Balloon-Expandable Covered Stent Therapy of Complex Endovascular Pathology
Article Outline
The current study was designed to investigate our hypotheses that balloon-expandable covered stents display acceptable function over longitudinal follow-up in patients with complex vascular pathology and provide a suitable alternative for the treatment of recurrent in-stent restenosis. All stents were Atrium iCast, which is a balloon-mounted, polytetrafluoroethylene-covered stent with a 6F/7F delivery system. A retrospective review was performed of 49 patients with 66 stented lesions. Data were analyzed with life tables and t-tests. The most commonly treated vessels were the iliac (61%) and renal (24%) arteries. Indications for covered stent placement were unstable atheromatous lesions (50%), recurrent in-stent restenosis (24%), aneurysm (8%), aortic bifurcation reconstruction (7.5%), dissection (4.5%), endovascular aneurysm repair-related (4.5%), and stent fracture (1.5%). Patency was assessed by angiogram or duplex ultrasonography. The primary end point was patency and secondary end points were technical success and access-site complications. Mean follow-up was 13 months (range 1.5-25). The technical success rate was 97%. Unsuccessful outcomes were due to deployment error (n
=1) and stent malpositioning (n=1). The cohort (n=64) 6- and 12-month primary patency rates were 96% and 84%, respectively. Twelve-month assisted primary patency was 98%. Iliac artery stents (n=38) had a primary patency of 97% at 6 months and 84% at 12 months with an assisted primary patency of 100% at 12 months. Renal artery stents (n=16) had a primary patency of 92% at 6 months and 72% at 12 months with an assisted primary patency of 92% at 6 and 12 months. Stents placed for recurrent in-stent restenosis (n=16) had a primary patency of 85%, assisted primary patency of 93%, and a 15% restenosis rate at 12 months. Specifically, stents placed for renal artery recurrent in-stent restenosis (n=10) had a primary patency of 73%, assisted primary patency of 82%, and a restenosis rate of 27%. The restenosis rate included two renal artery occlusions in patients noncompliant with clopidogrel use and resulted in ipsilateral kidney loss in both patients. In-stent peak systolic velocities decreased significantly (p<0.05) from preoperation to 12 months in iliac stents and to 18 months in renal stents. Ankle-brachial index increased significantly in iliac stents from preoperation (0.62±0.18) to 18 months (0.86±0.16). Successful exclusion of atheromatous lesions and aneurysm/dissection/endoleak was 100%. Access-site complications occurred in 6%: pseudoaneurysm (n=2), dissection (n=1), and bleeding (n=1). Balloon-expandable covered stents have an acceptable primary patency with an excellent assisted patency after salvage angioplasty. The clinical utility of this technology is broad for the treatment of aneurysms, extravasation, unstable atheromatous lesions, and recurrent in-stent restenosis.
Introduction
Endovascular techniques have assumed the forefront of vascular surgery. Early contributions of Dotter and Gruentzig for angioplasty and Palmaz for balloon-expandable stents paved the way for endoluminal intervention. Once felt to be an alternative to open surgery for high-risk patients, endovascular therapy now serves as a minimally invasive, first-line treatment for many vascular conditions. Stents have proven to be most effective in rescuing angioplasty failures such as vessel recoil and dissection.1, 2, 3 Unfortunately, this explosion of endovascular technologies has resulted in a host of new problems, including endoleak and in-stent restenosis. In-stent restenosis is believed to be caused by neoimtimal hyperplasia that extends through the stent struts. Restenosis rates vary by anatomic bed, vessel size, and patient comorbidities.
Covered stents consist of a metallic stent skeleton covered with synthetic graft material and have primarily been used in the coronary arteries and nonvascular structures such as the biliary and tracheobronchial trees.4 Though first envisioned by Dotter in 1969, recent usage in the peripheral vascular system has shown promise for the exclusion of aneurysm-related pathology, arteriovenous fistulas, and vessel extravasations.5 Covered stents may also be beneficial when dealing with in-stent restenosis by providing a mechanical barrier to neointimal hyperplasia. Previous limitations included larger delivery systems due to the bulk of additional graft material and lack of precision due to foreshortening, inherent to the self-expanding design. Balloon-expandable stents offer high radial and hoop strengths and display minimal foreshortening. The iCast (Atrium Medical, Hudson, NH) is the only commercially available balloon-mounted covered stent in the United States. The aim of this study was to determine the function of the balloon-expandable covered stent in patients with complex endovascular pathology, including unstable atheromatous lesions, aneurysms, dissections, extravasations, and recurrent in-stent restenosis.
Materials and Methods
Approval for this study was obtained from the institutional review board of the University of Tennessee Chattanooga at Erlanger Health System. Patients were identified using the prospectively collected vascular surgery database at Erlanger Medical Center. Demographic and procedural information was collected by retrospectively reviewing both hospital and office medical records. From February 2005 to August 2007, 60 patients received a total of 80 iCast stents. For the purpose of this study, only arterial stents were included; therefore, exclusion criteria consisted of inadequate follow-up (no postoperative office or hospital visits) and venous stents. Two patients had inadequate follow-up and six patients had venous stents placed, for a total of 10 excluded stents. Three deaths, unrelated to stent placement, occurred in this study population: one respiratory failure, one cerebrovascular accident, and one sepsis from a mycotic aneurysm. Therefore, 49 patients (37 male, mean age 66 years, range 45-85) with 66 stented lesions remained in the primary study group. Patency calculations include only 64 stents due to two stent placement failures, resulting in one removal and one immediate bypass. The patients suffered from the typical vascular comorbidities found in Table I.
Table I. Demographics (n=66)
| Variable | Number | Percent |
|---|---|---|
| Male | 37 | 56 |
| Hypertension | 58 | 88 |
| Cigarette use | 54 | 82 |
| Coronary artery disease | 33 | 50 |
| Diabetes mellitus | 29 | 44 |
| Cerebrovascular disease | 19 | 29 |
| Renal insufficiency | 11 | 17 |
The Statistical Package for the Social Science (SPSS, Inc., Chicago, IL) was used to create life tables for patency analysis. Student's t-test was used for comparison of ankle-brachial indices (ABIs) and peak systolic velocities (PSVs). Statistical significance was defined at p<0.05. Standard error measurements for life tables are provided in “Results.”
The primary outcome measure was patency. Secondary outcome measures included technical success and access-site complications. Patency was assessed by review of either follow-up duplex ultrasound or repeat angiography. Duplex ultrasound was performed by Vascular Diagnostic Services (Chattanooga, TN), an Intersocietal Commission for the Accreditation of Vascular Laboratories–accredited laboratory. Ultrasounds were performed per protocol at 6 and 12 weeks, followed by every 6 months for 2 years and yearly thereafter. The criteria for stenosis reported in Table II are used for stent surveillance; however, they represent values established for native vessels. These criteria are used at our institution secondary to a lack of established values specifically for stents. All repeat angiograms were performed and interpreted by board-certified vascular surgeons at the University of Tennessee Chattanooga. Primary patency was defined as a patent stent without any reintervention. Assisted primary patency was defined as a patent stent despite endovascular reintervention. Secondary patency was defined as a patent stent after reintervention for occlusion. Patency ended with an untreatable occlusion. Technical success required proper deployment and placement of the stent. Access-site complications included pseudoaneurysm, dissection, and bleeding.
Table II. Ultrasound criteria for stenosis
| Vessel | Velocity (m/sec) |
|---|---|
| Peripheral | |
| Normal | <1.5 |
| 30-49% | 1.5-2 |
| 50-79% | 2-4 |
| ≥79% | >4 |
| Renal | |
| Abnormal | >2 with PST |
| Superior mesenteric artery | |
| Abnormal | >2.75 with PST |
The indications for covered stent placement were unstable atheromatous lesions (50%), recurrent in-stent restenosis (24%), aneurysm (7.5%), aortic bifurcation reconstruction (7.5%), iatrogenic dissection following angioplasty (4.5%); endovascular aneurysm repair–related (4.5%), and stent fracture (1.5%). Target arteries included common iliac (59.1%), renal (24.3%), abdominal aorta (6.1%), innominate (3%), external iliac (1.5%), subclavian (1.5%), superior mesenteric (1.5%), common femoral (1.5%), and tibial-peroneal trunk (1.5%). Target arteries were analyzed by group: iliac, renal, abdominal aorta, infrainguinal, and other (Table III). All eligible patients were placed on aspirin indefinitely and clopidogrel for 6 weeks postprocedure.
Table III. Target vessels and indications for placement
| Iliac | Renal | Abdominal aorta | Infrainguinal | Othera | |
|---|---|---|---|---|---|
| Indication | (n=40, 61%) | (n=16, 24%) | (n=4, 6%) | (n=2, 3%) | (n=4, 6%) |
| Unstable atheromatous lesion (n=33, 50%) | 23 | 2 | 4 | 1 | 3 |
| In-stent restenosis (n=16, 24%) | 4 | 10 | 0 | 1 | 1 |
| Aneurysm (n=5, 8%) | 3 | 2 | 0 | 0 | 0 |
| Aortic bifurcation reconstruction (n=5, 7.5%) | 5 | 0 | 0 | 0 | 0 |
| Dissection (n=3, 4.5%) | 3 | 0 | 0 | 0 | 0 |
| Endovascular repair-relatedb (n=3, 4.5%) | 1 | 2 | 0 | 0 | 0 |
| Stent fracture (n=1, 1.5%) | 1 | 0 | 0 | 0 | 0 |
aInnominate (n |
bPartial occlusion from aortic endograft (n |
The iCast is a balloon-expandable, Advanta polytetrafluoroethylene (PTFE) –covered stent with a 6F/7F delivery system.6 Advanta is an ultrathin, microporous PTFE coating designed for biocompatibility and uniform deployment.6 Film-cast encapsulation technology embeds the stainless steel struts in PTFE to prevent contact with the luminal wall and provide uniform expansion in an attempt to minimize tissue prolapse within the stent.6 The iCast also features a one-step, balloon-mounted deployment system, allowing minimal foreshortening and precision deployment.6
Results
The technical success rate of stent placement was 97% (64/66). Failure 1 occurred in a patient with high-grade stenoses of both common iliac arteries in which bilateral, self-expanding, bare metal stents had been placed. The proximal portion of the left common iliac stent fractured during an attempt to traverse the aortic bifurcation with a balloon. A decision was made to recreate the bifurcation with covered stents and exclude the fracture; however, as the left iCast stent exited the sheath, the previously fractured stent migrated over the wire into the aorta. Removal of the left iCast stent was attempted in order to retrieve the fractured stent; however, the iCast migrated off the balloon upon reentrance into the sheath. The iCast stent was removed via arteriotomy. Failure 2 occurred in a patient with aortoiliac stenoses. iCast stents were placed in the abdominal aorta and bilateral common iliac arteries; however, the left common iliac stent was malpositioned behind the aortic stent. A right to left femoral-to-femoral artery bypass was then performed secondary to flow limitation within the left common iliac artery. Successful exclusion of aneurysms, dissections, and endoleaks was 100% (9/9).
Mean follow-up was 13 months (range 1.5-25). Two stents (3%) were followed up for 24 months or longer, 30 (47%) for 12-23 months, 14 (22%) for 6-11 months, and 18 (28%) for less than 6 months.
Primary patency rates were 96% at 6 months, 84% at 12 months, and 77% at 18 months for the entire cohort (Fig. 1). Assisted primary patency rates were 98% at 12 months and 95% at 18 months for the entire cohort (Fig. 1). Standard error did not exceed 10% to 24 months for primary or assisted primary patency. The assisted primary and secondary patency rates were the same due to two unrevascularizable occlusions. Iliac artery stents (n=40) had a primary patency of 97% at 6 months, 84% at 12 months, and 77% at 18 months with an assisted primary patency of 100% at 18 months (Fig. 2). Standard error exceeded 10% after 20 months for primary patency and was less than 10% to 24 months for assisted primary patency. The average diameter stent placed in the iliac arteries was 8mm (range 7-10). Renal artery stents (n=16) had a primary patency of 92% at 6 months, 72% at 12 months, and 60% at 18 months with an assisted primary patency of 92% at 6 and 12 months and 79% at 18 months (Fig. 3). Standard error exceeded 10% after 6 months for primary patency and after 12 months for assisted primary patency. The renal arteries had an average vessel diameter of 5mm and the average diameter stent placed was 6mm (range 5-7).
Fig. 1
Life-table patency cohort.
Fig. 2
Life-table patency iliac.
Fig. 3
Life-table patency renal.
Recurrent in-stent restenosis includes restenosis of a previously placed stent after at least one revascularization. Stents placed for recurrent in-stent restenosis (n=16) had a primary patency of 93% at 6 months and 85% at 12 months with an assisted primary patency of 93% at 12 months (Fig. 4). Standard error exceeded 10% after 12 months for primary and assisted primary patency. The restenosis rate was 15% at 12 months. Recurrent in-stent restenosis involved the renal arteries in 63% (10/16) and revealed a primary patency of 73% and assisted primary patency of 82% at 12 months. Two occlusions occurred in renal stents placed for recurrrent in-stent restenosis at 1.5 months and 5 months, both resulting in functional loss of the ipsilateral kidney. One revascularization was performed due to recurrent in-stent restenosis at 8 months. No occlusions occurred in any other target vessel.
Fig. 4
Life-table patency recurrent in-stent restenosis.
Mean PSVs for iliac and renal arteries as well as ABIs can be found in Table IV. Mean PSVs of iliac vessels decreased significantly from 3.52±1.31m/sec preoperatively to 1.73±0.61m/sec at 6 weeks. Significantly lower velocities were maintained to 12 months with a value of 2.27±0.81m/sec. Mean ABIs in the ipsilateral extremity of iliac stents improved significantly from 0.62±0.18 preoperatively to 0.83±0.26 at 6 weeks. Significantly improved ABIs were maintained to 18 months with a value of 0.86±0.16. Mean PSVs for renal arteries decreased significantly from 3.44±1.73m/sec preoperatively to 1.57±0.04m/sec at 6 weeks. Significantly lower velocities were maintained to 18 months with a value of 1.37±0.28m/sec.
Table IV. PSV and ABI
| Iliac | Renal | |||||
|---|---|---|---|---|---|---|
| velocity (m/sec) | p | ABI | p | velocity (m/sec) | p | |
| Preoperation | 3.52±1.31 | 0.62±0.18 | 3.44±1.73 | |||
| 6 weeks | 1.73±0.61∗ | 0.0002 | 0.83±0.26∗ | 0.0045 | 1.57±0.044∗ | 0.0028 |
| 3 months | 2.21±0.54∗ | 0.0046 | 0.92±0.16∗ | 0.0001 | 1.49±0.73∗ | 0.007 |
| 6 months | 2.13±0.92∗ | 0.0083 | 0.84±0.22∗ | 0.0014 | 1.63±1.16∗ | 0.01 |
| 12 months | 2.27±0.81∗ | 0.017 | 0.84±0.21∗ | 0.0016 | 1.32±0.4∗ | 0.007 |
| 18 months | 2.11±1.24 | 0.1 | 0.86±0.16∗ | 0.0006 | 1.37±0.28∗ | 0.04 |
| 24 months | 1.61±0.81 | 0.1 | 0.75±0.1 | 0.096 |
Access-site complications occurred in 6% (4/66). Two pseudoaneurysms were identified, with one requiring operative fixation and one receiving thrombin injection. One dissection occurred, which resulted in an intimal flap and thrombosis of the common femoral and superficial femoral arteries. This required open thromboendarterectomy and repair with a saphenous vein patch. One major bleeding event occurred in the brachial artery that required cutdown and primary repair. No further morbidity related to access-site complications was noted at follow-up.
Discussion
This study evaluated the efficacy and clinical utility of the iCast balloon-expandable covered stent. Technical success was high (97%), with only two failures, both of which were iatrogenic and likely not due to the stent itself.
The primary patency of 77% and assisted primary patency of 95% at 18 months for the entire cohort show this stent to be durable in a variety of peripheral and visceral vessels. Subcategorizing into iliac and renal vessels allowed for analysis of stent performance and durability in different anatomic beds. Iliac arteries have high elastin content within their media, which makes them elastic and thus potentially less likely to undergo restenosis.7 The reported average 12-month patency for iliac artery stents is 90% (78-97%).8 Our primary patency of 84% at 12 months was lower than literature averages, which is likely due to the complex pathologic indications for placement. Twenty-three stents were placed for unstable atheromatous lesions, which included critical stenoses felt to be high risk for emoblization or coral reef in nature (n=15), lesions persisting after subintimal angioplasty (n=5), and penetrating ulcers (n=3). Four stents were placed within another stent for in-stent stenosis. The assisted patency of 100% at 18 months, however, displays the salvageable characteristics of this stent. PSVs decreased significantly (p<0.05) from preoperative values to 12 months, and ABIs increased significantly from preoperative to 18 months. The reintervention rate for iliac vessels was 15% (6/40) overall, with no occlusions.
Renal artery stenosis has been studied extensively. The Dutch Renal Artery Stenosis Intervention Cooperative (DRASTIC) trial revealed a 48% restenosis rate for percutaneous transluminal renal artery angioplasty.9 Van de Ven showed that the restenosis rate for ostial lesions could be decreased significantly to 14% with percutanous transluminal renal artery stenting.10 This decrease resulted in percutaneous intervention becoming the treatment of choice for renal artery stenosis. Numerous studies have subsequently been performed yielding a renal artery restenosis rate of 15-25%.7 Bates et al.11 described predictive factors for the need for target vessel revascularization following renal artery stenting, which included age ≤67 years, stent diameter ≤5mm, solitary functioning kidney, history of lower extremity peripheral artery disease, and antecedent history of stroke. Vessels with a diameter <4mm have a higher rate of restenosis, which is as high as 40% in some studies.7, 12
The majority of renal artery stents in our series were placed for recurrent in-stent restenosis (10/16). These patients had PTRAS with subsequent recurrence of symptoms and stenosis. The stenosis was treated with angioplasty; however, recurrent in-stent restenosis led to placement of the balloon-mounted covered stent. Our primary patency in renal artery stents of 75%, assisted primary patency of 92%, and 25% restenosis rate at 12 months compare favorably with reported average rates. The literature average, however, corresponds to the stenting of native vessels; and 63% (10/16) of our stents were placed for recurrent in-stent restenosis. Our native vessel restenosis rate was 0% (0/5). We attempted to collect blood pressure and renal function changes for these patients; however, due to the retrospective nature of this study, insufficient data were found for meaningful analysis. Zeller et al.13 reported a series of 33 patients with at least a second reoccurrence of in-stent restenosis. Despite small group numbers, this study supported the use of covered (six patients, 17% restenosis) and drug-eluting (10 patients, 0% restenosis) stents in order to halt the excessive neointimal hyperplasia likely present in these patients. The recurrent restenosis rate of 27% (3/11) in our study is high; however, this includes two unrevascularizable occlusions, occurring at 1.5 and 5 months. Both occlusions occurred in patients noncompliant with clopidogrel use and considered high risk for restenosis with vessel sizes ≤4mm. Surveillance yielded significantly lower postoperative PSVs in renal arteries 18 months. The two high-risk occlusions likely skewed our recurrent in-stent restenosis data in that only one of the remaining 14 total patients developed a stenosis within the iCast stent.
Stent diameter differs among anatomic beds. The average renal stent diameter was 6mm, and the average iliac stent diameter was 8mm. The higher patency rate for iliac stents would suggest that the larger diameter stents are less likely to undergo stenosis. However, the two renal occlusions previously mentioned combined with small renal numbers make these data misleading. Stents placed in the tibioperoneal trunk (5mm) and superior mesenteric (6mm), subclavian (6mm), and innominate (7mm) arteries had diameters consistent with those placed in the renal arteries, yet none had evidence of stenosis. Although the numbers are too small to make definitive conclusions, this would suggest that smaller stent diameters combined with the inability to anticoagulate places the patient at higher risk of stenosis and occlusion.
Balloon-expandable stents display minimal foreshortening, which provides the precision needed for ostial lesions and lesions in close proximity to branching vessels. The stainless steel struts are more rigid and provide higher radial and hoop strengths than self-expanding stents. Although this rigidity provides resistance against elastic recoil, it also makes the stent susceptible to deformity from external pressure, thus limiting the number of available target vessels. Balloon-expandable stents are not suitable for placement in vessels traversing or in close proximity to a joint which could cause kinking and eventual failure. Three stents were placed in areas considered to be higher risk for potential deformity (one common femoral, one subclavian, one tibioperoneal trunk); however, all maintained patency without reintervention.
Our 6% rate of access-site complications is within the reported rates of 1-6% with percutaneous interventions.14 However, in this regard, our study has smaller numbers than many other reported series. The smaller delivery system potentially improves safety and allows for the use of vascular closure devices which could shorten operative time.
Conclusion
This study demonstrates the safety, precision, and efficacy of the iCast balloon-expandable covered stent. Intermediate follow-up suggests that patency is sustained with longitudinal surveillance and failures exhibit excellent salvage with endovascular treatment. The data will need to be reanalyzed in the future to determine long-term durability. The clinical utility of this technology is broad for the treatment of aneurysm, extravasation, unstable intimal lesions, and recurrent hyperplasia.
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PII: S0890-5096(08)00339-7
doi:10.1016/j.avsg.2008.09.001
© 2008 Annals of Vascular Surgery Inc. Published by Elsevier Inc All rights reserved.
