Stent Therapies in the Lower Extremity: Bare-Metal, Drug-Eluting, and Reabsorbable Stents

Kwan S. Lee, MBBCh

Ahmed Harhash, MBBCh

Tze-Woei Tan, MBBS

I. Introduction

A. History Percutaneous endovascular therapies for peripheral arterial disease have continued to evolve since Charles Dotter performed the first percutaneous transluminal angioplasty (PTA) of a superficial femoral artery (SFA) in 1964. In deciding optimal therapy from the myriad of devices and techniques currently available, the unique properties of specific arterial distributions must also be accounted for. Balloon angioplasty is overall effective and has the advantage of simplicity and cost-effectiveness, but results are limited by recoil, acute dissection, and significant rates of restenosis. There are numerous adjunctive therapies to angioplasty, including scoring balloon angioplasty, atherectomy in its many forms (rotational, excisional, laser), drug-coated balloons, and a variety of stents. Many permutations therefore exist for therapy and can be both confusing and complementary. Familiarity with the Inter-Society Consensus for the Management of Peripheral Arterial Disease (TASC) guidelines is important, as they categorize heterogeneous anatomical presentations of disease and guide suitability of surgical versus endovascular therapy and choice of endovascular intervention.¹
B. Technology Given the central and important role of stenting in endovascular intervention, it is important to have an appreciation of the technologies involved and the relative benefits and drawbacks of each. Stents were developed to improve outcomes over angioplasty. They improve vessel patency by acting as a scaffold against recoil and dissection. The primary structural challenge in peripheral vascular disease is compressional and torsional forces acting upon the stent in specific anatomic territories during daily motion, which may crush the stent or contribute to strut fractures. In addition to the challenges posed by daily external forces, stents induce proliferative reactions, contributing to long-term restenosis.
C. Chapter Outline This chapter aims to provide a framework and contextual understanding of stents by discussing the technical and clinical performance of different stents used in the lower extremity. The use of stents for other vascular territories including venous intervention will be covered in their individual chapters.

II. Bare-Metal Balloon-Expandable Stents

A. General Principles In lower extremity arterial stenting, bare-metal balloon-expandable stents are primarily used in the iliac territory, given the lower degree of stress and torsional movement to which the iliac artery is exposed. Balloon-expandable stents should not be used in areas where repeated flexing of the artery occurs. Balloon-expandable stents are constructed from stainless steel or cobalt chromium and in general have greater radial strength and radiopacity but poorer flexibility and trackability compared with self-expanding stents. Given the mechanism of deployment, they can be positioned with greater accuracy.
B. Common Iliac Artery The common iliac artery is a straight and immobile vessel, being fixed to the sacral promontory, while the external iliac artery has a much more tortuous course, stretching during hip extension. Open surgical procedures have excellent patency rates but are associated with significant morbidity and mortality. Several randomized studies comparing endovascular stenting with stand-alone angioplasty demonstrated stenting to be superior in both hemodynamic parameters and Rutherford classification. A meta-analysis of iliac intervention showed that stent placement reduced the risk of long-term failure by 39% when compared with PTA alone.² Table 13.1 lists all current FDA (U.S. Food and Drug Administration)-approved balloon-expandable stents.

III. Balloon-Expandable Covered Stents

A. General Principles Covered stents can be used to treat aneurysms, perforations, and arteriovenous fistulae in the aortoiliac region. Open surgery is the preferred approach for complex and severe iliac artery occlusive disease in low-risk patients. More recently, endovascular intervention has been performed with acceptable outcomes even in TASC C and D lesions. The use of covered stents has been proposed to reduce intimal hyperplasia and to improve long-term patency by reducing restenosis. The Atrium iCAST covered stent (Maquet Getinge Group, Rastatt, Germany) and the Viabahn VBX endoprosthesis (W.L. Gore & Associates, Flagstaff, AZ) are two commercially available balloon-expandable polytetrafluoroethylene (PTFE) covered stents. Although the iCAST stent is currently approved by the FDA to treat tracheobronchial strictures, it has been used “off-label” in aortoiliac, mesenteric, and renal arterial occlusive disease. The Viabahn VBX stent was recently approved by FDA as the first balloon-expandable stent graft for use in the iliac artery.
- 1. The iCAST covered stent is a 316L stainless steel balloon-expandable stent, which is encapsulated entirely with a thin PTFE film. The stent graft is premounted on a noncompliant balloon and is compatible with a 6 or 7 Fr sheath. It is available in 5 to 10 mm diameter and in 16, 22, 38, and 59 mm lengths. The device is available in 80 cm and 120 cm shaft lengths and is delivered on a 0.035-in wire platform. The stent can be postdilated to 4 mm larger if necessary after deployment, but this results in stent foreshortening. The iCAST stent is stiffer than a bare-metal stent (BMS) of comparable size, and care should be taken when it is advanced through severely stenotic or occlusive lesions owing to the risk of stent dislodgement.
- 2. The Viabahn VBX endoprosthesis is composed of a stainless steel stent fully covered in a fluoropolymer, which is coated with bioactive heparin surface. The stent is premounted on a 0.035-in guidewire-compatible delivery system and requires a 7 or 8 Fr sheath. It is available in 5-10 mm diameter and in 15, 19, 39, and 59 mm stent lengths. Similar to the iCAST stent, the Viabahn stent can be postdilated to a larger diameter if needed.

B. Comparison with Bare-Metal Stents There are a few retrospective series and only one randomized controlled trial comparing the outcomes of balloon-expandable covered versus bare-metal stents (BMSs) in the treatment of severe aortoiliac occlusive lesion. One retrospective review, consisting of 54 patients showed superior patency at 2 years with the use of covered balloon-expandable stents for aortic bifurcation occlusive disease.³ Others, however, showed comparable or worse outcomes of covered stents in severe iliac artery obstructive lesions.⁴^,⁵ In a retrospective series, which included 128 patients from Italy, early and midterm outcomes were comparable between covered and bare-metal balloon-expandable stents. The only benefit for covered stents was in TASC II D lesions with long lesions of the common and external iliac arteries.⁴ In another series, BMS was reported to have significantly better patency compared with covered balloon-expandable stents.⁵

Table 13.1. FDA-Approved Balloon-Expandable Bare-Metal Stents in Lower Extremity Peripheral Arterial Disease

Device Name	Manufacturer	Material Used	Guidewire Size/Endhole (inch)	Introducer Size (F)	Stent Diameter (mm)	Stent Length (mm)	Delivery System Length (cm)	FDA Approval
Omnilink elite	Abbott Vascular	Cobalt chromium	0.035	6, 7	6-10	12, 16, 19, 29, 39, 59	80, 135	Iliac
Express LD Iliac/Biliary Premounted	Boston Scientific	316L Stainless steel	0.035	6 (up to 8 × 37 mm), 7 (up to 10 × 57 mm)	6-10	17, 25, 27, 37, 57	75, 135	Iliac and biliary
Palmaz Iliac	Cordis	316L Stainless steel	0.035	10	8-12	30	NA	Iliac
Palmaz Iliac and Renal	Cordis	316L Stainless steel	0.035	6, 7	4-8	10, 15, 20, 29	NA	Iliac and renal
Assurant Cobalt Iliac	Medtronic	Cobalt chromium alloy	0.035	6	6, 7, 8, 9, 10	20, 30, 40, 60	80, 130	Iliac
Visi-Pro	Medtronic	316L Stainless steel	0.035	6 (5-8 mm), 7 (9-10 mm)	5, 6, 7, 8, 9, 10	12 (5-7 mm diameters), 17, 27, 37, 57	80, 135	Iliac and biliary
FDA, U.S. Food and Drug Administration.

1. Covered Versus Balloon-Expandable Stent Trial (COBEST) The Covered Versus Balloon-Expandable Stent Trial (COBEST) enrolled 125 patients at 8 major Australian centers to determine whether covered stents were superior to BMS in the treatment of aortoiliac occlusive disease with both 1-year⁶ and subsequent 5-year outcomes reported.⁷ Overall, stent patency was found to be similar with covered stents and BMS. In subgroup analysis, the patency rate for covered stents was significantly higher than BMS at 18 months, and this persisted up to 60 months in TASC C and D lesions.⁶^,⁷ Although patients who received covered stents received fewer revascularization procedures in the trial, the choice of covered stent versus BMS did not affect the rate of limb amputation.⁷ A recent meta-analysis that included 255 diseased arteries in 182 patients showed no significant improvement in primary patency with covered stents.⁸

C. Summary In summary, the results of balloon-expandable covered stents and BMS are similar in aortoiliac occlusive disease. Current literature does not support the routine use of balloon-expandable covered-stent grafts, although there is some evidence to suggest improved patency for more advanced TASC C and D lesions. They remain an important therapeutic option, however, for bailout of perforations or for complex lesions at risk of rupture during intervention.

IV. Self-Expanding Stents

A. Wallstent
- 1. Self-expanding Wallstents (Boston Scientific, Marlborough, MA) were first deployed in the SFA in the late 1990s but were quickly shown to be inferior to nitinol self-expanding stents with a relatively high rate of strut fracture.⁹ Wallstents are made of Elgiloy, a “superalloy” combining cobalt, chromium, nickel, molybdenum, manganese, and a relatively small amount of iron. It is therefore nonferromagnetic and magnetic resonance imaging (MRI) compatible. The platinum core renders this stent radiopaque. It has a braided, tubular woven-mesh design, which imparts flexibility and an outward self-expanding force to the stent. These unique design characteristics impart an ability to recapture the stent even when 87% deployed, allowing it to be repositioned during deployment. The woven-mesh configuration of the Wallstent causes it to adapt its diameter to the width of the vessel lumen compared with nitinol stents, which expand to a predetermined diameter. The length of a deployed Wallstent is more variable, being dependent on the vessel diameter, therefore susceptible to foreshortening or the opposite.
B. Nitinol Self-Expanding Stents
- 1. The introduction of nitinol self-expanding stents changed the treatment of femoropopliteal endovascular disease, moving stent implantation from primarily a bailout procedure after failed balloon angioplasty to a reasonable initial approach. Nitinol is a metal alloy of nickel and titanium, which exhibits two unique and closely related properties: shape memory effect and superelasticity. Shape memory is the ability of nitinol to undergo deformation at one temperature and then recover its original shape at a higher temperature. Nitinol stents have improved radial strength and reduced foreshortening and crush resistance owing to the above unique properties, making them well suited for use in the SFA, the longest artery in the human body, subject to flexion, extension, lateral compression, and torsional forces during daily activity.
- 2. Nitinol self-expanding stents have not been shown to be superior over angioplasty for short, focal lesions. The FAST (Femoral Artery Stenting Trial) randomized 244 patients with lesions between 1 and 10 cm to angioplasty versus primary stent implantation. Despite higher initial technical success, no differences were seen at 12-month follow-up in ultrasound-assessed binary restenosis rates, target lesion revascularization, or Rutherford category.¹⁰
- 3. For lesions of intermediate length, however, self-expanding nitinol BMSs have been shown to be superior to balloon angioplasty. An important early trial, the Vienna Absolute study, randomized 104 patients to primary nitinol stenting versus balloon angioplasty with optional secondary stenting. Mean lesion length was 13.2 cm in the stent group, and secondary stenting was performed in the angioplasty group in 32% of patients. The restenosis rate and treadmill walk distance were significantly superior in the primary stented group at 6- and 12-month follow-up.¹¹ Subsequent studies have supported this initial finding.¹²^,¹³
C. Summary In routine clinical practice, many patients have much longer SFA lesions than those studied in randomized trials, which are associated with high restenosis rates.¹⁴ Restenosis is difficult to treat and associated with worse clinical outcome. For this reason, self-expanding nitinol BMSs are currently mainly used in conjunction with plain balloon angioplasty or drug-coated balloon angioplasty to spot stent segments with unsatisfactory angioplasty results in external iliac, superficial femoral, or popliteal arteries. All self-expanding, FDA-approved stents for the lower extremities are listed in Table 13.2.

V. Self-Expanding Covered Stents

A. General Principles Despite continued advances in SFA endovascular intervention, TASC II C and D lesions remain a technical challenge with poor primary patency rates due to in-stent restenosis and stent fracture. The length and complexity of an SFA lesion are the most important factors linked to such failure. Theoretically, a stent graft in the SFA with continuous exclusion of the vessel wall from luminal flow may prevent neointimal tissue growth, reducing the risk of in-stent restenosis and thrombosis commonly seen with BMSs in addition to providing protection from distal embolization during deployment. One potential drawback of a stent graft, however, is the possible exclusion of important collateral branches, which may worsen pretreatment symptoms or lead to limb ischemia in the event of a graft thrombosis. This property of exclusion is used specifically, however, in the treatment of popliteal artery aneurysms.

Table 13.2. FDA-Approved Self-Expandable Bare-Metal Stents in Lower Extremity Peripheral Arterial Disease

Device Name	Manufacturer	Material Used	Guidewire Size/Endhole (inch)	Introducer Size (F)	Stent Diameter (mm)	Stent Length (mm)	Delivery System Length (cm)	FDA Approval
Absolute Pro	Abbott Vascular	Nitinol	0.035	6	6, 7, 8, 9, 10	20, 30, 40, 60, 80, 100	80, 135	Iliac
Supera	Abbott Vascular	Nitinol	0.018	6	4.5, 5, 5.5, 6, 6.5	20, 30, 40, 60, 80, 100, 120, 150	120	SFA and proximal popliteal
E-Luminexx Vascular and Biliary	Bard Peripheral Vascular	Nitinol	0.035	6	7, 8, 9, 10	20, 30, 40, 60, 80, 100	80, 135	Iliac and biliary
LifeStar Vascular and Biliary	Bard Peripheral Vascular	Nitinol	0.035	6	7, 8, 9, 10	20, 30, 40, 60, 80, 100	80, 135	Iliac and biliary
LifeStent Solo Vascular	Bard Peripheral Vascular	Nitinol	0.035	6	6, 7	200	100, 135	SFA and full popliteal
LifeStent and LifeStent XL Vascular	Bard Peripheral Vascular	Nitinol	0.035	6	5, 6, 7	20, 30, 40, 60, 80, 100, 120, 150, 170	80, 130	SFA and full popliteal
Astron	Biotronik (distribued by Getinge/Macquet)	Nitinol	0.035	6	7, 8, 9, 10	30, 40, 60, 80	72, 130	Iliac
Epic	Boston Scientific	Nitinol	0.035	6	6, 7, 8, 9, 10, 12	20, 30, 40, 50, 60, 70, 80, 100, 120	75, 120	Iliac
Innova	Boston Scientific	Nitinol	0.035	6	5, 6, 7, 8	20, 40, 60, 80, 100, 120, 150, 200	75, 130	SFA
Wallstent Endoprosthesis	Boston Scientific	Elgiloy	0.035	6	6, 7, 8, 9, 10	18, 20, 23,24, 34, 35, 36, 38, 39, 46, 47, 49, 52, 55, 59, 61, 66, 67, 69	75, 135	Iliac
Zilver 518	Cook Medical	Nitinol	0.018	5	6, 7, 8, 9, 10	20, 30, 40, 60, 80	125	Iliac
Zilver 635	Cook Medical	Nitinol	0.035	6	6, 7, 8, 9, 10	20, 30, 40, 60, 80	80, 125	Iliac
Smart Control Iliac	Cordis	Nitinol	0.035	6	9, 10	20, 30, 40, 60	80, 120	Iliac
Smart Control Vascular	Cordis	Nitinol	0.035	6	6, 7, 8	20, 30, 40, 60, 80, 100	80, 120	Iliac and SFA
Smart Vascular	Cordis	Nitinol	0.035	6	6, 7, 8	120, 150	120	SFA
Gore Tigris	Gore	Nitinol/ePTFE	0.035	6	5, 6, 7	40, 60, 80, 100	120	SFA and proximal popliteal up to 240 mm in length
Complete SE	Medtronic	Nitinol	0.035	6	5, 6, 7, 8, 9, 10	20, 40, 60, 80, 100, 120, 150	80, 130	Iliac, SFA, and proximal popliteal
EverFlex—SFA and PPA	Medtronic	Nitinol	0.035	6	6, 7, 8	20, 30, 40, 60, 80, 100, 120, 150, 200	80, 120	SFA and proximal popliteal
EverFlex—Iliac	Medtronic	Nitinol	0.035	6	6, 7, 8	20, 30, 40, 60, 80, 100, 120	80, 120	Iliac
Protege GPS	Medtronic	Nitinol	0.035	6	9, 10, 12	20, 30, 40, 60, 80	80, 120	Iliac
Misago RX	Terumo	Nitinol	0.035	6	6, 7, 8	40, 60, 80, 100, 120, 150	135	SFA and proximal popliteal
FDA, U.S. Food and Drug Administration; PPA, proximal popliteal artery; SFA, superficial femoral artery.