Using Coiled Stents to Treat Arterial Occlusive Disease
Nitinol stent platforms, such as aSpire and Intracoil, may emerge as the most patient- and physician-friendly long-term solution for this troubling disorder.
A multitude of options exists for the treatment of arterial occlusive disease. Although exercise is often touted as an effective therapy for this patient population, the studies supporting this therapy enroll few patients with short follow-up. Only highly motivated individuals are able to comply with the rigors necessary for sustained clinical improvement. These poor results should not be surprising; studies in cardiac patients have shown less than impressive long-term commitment by the majority of patients as well.1
Pharmacologic agents, primarily cilostazol, have improved the walking distance in up to 50% of claudicants. However, full-dose therapy may not be tolerated in up to 20% of patients and an increase in walking distance of 50% is not sufficient for many patients.2
Surgical bypass is certainly effective in improving ambulation. Use of surgical therapy is usually reserved for limb-threatening ischemia due to the significant morbidity and mortality associated with bypass.3,4
Percutaneous therapy, such as angioplasty, offers low-risk, clinically effective therapy for many patients with focal arterial obstructions but stand-alone angioplasty of more complex disease may be associated with less optimal results. Acute vessel closure, residual luminal narrowing, and a significant need for repeat procedures may be seen.5,6
In an effort to improve the acute procedural results, as well as improve long-term patency, metallic stents are being utilized. Certainly in the iliac artery, the development of stenting has led to improved acute and long-term results. Whether this success is transferable to the femoral artery is a matter of significant ongoing controversy. The early data on stainless-steel stents revealed the tendency for balloon-expandable stents to crush, whereas the self-expanding, stainless-steel stents appear to have a significant restenosis rate.7-9 These early restenosis data should be evaluated in light of the poor study designs, significant stent oversizing, and a lack of focus on antiplatelet therapy that occurred. New self-expanding stent designs have focused on the use of a nickel titanium alloy (nitinol) with a temperature-dependant memory.
The first nitinol stent evaluated, and to date the only FDA-approved femoral artery stent, is the open-coil designed Intracoil (ev3 Inc., Plymouth, MN) (Figure 1).
A coil stent design allows for bending, as well as the shortening/elongation that may occur in the femoral artery with limb movement (Figure 2). The data for the “role in patients” showed excellent safety and promising efficacy.10 Although yet to be published, data on this randomized study comparing stenting to balloon angioplasty presented at an FDA panel review showed no statistical improvement in 9-month repeat revascularization (14.3% stent versus 16.1% balloon). However, more than 10% of the balloon angioplasty patients crossed over to stent and a significantly greater improvement in ankle-brachial index (.19 versus .08) and lower early complication rate (1.5% versus 8.4%, P<.01) were seen.11 Widespread utilization of this coiled stent has been limited by the short stent length available (&Mac178;60 mm), nonuniform release, and varied appearance when first deployed. A new deployment mechanism under development appears to significantly improve these current shortcomings. However, the current device appears to be the stent of choice for placement in proximity to a joint space where flexibility is required.
The next evolution in nitinol stent designs are based on tubular self-expanding stents. These designs have anecdotally improved stent delivery and may decrease restenosis. Although no large clinical trials randomizing between angioplasty and tubular nitinol stents has been completed, nonrandomized series appear to show significant restenosis reduction.12,13 Stent fracture, however, has become a concern recently. The long-term clinical significance of these fractures is not known.
In an effort to decrease restenosis, material-covered stents have been developed and are currently being clinically evaluated. These covered stents have the theoretical advantage of reducing tissue in-growth at the angioplasty site. The first of these covered stents included a full-length, expanded polytetrafluoroethylene (ePTFE) covering. Published data appear to show decreased restenosis in carefully selected patient populations with good runoff (2 vessel).13 The drawback to these stents will be the potential for increased limb-threatening ischemia if important collaterals are covered and thrombosis occurs.
The aSpire covered stent (Vascular Architects, Inc., San Jose, CA) is the most recent stent design to be developed. The stent was designed with several features in mind: flexibility for bending, including elongation/shortening; controlled release; and spiral design with partial ePTFE covering of the vessel wall that allows important side branch patency. This stent is fashioned by chemical etching a sheet of nitinol into a spiral-shaped stent. The aSpire stent is fully encased inside a sleeve of ePTFE covering. The stent is wound down by the physician onto a delivery catheter (100 cm), which is currently 8F compatible for femoral sizes (Figure 3). The delivery of the stent is completed by slowly unwinding the stent at the site of the stenosis. During the delivery process, winding and unwinding may be completed. This feature allows the stent to be rewrapped onto the delivery catheter for controlled positioning of the stent at the desired site of placement. Because the stent has an open spiral design, it may be adjusted during deployment to allow for patency of important side branch channels (Figure 4). The delivery system allows the aSpire stent to be delivered either from antegrade or contralateral femoral access. To complete the stenting procedure, multiple balloon dilations with a short balloon are completed. Both the European and American feasibility trials evaluating the aSpire stent in the femoral and iliac arteries have completed enrollment and 9-month follow-up will be completed in September 2003. A more specific trial (Valiant) evaluating this stent as an adjunctive procedure to balloon angioplasty for long superficial femoral and popliteal arterial occlusive disease is currently underway.
This stent graft platform also holds promise as an excellent drug-eluting platform. The ePTFE sleeve can act like a tea bag allowing for slow controlled release of the pharmacologic agent placed between the ePTFE layers. Coiled nitinol stents continue to hold promise as effective technology for the treatment of patients with femoropopliteal arterial occlusive disease.
Gary M. Ansel, MD, is Director of Peripheral Vascular Intervention at Midwest Research Foundation in Columbus, Ohio. He is on the advisory board of Vascular Architects. Dr. Ansel may be reached at 614-262-6772; firstname.lastname@example.org.
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