Different Stent Types

There are several different stent types and models. Stent may differ in the metallic composition of the struts (e.g. stainless steel, nitinol, cobalt-chromium), the geometric configuration of the struts (e.g. open cell vs. closed cell design), but most importantly, to the delivery system of the stent (e.g.

balloon-expandable vs. self-expanding). Finally, several small differences may also be observed according to the company manufacturing the stent (e.g. Cordis, Abbott, Bard, Cook, ev3, Medtronic/Invatec, etc.). Herewith, a short summary of the main differences of several stent models 4.1.4.1 Balloon-expandable stents: (Figure 24):

The slotted tube stainless steel Palmaz stent is the prototype for balloon-expandable stents.

These devices have several advantages in the aortoiliac circulation compared with self-expanding stents. Their high radial force makes them suitable for heavily calcified lesions. Minimal foreshortening at deployment and good visibility allow for precise placement in the treatment of ostial lesions. Finally, balloon-expandable stents may be further expanded (typically by 1-2mm) after initial deployment by using larger balloons until the desired diameter is achieved.

Currently, the vast majority of stents are pre-mounted on a balloon and stent flexibility has improved, allowing for stent delivery using the crossover technique. One of the potential disadvantages of these devices in the iliac circulation is their propensity to create edge dissections, in particular in heavily calcified vessels or if the stent diameter is oversized. Since balloon-expandable stents are not elastic, they may crush in the presence of extensive compressive forces. Therefore, these devices should not be placed in the common femoral artery and should also be avoided in the distal part of the EIA, where significant conformational changes may occur. Furthermore, several of currently available peripheral balloon-expandable stents are still made of stainless steel, thus causing significant artifacts (i.e., signal loss) on MRA.

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Figure 24 : balloon-expandable stent (Formula™ 414RX, Cook medical) [reproduced from the Cook Medical’s website]

4.1.4.2 Self-expanding stents: (Figure 25):

The most distinguishing features of self-expanding stents are their elasticity and flexibility.

These devices expand to their nominal diameter when released from a constrained state within the delivery system. Typically, a slightly oversized stent (i.e. 1-2mm) is chosen to allow for optimal vessel wall apposition. The flexibility facilitates stent delivery using the crossover technique and allows for excellent trackability and conformability in tortuous iliac vessels. After stent release, post-dilatation is usually performed to achieve good stent apposition to the vessel wall. Disadvantages of self-expanding stents include suboptimal radial strength and varying degrees of foreshortening at the time deployment. Therefore, this stent type is less suitable for the treatment of ostial lesions. Self-expanding stents should be considered for the treatment of non-ostial lesions of the CIA and all EIA lesions.

While the prototype of this stent class (Wallstent) is made of stainless steel, newer generation devices (e.g., SMART Cordis, Absolute Abbott) are composed of nitinol, an alloy of nickel and titanium. Compared with the stainless steel counterparts, nitinol stents allows for increased radial strength and minimal foreshortening at deployment. The radial strength of nitinol stents has further increased in third generation devices (e.g., Supera Stent [IDev Tech., Texas, USA]), allowing for optimal stent expansion also in severely calcified lesions. In addition, the superior conformability is important in vascular segments with abrupt changes in vessel diameter, as might be encountered at

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the transition from the common and external iliac arteries. A further advantage of nitinol over stainless steel stents is their magnetic resonance compatibility, achieved however at the cost of a reduced x-ray visibility.

To our knowledge, only one randomized head-to-head comparison between different self-expanding stents in the iliac circulation has been performed. Beteween 1998 and 2001, the CRISP (Cordis Randomized Iliac Stent Project) trial randomized a total of 203 patients with symptomatic iliac disease and suboptimal results following angioplasty to treatment using a nitinol stent (SMART) or a stainless steel stent (Wallstent) (137). Acute procedural success was significantly higher with the nitinol stent compared with the stainless steel stent (98% versus 87%, respectively), however, the primary vessel patency at 12 months was comparable (95% and 91%, respectively).

Figure 25 : self-expanding stent (Zilver PTX™, Cook Medical) [reproduced from the Cook Medical’s website]

4.1.4.3 Covered stents: (Figure 26-27):

Covered stents are composite devices consisting of a metallic skeleton covered with synthetic graft material (e.g. Dacron, ePTFE). Because of the bulky graft material, these devices require larger delivery systems. They are currently not FDA approved for iliac occlusive disease. Nevertheless, covered stents have been used in this vascular bed, mainly for the treatment of aneurysms but also for bailout of iatrogenic iliac ruptures or arteriovenous fistulas. Isolated aneurysm of the iliac arteries are relatively uncommon, accounting only for 2% to 7% of all intra-abdominal aneurysms (212).

The most frequently used covered stents in the iliac circulation include the self-expanding Wallgraft (Boston Scientific), the Viabahn (Gore) and the Fluency (Bard) stents, and the balloon-expandable iCAST (Atrium Medical, Hudson, NH, USA) stent. The self-expanding covered stents are

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more flexible than the iCAST stent, but require larger caliber access sheaths for delivery. This can be particularly problematic when treating iliac perforations, where the clinical situation may not permit time to change access sheaths.

The iCAST balloon-expandable stent consists of a 316L stainless steel stent that is encapsulated with ePTFE such that the stainless steel is not exposed to the luminal wall. Although this stent is very stiff, it has the advantage over self-expanding covered stents that stents up to 10mm in diameter may be delivered through a 7Fr sheath. This stent is being used increasingly for the treatment of occlusive iliac disease, and preliminary data suggests that restenosis rates with this stent may be lower than with uncovered balloon-expandable stents.

Figure 26: balloon-expandable covered-stent (iCAST™, Atrium) [reproduced from the Atrium Medical’s website]

Figure 27: self-expanding covered-stent (Viabahn™, Gore Medical) [reproduced from the Gore Medical’s website]

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