stent graft itself creating a product that promises to be vastly superior to the original Talent stent graft. That being said, the original Talent stent graft was the first endovascular device that we had available to treat thoracic aortic pathology when we began our thoracic endovascular program at the Hospital of the University of Pennsylvania in 1998. The customization features of that first generation Talent device presented us with our first opportunity to offer novel endovascular options to patients who were being managed with "watchful waiting." The preliminary outcomes in our first 50 patients using that early Talent design were largely extraordinary. Minor changes to that original Talent device culminated in the Talent Stent Graft System used in the Valor Trial (Figure 1).
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Figure 1: The Talent stent graft
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Figure 2: Competion angiogram following the placement of a Talent stent graft
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The system is composed of a pre-loaded stent graft and the CoilTrac delivery system. For the purposes of the Valor Trial, the delivery system was re-designed without the balloon in the tip. This change was made based on previous clinical trial experiences. Elimination of the integral balloon served to reduce the potential for kinking thereby reducing the deployment forces. The implanted Talent Thoracic endoprosthesis is composed of a polyester graft fabric sewn to a self-expanding Nitinol wire frame. The design concept is modular and although the Talent Thoracic stent graft has been viewed as a customized device, only catalog sizes were available in the Valor Trial. Proximal and distal stent graft diameters range from 22mm-46mm and total covered length of device range from 112mm-116mm. Bare Spring (proximal device diameter<24mm) and Freeflo (proximal device diameter >24mm) configurations are available proximally, which have a terminating spring without fabric coverage. Bare Spring configurations are also available distally.
Both proximal and distal Bare Spring configurations allow for crossing the great vessels of the arch as well as the celiac artery respectively. An accessory Reliant stent graft balloon, packaged separately is intended for use following stent graft deployment to facilitate modeling of the covered springs and to remove fabric pleats from the graft material.
The Valor Trial is a prospective, multi-center, non-randomized evaluation of the safety (rate of "all cause" mortality) and efficacy (successful aneurysm treatment at one year) of the Talent Thoracic Stent Graft System when used in patients with thoracic aortic aneurysms (Test Arm). The Test Arm consists of patients diagnosed with thoracic aortic aneurysms that are considered candidates for open surgical repair whom are low to moderate risk based on SVS/ISCVS criteria. Additionally, two observational treatment group registries were conducted concurrently, serving to record descriptive information that may serve as the basis for future clinical investigations. The Registry and High Risk Arms include patients diagnosed with dissections, traumatic injury, pseudoaneurysms, as well as aneurysms without a distinct proximal or distal aneurysm neck of 20mm or greater in length.
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Figure 3
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Figure 4: The Xcelerant delivery system allows stable and accurate deployment
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Although there were 40 active sites in the Valor Trial, eight sites in the US trial enrolled 57% of the test group patients and 66% of the high risk/registry arm. At the time of this trial, thoracic stent grafting was performed largely in a handful of centers. Most of the pathology treated throughout all arms of the trial consisted of fusiform or saccular aneurysms; in the high-risk arms, this pathology was present in 76% of the patients enrolled. The demographics revealed that 40% of the patients enrolled were females, a percentage not dissimilar from the phase II multi-centre trial of the Gore Tag thoracic endoprosthesis. With a greater percentage of female patients compared to abdominal aortic aneurysmal disease, issues of iliac access assume critical importance.
In the Valor Trial, surgically placed conduits were necessary in upwards of 15% of patients, demonstrating the need for delivery systems smaller than 22-25 Fr (O.D.). The Bare Spring or Freeflo proximal design as well as the availability of stent graft devices with diameters as large as 46 mm, opened the door to endovascular thoracic aortic options for a broader range of patients in Valor than with any other industry sponsored thoracic device trials (Figure 2). Due to device sizing constraints based on thoracic aortic anatomy, 35% of the patients treated with the Talent Thoracic stent graft in the high risk arm of Valor could not be treated with any other industry sponsored device. The preliminary results of the high risk arm were presented in June, 2005 at the Vascular Annual Meeting in Chicago.
The Talent Thoracic experience has resulted in a number of consistent observations that are relevant to not only the Talent Thoracic device, but to all endovascular therapies in the thoracic aorta. Although rigid stent grafts can function well in the abdominal aorta, flexible designs that conform to the aorta are paramount in the thoracic aorta (Figure 3). Thoracic devices need to conform to the aortic arch as well as the tortuosity inherent in the atherosclerotic thoracic aorta. Although one can accurately deploy a Talent Thoracic device in the proximal descending thoracic aorta, controlled deployment in an angulated arch or in an area of marked tortuosity is difficult. These issues are addressed with the Xcelerant delivery system (Figure 4) which has been available to physicians in the US for the AneuRx AAA stent graft, and has been modified for the Valiant device. The new thoracic version of the delivery system has a slightly smaller profile than Coiltrac (and tapers near the handle) and allows for controlled ratcheted precise deployment.
Stabilization of the delivery system when deploying in the arch is of fundamental importance in preventing embolic stroke. In order to optimize ease and accuracy of deployment as well as conformability, the long connecting bar of the Talent Thoracic device has been removed in Valiant, while columnar support has been optimized through stent spacing in the exoskeleton. The removal of the connecting bar has eliminated the need to orient the device in vivo and results in improved flexibility. The proximal uncovered bare spring has been increased from five to eight peaks, which distributes the force of the spring over more apexes, and in addition the proximal stents have been inset into the fabric slightly to reduce radial flare. Experience to date has shown that this change does result in more stable deployment, which may prevent the rare instances of "bare spring flip" observed when deploying in an extremely angulated arch.
Furthermore, longer stent grafts are particularly desirable when treating most pathology in the thoracic aorta. The great majority of thoracic aortic conditions require stent graft coverage of up to 200 mm. Although shorter stent grafts are fine for treating focal disease processes such as penetrating ulcers, transections, or saccular aneurysms; in most instances we are treating fusiform longer segments of disease. Longer endografts result in less modular junctions and fewer passes of large delivery systems through small diseased iliac arteries.
A consistent observation is that it is difficult to identify proximal and distal aspects of modular components once inserted. The new Valiant device has distinct "figure of eight" radiopaque markers proximally and "zero" markers distally which provide enhanced visibility and result in more precise overlap at modular junctions. The Talent Thoracic and Valiant devices are compared side by side in Figure 5. Although the preliminary outcomes of the Valor High Risk Arm using the Talent Thoracic Stent Graft System are encouraging and reveal high procedural success in the setting of low operative mortality, stroke incidence and paraplegia rates, enhancements in stent graft design are evolving. The delivery system and stent graft changes culminating in the Valiant design will allow more precise placement of endografts and should further reduce deployment related complications.
The Valor Test Arm is now in the follow-up phase and the PMA will be filed with the FDA very soon. Medtronic is currently initiating their US clinical trial using the Valiant Stent Graft System, called Valor II.
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Figure 5
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