Balloon-Expandable Stents

Many characteristics of balloon-expandable stents require consideration in the choice of device to use for a given patient, including deployment accuracy, crush resistance, length and expandable size, foreshortening, deliverability, tapering, shape, recoil, radiopacity, and corrosion resistance.

Because of the perception that balloon-expandable stents have a higher degree of precision in deployment, they are sometimes selected for treatment of lesions near critical branch vessels. These stents are often favored in aortic applications because they have sufficient “hoop strength” or compression resistance to enable them to overcome strong elastic recoil after balloon expansion of stenotic aortic tissue. Hoop strength helps maintain an adequate lumen during treatment of resilient calcified lesions. Aortic uses require stent sizes larger than most other endoprosthesis applications, and many balloon-expandable stents are dilated to a significantly larger extent than the labeled maximum size, particularly for endovascular repair of aortic aneurysms, where diameters above 25 and even 30 mm are required. At these large diameters, balloon-expandable stents undergo severe foreshortening. A large balloon-expandable stent that is commonly used in the aorta is the Palmaz XL Transhepatic Biliary Stent, (P4010; Cordis Corporation, Bridgewater, NJ) (Fig. 95-1 right).

Deliverability of large balloon-expandable stents is a major issue. They often require large sheath access across the lesion and are usually unmounted. The technical art of delivering unmounted stents is rarely practiced in the contemporary era of premounted stents for most small and medium-sized applications. An unmounted stent must be crimped tightly by the physician onto a large balloon (such as an aortic valvuloplasty balloon), delivered to the target site, and then expanded precisely. Such a large stent is inflexible and difficult to crimp because of the thickness of the struts, so it can easily slip onto the hub end of the balloon catheter during introduction or migrate off the tip end of the balloon during deployment. Specific techniques that must be used to prevent or recover from these possible catastrophes are discussed in Chapter 132.

Another important characteristic is the ability of a stent to be tapered or molded for treatment of a focal stenosis or coarctation of the aorta. Many physicians prefer to dilate these lesions cautiously to a diameter just large enough to eradicate a significant pressure gradient, because full expansion might result in higher risk of aortic rupture. For example, if the focal stenosis and aortic stent are dilated to 10 mm but the adjacent aorta is 18 mm above and below the lesion, tapering the stent into an hourglass shape is favored to minimize later stent migration and avoid leaving bare metal suspended in the aortic lumen. The design pattern of different stents allows them to be tapered to a variable degree. Specific shaped stents are emerging for niche applications in aortic branches.

All balloon-expandable stents demonstrate some degree of recoil after expansion, but it is usually small and clinically negligible. Clinicians should be aware that some newer metals, such as cobalt-chromium alloys, have greater recoil than their stainless steel predecessors, and slightly greater balloon sizes and target inflation pressures may be needed to attain the nominal size after recoil. As long as the underlying lesion has similar elastic recoil, rupture or inadequate apposition should not be a clinical issue. Radiopacity is a strong attribute of most balloon-expandable stents, particularly the large sizes used for aortic applications. Finally, clinicians should be aware that the biocompatibility of different metals has not been tested for off-label indications, and the potential exists for accelerated corrosion and metal ion leaching, with unknown consequences.

Self-Expanding Stents

Self-expanding stents suitable for off-label use in the aorta are composed of stainless steel, nitinol, or Elgiloy. Detailed characteristics of each of these metals are presented in Chapter 96. Nitinol and Elgiloy self-expanding stents labeled for iliac use are generally available in diameters up to 10 mm and lengths of 6 to 7 cm for that diameter. Nitinol stents have relatively little foreshortening, high radial strength, excellent conformability to eccentric lesions, and good deliverability, and most have radiopaque markers on their ends to improve fluoroscopic visualization.

Elgiloy stents are more radiopaque but less conformable. They have the additional advantage of reconstrainable deployment, such that up to 80% of the stent can be deployed and then reconstrained, and the deployment location readjusted up to three times. This characteristic is useful because of the less predictable foreshortening of Elgiloy stents. There is a more limited selection of stents larger than 10 mm, which are often required for aortic use, and they all require sheaths larger than 6F. The S.M.A.R.T. CONTROL nitinol biliary stents (Cordis Corporation) come in diameters up to 14 mm and lengths up to 80 mm. The Elgiloy tracheobronchial Wallstent (Boston Scientific, Natick, Mass) is available in diameters up to 24 mm and lengths of 90 mm. The stainless steel Cook Z-Stent: Gianturco-Rosch Tracheobronchial Design (GTZS-40-5.0; Cook Medical, Inc., Bloomington, Ind) (Fig. 95-1 left) is available in diameters up to 40 mm. These larger sizes are useful in fashioning custom-made stent-grafts for some aortic applications. These stents also come as a two-stent complex composed of two 2.5 cm–long devices sutured together with nylon suture and with welded barbs for fixation.

Positive fixation describes the use of metal hooks, barbs, anchors, or supplemental staples that embed in the aortic wall. In endografts with stents that run the full length of the device, column stiffness helps hold the cranial end of the device in place by “standing” it on the iliac arteries or aortic bifurcation. This concept is supported by the finding that long iliac seal zones reduce infrarenal migration.⁸ The outward radial force of the stents themselves creates friction that retards migration. Some devices have polyester fuzz or other prosthetic material that induces a fibrotic reaction in the necks that helps hold a device in place. These methods are not exclusive, and several devices include a combination of these characteristics. There is some evidence that positive fixation provides high fixation force, which may lead to low rates of long-term migration.⁹

Infrarenal versus Suprarenal Fixation

Suprarenal fixation is an option in which the fixation component of a bare-metal stent is separated from the sealing component of the infrarenal neck portion of the stent-graft. The suprarenal aortic neck is more resistant to late neck dilatation, and long-term fixation may be improved.^10,11 Concern has been raised about late renal infarction, but data are insufficient to assess the relationship between late renal dysfunction and suprarenal bare-metal stents.¹² If open conversion and complete graft explantation should become necessary, suprarenal fixation may present more difficulties.