This article introduces the concept of radial fit—the optimization of force and conformability of a device in a disease- and anatomy-specific manner. Ideally, we would like to see a device abut the entire circumference of the aorta with enough radial force to provide a seal without adversely affecting the vessel wall with excessive force. In maintaining these elements, the device should be able to conform to nonuniform circumferential anatomies and a variety of longitudinal aortic shapes and be usable in a range of aortic pathologies with variable vessel wall characteristics. A device is only able to appose the wall along its entire surface if it has features that permit adaptability.

RADIAL FIT: EXPANDED OVERSIZING RANGE

A key example of a device's adaptability is the expanded oversizing range of the Conformable GORE® TAG® Device (Gore & Associates), which can accommodate a larger treatment range with a single device. In preoperative planning, computed tomographic angiography is the most commonly used modality. As we gain significant experience in cardiac (dynamic) magnetic resonance imaging (MRI) and electrocardiography-gated computed tomography, we realize that there can be up to 27.5% variation between systolic and diastolic aortic diameters.1 It is difficult to calculate the most appropriate fit in preoperative imaging, where dimensions are measured during diastole. In this case, radial fit is the ability of a stent graft to manage a range of diameters without losing radial apposition. Table 1 shows the expanded treatment range of the Conformable GORE® TAG® Device, which can accommodate a potential difference between systolic and diastolic in aortic diameters of up to 8 mm. These expanded treatment ranges are also important when considering tapered aortas, traumatic disruptions in younger patients, and aortic coarctation, in which a difference of up to 9.5 mm between the proximal and distal diameters can be accommodated with a tapered device.

RADIAL FIT: CONFORMABILITY

In preoperative measurements, we tend to identify tortuosity and sharp angulations at the arch or diaphragm as cautionary landing zones, generally trying to move that landing zone into straighter segments. Proper radial fit improves conformability so that the device is able to seal in territory that was previously hostile because of tortuosity, including difficult landing zones distal to the left subclavian artery. There, radial fit has the potential of avoiding a significant percentage of left carotid-subclavian bypasses, which have become fairly standard when coverage of the left subclavian artery is required.

There are several engineering elements that contribute to the radial fit of the device. This includes outward radial force (amount of force the device applies to the vessel wall due to oversizing), minimum bend radius (device flexibility without kinking), and spring-back force (amount of force the device exerts on the aortic wall due to device straightening). Engineers use these terms to study devices in a preclinical setting to predict device behavior when implanted in patients. In reality, the engineering inputs have not affected device selection, because this level of detailed patient-specific measurement is not currently available in humans. We have been interested in using dynamic MRI to evaluate many of these parameters on a patient-specific basis for device deployment in the thoracic aorta. Dynamic MRI provides anatomic and physiologic data acquired in four-dimensional mode. With the addition of patientspecific computational fluid dynamics, we can solve the flow conditions in which the devices are being deployed (Figure 1). Cardiac MRI techniques, which include four-dimensional flow, are completely unique and allow an evaluation of flow directional changes and vessel wall interactions. There is some evidence that changes in spiral flow with increased eccentric flow vectors and consequent elevated shear stress have a link to aortic aneurysm development.2 There is also an indication that decreased spiral flow patterns are linked to increased atheromatous disease in the carotids, as well as coronary artery disease at all ages.3,4 In light of these findings, we submit that radial fit is extremely important, particularly as clinicians continue to push the envelope and bring aortic endografts into more proximal segments of the aortic arch. Poorly apposed endografts are more likely to cause flow disturbance with ensuing elevated shear stress and potential clinical consequences, such as stent collapse and fracture.5 Currently, we evaluate all our aortic endografts with these techniques, including computational fluid dynamics, in an effort to understand the effects of flow on the grafts and how devices promote conformational changes of the aorta versus stents that conform to the patient's anatomy.

RADIAL FIT: RADIAL FORCE

The early results of a regulatory study originally published in 2005 led to the US Food and Drug Administration's approval of the GORE® TAG® Device as the first stent graft approved for the repair of descending aortic aneurysms of the thoracic aorta.6 Since then, the Conformable GORE® TAG® Device, along with multiple other devices, have undergone numerous modifications to address some of the issues initially encountered— bird-beaking, collapse, and stent fatigue with fracture. Radial fit potentially addresses many of these issues by giving the interventionist an opportunity to dictate the radial force of the endograft, tailoring it to a specific pathology or anatomy. For example, a patient with a 29-mm aorta can be treated with three different Conformable GORE® TAG® Devices (31, 34, and 37 mm) that will provide different amounts of radial force (Figure 2).

RADIAL FIT IN THE CLINICAL SETTING

The need for pathology-specific grafts that increase conformability and trackability was well noted in a recent European position statement on TEVAR.7 In modern endovascular aortic repair, pathologies encountered include degenerative aortic disease (aneurysm, penetrating ulcer), aortic dissection, intramural hematoma, aortic trauma, and coarctation. In each pathological process, the aortic wall will behave differently upon being manipulated by endovascular devices, and therefore, a clinician must be able to identify a disadvantaged aortic wall and dictate the most appropriate radial force. The risk of stroke can also be affected by proper radial fit. Stroke is a consequential complication of TEVAR and still persists at a rate of 4.3% to 5.8%.8,9 By using transcranial Doppler (TCD) monitoring during all our TEVAR procedures, we found that there was a significant association between the total number of microemboli and postoperative stroke, transient ischemic attack, and death.10 TCD monitoring's objective is to estimate which devices and steps of the procedure provoke the most microembolization. This information implores changes in the approach toward stent grafting: How can certain parts of the procedure, or the actual device choice, be modified to limit the degree of microembolization and ensuing stroke? One of the most common issues with the early generation of thoracic devices was “bird-beaking,” a process that exacts a degree of deformation to the aorta.11,12 The ideal condition is to devise an endograft that conforms to the vessel and does not deform the aorta. The Conformable GORE® TAG® Device does exactly that, as it affords better wall apposition and low spring-back force.

In our analysis of the TEVAR procedure, device deployment generated the greatest number of microemboli. We have noted that the microemboli count has been greatly reduced when using the Conformable GORE® TAG® Device (Figures 3 and 4). This observation is inherently intuitive when one considers the potential adverse impact that a non-conformable stent could have on the vessel. With a conformable stent, forces are naturally distributed more evenly on the entire stent-to-wall interface, displaying an ideal radial fit of the stent. Because radial fit is maintained across greater degrees of oversizing, the clinician gains significant control in how he or she chooses a stent based on the ideal radial force desired. Furthermore, we have identified a major modification in the way a majority of procedures are performed in our practice. Proper radial fit may obviate the need for post-delivery ballooning in cases where the device is appropriately apposed to the aortic wall. Ballooning is a process that can produce significant embolization, as well as cerebral flow changes (Figure 5).10

CONCLUSION

Endovascular management is only as successful as the devices we implant. Therein lies the paradox—for years, we have implanted devices that were not designed for all thoracic applications. Therefore, it only requires an occasional twist of fate for devices to show signs of failure, requiring further intervention. Imaging modalities such as TCD and MRI have allowed us to ascertain that positive changes can come with device improvements, such as those of the Conformable GORE® TAG® Device. Learning from incidents and clinical observations over a longer period of time will establish whether these encouraging outcomes will be maintained.

Alan Lumsden, MD, is Chair of the Department of Cardiovascular Surgery and Medical Director of the Methodist DeBakey Heart and Vascular Center at the Methodist Hospital. He is also a professor of cardiothoracic surgery at Weill Cornell Medical College of Cornell University in Houston, Texas. Dr. Lumsden has disclosed that he is a speaker and receives education and training support from Gore & Associates.

Jean Bismuth, MD, is an assistant professor of surgery at Weill Cornell Medical College of Cornell University and with the Department of Cardiovascular Surgery at the Methodist DeBakey Heart and Vascular Center in Houston, Texas. He has disclosed that he is a speaker and consultant for Gore & Associates. Dr. Bismuth may be reached at JBismuth@tmhs.org.

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