Each year in Europe, an estimated 16,000 patients are diagnosed with descending thoracic aortic lesions. The thoracic aorta is vulnerable to the development of aneurysms and dissections, which can be signalled by chest or back pain, but are usually present without symptoms. Associated pain may indicate acute expansion or leakage of the lesion. The thoracic aorta can also be impacted by outside trauma, such as injuries sustained in an automobile accident. If the thoracic aorta becomes dissected or severed during an accident, patients invariably face high mortality rates if normal blood flow is not restored quickly.
Thoracic stent grafts are used for the minimally invasive repair of the thoracic aorta for several disease states, including aneurysms, penetrating ulcers, acute or chronic dissections, and contained or traumatic ruptures. Stent grafts offer a valid alternative to open surgical repair as many patients also have serious co-morbidities such as coronary artery disease, emphysema, high blood pressure or diabetes. Open surgery is a risky, taxing procedure that can lead to death in many frail and elderly patients. For those patients, considered ineligible for open surgical repair, conservative medical management, or "watchful waiting," is often used as a treatment option and can lead to increased mortality and morbidity in many elderly patients due to the risk of rupture.
One year after Gore launched its modified thoracic endoprosthesis, the Gore TAG Thoracic Endoprosthesis and following the launch of Cook's TX2 device, Medtronic launched its Valiant Thoracic Stent Graft with Xcelerant Delivery System for commercial release in Europe, at the Charing Cross International Symposium, London, UK.
Valiant is seen as a robust, next-generation stent graft built on the company's strong thoracic endovascular clinical history, which includes more than 16,000 implanted grafts. Medtronic claims its Valiant system provides more options and increased control, being based on a platform that has a strong history of proven results - essential benefits physicians need to confidently treat challenging thoracic anatomy. In the pre-clinical models the device has shown improved performance, especially regarding key attributes such as conformability, trackability, and deployment.
Johannes Lammer, Professor of Radiology at University Hospital Vienna, discussed current stent design and highlighted the current limitations of stent TAA design. "At the moment, we have insufficient diameter of stent grafts, adaptablity of the stent grafts once we come to the aortic arch and the aortic arch itself. We need more flexible devices."
The Valiant addresses a variety of anatomies with the breadth of sizes and configurations available. Physicians have requested a device that is available with either a proximal bare spring or a closed web, and Valiant is offered with both configurations to address a wider variety of anatomies. Furthermore, Valiant is offered in lengths up to 227mm and in diameters from 24 to 46mm. In addition, both straight sections as well as tapered sections are available.
"Initially, we only had a 10-12cm device, now we have longer devices, which is extremely important as previously we may have to implant two devices, which increases the risk of endoleak and the risk of complications of trauma to the excess vessels," said Lammer.
Based on a global experience of more than 16,000 implants worldwide with the Talent Thoracic platform, Valiant is the first endovascular solution with such extensive clinical data as a foundation for design. Dr Lars Lonn, University of Gothenburg, Sweden, discussed his experience with the Valiant stent graft, "Previous systems were very demanding and they were stiff and difficult to deliver. For the interventional radiologist, there was a big learning curve."
Valiant takes the best of the Talent thoracic device and adds significant improvements. For instance, Valiant is more conformable than Talent Thoracic. The removal of the longitudinal connecting bar and optimisation of stent spacing in the new device allow the device to fit better in anatomies with significant three-dimensional curvature. In addition, the new 8-peaked stent rings on each end (compared to 5-peaks in Talent) more evenly distribute the radial force around the circumference of the aorta, leading to gentler but equally strong fixation as with Talent.
"The Valiant stent graft improves on its predecessors with superior deployment. The new deployment system allows more much precisely and easier, less traumatic, for the patient and physician," commented Lammer. Furthermore, Lammer welcomed the incorporation of the proximal bare spring, "It is useful in certain patients when coming to the arch, as it helps the conformability of the stent graft to the arch."
Xcelerant has been optimised even further, for thoracic use, from its design for Talent AAA and AneuRx. Its newest version is designed to minimise kinking of the catheter within the section in which the graft is held and a new stent stop makes release from the delivery system even easier. In addition, the Xcelerant tip is a centimeter shorter than the CoilTrac tip and more flexible, allowing Xcelerant to effectively deliver the stent graft into a broader range of anatomies. The radiopaque markers in Valiant also have been improved compared to Talent Thoracic. The distal end markers now have a different shape to help differentiate which end a physician is examining in patients in whom multiple sections have been used.
"For me the radiopaque markers still have great relevance, and the Valiant has improvements that allows us to distinguish elements of the graft," added Lonn. Dr Rob Morgan of St George's Hospital, UK, agrees that the radiopaque markers have changed and that this addition allows one to tell apart the top end from the bottom end.
In bench testing, the force required to deploy the stent graft was decreased by more than 50%, compared to similarly sized Talent Thoracic devices. A coating on the inside of the graft cover helps minimize deployment forces. In addition, the design of the delivery system gives a mechanical advantage that makes the force the physician feels less than 5% of the force felt with Talent deployment. In addition, the tracking force required to advance the delivery system through the iliacs is significantly reduced. This change is due in part to an improved catheter, which tapers towards the handle, reducing the average profile.
"I have used the Valiant stent graft in several cases and found it much improved in terms of deliverability. Previously, there was a need to use a lot of force involved in the placement and you had to keep both hands on the delivery system, this is not the case with the Valiant stent graft system," said Lonn.
Dr Morgan added, "For me a big difference is the control over release. Before there was a jerking release. Now we have a rotating handle which allows a controlled release. This is still movement, but it is predictable, which has resulted in a high technical success rate."
Lonn continued, "Not only has the deployment improved but also the accuracy. There was a tendency in previous systems for the stent graft to move upward, but this has been significantly reduced."
The stent rings also have been moved to the outside of the graft. In histological studies of both Talent AAA and AneuRx, Medtronic has found that the aortic wall incorporates the metal structure when the stents are on the outside of the graft, which may assist in long-term fixation. In addition, by moving the stents to the outside, the inner lumen is smoother and more closely resembles a healthy aorta.
Dr Morgan also added that the tapered sections of the device has allowed the overlapping of two devices, resulting in a much improved seal. He also acknowledged the utilization of a larger device, with a top end of 42mm and a bottom end of 38mm.
Medtronic will be sponsoring clinical studies to learn about potential advantages of endografting. For example, in Europe, the INSTEAD study completed enrolment earlier this year. The company also has plans to explore the health economics of thoracic endografting compared to open procedures. Finally, Medtronic has also begun market research for the thoracic endograft that will be Valiant's successor.
As for the next-generation of TAA stent grafts, Lammer said such devices require side branches in order to treat patients with arch pathology, "The majority of patients have type B dissections and for this we need special devices which are shorter and with even larger diameters."
Dr Lonn added, "For the next generation of TAA stent grafts we would like to see fenestrated stent grafts systems. In Scandinavia, we are treating patients with longer stents grafts, so I agree with Johannes that an increase in diameters in also required."
Dr Morgan concluded, "Of course there is no perfect device and deployment and accuracy can always be improved and I think will be in the next generation. There may also be the need for smaller devices, for example in young people in car accidents. I agree with both Johannes and Lars in that both fenestrated and branch stent grafts would be welcomed as this would allows us to treat a broader range of patients, from the celiac arteries to the subclavian left common carotid."
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