History of Endoluminal Treatment

The concept of endovascular therapy of atherosclerotic occlusive disease was introduced in the 1960s when Dotter performed the first transluminal angioplasty in a patient with ischemic extremities.¹ His novel technique, however, could not reliably maintain luminal patency and did not receive wide acceptance as a treatment of vascular occlusive lesions. Subsequent device modifications led to the development of an angioplasty balloon composed of latex material. The clinical success of this new construct was limited, in part because of its improved compliance, and could not fully dilate a calcified lesion. The introduction of an angioplasty balloon made of polyvinyl chloride in 1976 by Gruntzig and Hopff,² followed by rigid balloons composed of polyethylene and polyethylene terephthalate, marked a significant advance in endovascular therapy. The latter angioplasty balloons displayed a low compliance and high radial force in a fully inflated condition. As the technology and techniques of endovascular therapy continued to improve as well as become more widely accepted as outcomes data became available, transluminal balloon angioplasty evolved to an important modality in the treatment of peripheral vascular disease.

Despite numerous studies demonstrating the short-term clinical success of transluminal angioplasty, it has several limitations, in part because of the architectural variation of atherosclerotic lesions. Early restenosis or occlusion following angioplasty can occur because of either elastic recoil of the arterial wall or intimal dissection.^3,4 Vessels that are heavily calcified, completely occluded, or that contain ulcerated plaques might not be amenable to balloon angioplasty. Moreover, residual luminal irregularities following angioplasty occasionally trigger mural thrombus formation, leading to thrombotic occlusion.^5,6 Late restenosis following angioplasty can occur as a result of intimal hyperplasia or atherosclerotic progression.^7–9

In an effort to deal with these limitations of transluminal angioplasty, researchers proposed the concept of intravascular stents as a means of maintaining vessel patency against the restenotic process, as well as improving the clinical outcome of balloon angioplasty. Since Dotter first reported his successful deployment of intravascular stents in canine femoral and popliteal arteries in 1969,¹⁰ a variety of stent devices composed of various materials have been introduced.^11–14 Intravascular stenting is recognized as a potentially effective modality in overcoming elastic recoil of the arterial wall, stabilizing intimal dissection at the site of angioplasty, and maintaining the luminal patency of arteries with calcified and eccentric atherosclerotic plaques.

Classification of Aortoiliac Occlussive Disease

The 2007 Trans-Atlantic Inter-Society Consensus (TASC II) guidelines were developed to assist physicians in the prevention and management of peripheral arterial disease.¹⁵ The classification of inflow of aortoiliac lesions was updated in 2007 to reflect the advances in endovascular technology. General guidelines included in the TASC II document state that lesions classified as TASC A (Table 22-1) should be treated preferentially with endovascular techniques, whereas lesions classified as TASC D should be treated preferentially with open surgery. Lesions classified as TASC B or C have less clear recommendations. Good surgical judgment including surgical experience, available technology, and patient overall health needs to be considered when deciding on the appropriate initial treatment plan.

TABLE 22-1 TransAtlantic Inter-Society Consensus II: Morphologic Stratification of Iliac Lesions

General Principles of Endoluminal Stents

The desired characteristics of an intravascular stent depend on the anatomic placement and lesion characteristic for which it will be used. In general, the optimal stent needs to be encased in a low-profile device to cross high-grade lesions, and the stent should be deployed easily and accurately. Flexibility and durability are important characteristics that need to be combined with a low thrombogenic nature and high radial force. Moreover, the stent should have noticeable radiographic opacity to facilitate visualization. Finally, the ideal stent would be biocompatible, to promote endothelialization without causing intimal hyperplasia.

Intravascular stents can be separated into three distinct types: self-expanding stents, balloon-expandable stents, and covered stents (Table 22-2). Within each of these categories, there are numerous stents with different structural materials, deployment devices, and biocompatible characteristics.^{11,12,16–19} In addition, there are many stents undergoing clinical trials. However, the Wallstent (Schneider, Minneapolis, Minn.), the Palmaz stent (Cordis, Warren, N.J.), the Medtronic Vascular Complete SE Vascular Stent System (Medtronic Vascular, Santa Rosa, Calif.), the Express LD Iliac Premounted Stent System (Boston Scientific Corp, Natick, Mass.), the E-LUMINEXX Vascular Stent (Bard Peripheral Vascular, Tempe, Ariz.), the Zilver Vascular Stent (Cook, Bloomington, Ind.), the S.M.A.R.T. Nitinol Stent System (Cordis, Miami Lakes, Fla.), the Viabahn endoprosthesis (W.L. Gore, Flagstaff, Ariz.) have been approved by the U.S. Food and Drug Administration (FDA) specifically for iliac artery placement.

TABLE 22-2 Characteristics of Stents

Self-Expanding Stents

Self-expanding stents are generally manufactured so that the stents are constrained within a delivery sheath. The prosthesis is placed intravascularly through an introducer sheath that is advanced over a guidewire for deployment. Deployment is achieved by withdrawing the constraining sheath while keeping the stent in position, thus permitting the stent to self-expand and anchor to the vessel lumen (Figure 22-1). Stents in this category are generally noted for their ease of deployment and high degree of flexibility meandering around the curves typically found in an iliac artery. These characteristics are advantageous because they tend to be more forgiving if they are not placed exactly accurately. Compared with balloon-expandable stents, however, they may have lower hoop strength or radial force, which is the resistance to radial compression. These stents require oversizing by 2 to 3 mm to maintain outward radial force after deployment. Most delivery systems of self-expanding stents have a smaller diameter compared with those of balloon-expandable stents. Besides, the Wallstent and the Gianturco-Z stent (Cook, Bloomington, Ind.) are available.

FIGURE 22-1 A, Deployment of the Wallstent is achieved by withdrawing the constraining sheath. B, The fully deployed Wallstent.

Another type of self-expanding stent is the nitinol stent, which was first introduced by Dotter and colleagues in 1983.²⁰ Nitinol is a nickel-titanium alloy that has a unique temperature-associated memory property. The nitinol wires can be shaped into a coil spring configuration to serve as a stent when heated to 500°C. As it cools down to 0°C, the nitinol coil straightens into a linear alignment that can be constrained into a delivery catheter for stent deployment. The exposure to warm body temperature after deployment causes the nitinol stent to resume its original coil spring shape. Several clinical studies have evaluated the application of these stents in vascular occlusive lesions.^20–23 Common examples of nitinol stents include the Symphony stent (Boston Scientific Vascular, Watertown, Mass.), Cragg stent (Mintech, France, Elsenfeld, Germany), S.M.A.R.T. stent (Cordis, Miami, Fla.), and Memotherm stent (Bard/Angiomed, Karlsruhe, Germany).

Balloon-Expandable Stents

The balloon-expandable stent is first mounted and compressed onto an angioplasty balloon. The original stents were all operator mounted; newer stents are premounted by the manufacturer. Once the stent is positioned intraluminally, it is deployed by inflating the balloon to expand the stent (Figure 22-2). Unlike a self-expanding stent, the balloon-expandable stent can be expanded further by a larger angioplasty balloon catheter beyond its predetermined diameter. Most balloon-expandable stents are characterized by excellent hoop strength owing to intrinsic stent rigidity once deployed, which can be advantageous in treating stenotic vessels containing calcified plaques. The rigid property of such a stent, however, can create a technical dilemma when treating lesions in tortuous vessels. Moreover, it makes a contralateral approach for iliac artery stenting a challenge, although newer stents are more trackable and flexible. Commonly used examples of balloon-expandable stents include the Palmaz stent (Johnson and Johnson Interventional Systems, Warren, N.J.), Strecker stent (Boston Scientific Vascular, Watertown, Mass.), Gianturco-Roubin stent (Cook, Bloomington, Ind.), Ave stent (Arterial Vascular Engineering, Santa Rosa, Calif.), Absolute Stent (Medtronic, San Diego, Calif.), and Intrastent (Intratherapeutics, Minneapolis, Minn).

FIGURE 22-2 A, The Palmaz stent is crushed to a small caliber and mounted on an angioplasty balloon. Sizes most commonly used for iliac interventions are already premounted by the manufacturer. B, The Palmaz stent is deployed by inflating the angioplasty balloon.

Indications for Stent Placement

Indications approved by the FDA for iliac stent placement include stenotic or occlusive atherosclerotic lesions or failed or inadequate balloon angioplasty in the iliac artery (Box 22-1). The latter condition can be assessed by angiography, intravascular ultrasound (IVUS), or hemodynamic means. Angiographic or IVUS detection of residual stenosis of 30% or greater, unstable intimal flaps, and dissection along the subintimal or medial layers are all considered inadequate angioplasty results that warrant intraluminal stent placement. Furthermore, a transstenotic pressure gradient of 10 mm Hg or greater following angioplasty is considered an indication for iliac stent placement. Provocative testing using pharmacologic agents that stimulate vasodilatation may be necessary to identify a hemodynamically significant lesion. Up to 75% of patients without a translesion pressure gradient will have a significant gradient following injection of a vasodilator. Either 100 to 200 µg/mL of nitroglycerin or 30 to 60 mg of papaverine can be injected directly into the vessel in question. Immediate arterial dilatation is induced, thus mimicking exercise. A pressure drop of 10 mm Hg or more is considered indicative of a hemodynamically significant stenosis.

In addition to the FDA-approved indications for iliac artery stenting, certain other arterial conditions may be considered for stent placement. In the case of early restenosis following balloon angioplasty, stent placement is appropriate rather than repeated angioplasty alone. The 2-year patency rate of the iliac artery following angioplasty is between 65% and 81% and is affected by numerous factors, such as the degree of stenosis, the length of narrowing, and the distal vessel patency.^3,29,30 Restenosis of the iliac artery frequently occurs at the original angioplasty site,^7,9,11,31 which can be caused by neointimal hyperplasia or rapid atherosclerotic progression. Iliac artery stenting in such a condition can delay the restenotic process.

Primary stent placement, as opposed to stenting following inadequate angioplasty, has been used more frequently in the treatment of difficult iliac lesions. Nearly or totally occluded iliac arteries may be appropriate for primary stenting because angioplasty alone in these situations generally yields poor long-term patency rates.^14,32–36 Stenotic iliac vessels with ulcerative plaques can cause distal embolization when treated with balloon angioplasty.^6,37 It is appropriate to perform primary stent placement in such a condition to prevent plaque dislodgment. Moreover, primary stenting can be used as an adjunct procedure to improve iliac inflow when combined with a planned infrainguinal bypass operation.^38–40 A randomized controlled trial comparing primary stent placement versus primary angioplasty followed by selective stent placement found similar cumulative patency at 2 years (71% vs. 70%, respectively; p = 0.6).⁴¹ The authors concluded that because angioplasty followed by selective stent placement is less expensive than primary stent placement, the former seems to be the treatment of choice for intermittent claudication owing to iliac artery occlusive disease.

Contraindications to Stent Placement

Box 22-2 summarizes the contraindications for intravascular stent placement. Arterial perforation as a result of balloon angioplasty, as evidenced by contrast extravasation, is considered a contraindication for intraarterial stent placement. Stent placement in such a condition can lead to severe hemorrhagic complications or pseudoaneurysm formation. In these challenging situations, placement of a covered stent may be more appropriate.^42,43 Stent placement should also be avoided in aneurysmal arteries, because persistent blood flow around the stent can lead to further aneurysm expansion unless a covered stent is used. Because balloon-expandable stents, such as the Palmaz stent, are relatively rigid, they are less than ideal in tortuous vessels. Self-expanding stents, such as the Wallstent, have greater flexibility and are more appropriate in vessels with marked tortuosity. Severely calcified arteries often are not amenable to balloon angioplasty; similarly, stent placement should not be expected to restore the normal vessel lumen, because stent deployment requires the same basic techniques as balloon angioplasty. Long-standing arterial occlusion is considered a relative contraindication to stent placement. In this setting, the risk of plaque embolization resulting from stent expansion may outweigh the potential benefit of arterial recanalization. Finally, stent deployment should be avoided in arteries that might serve as either the proximal or distal site of a bypass grafting procedure. In a stented artery, application of vascular clamps might not only cause significant damage to the arterial wall, but also crush the stent so that reexpansion is impossible. If vascular control is needed in a stented artery, intraluminal occlusion balloon catheter should be used rather than vascular clamps.

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