Applied Radiology

Over the past 20 years, percutaneous transluminal angioplasty (PTA) hasevolved into a widely used, reliable, minimally invasive method to treat iliacartery occlusive disease. Primary technical success rates have been reported tobe 90 to 98%, with 5-year patency rates of approximately 70%. In a definedpercentage of patients, suboptimal results immediately following PTA lead toearly failures. With the advent of stents, however, many of these suboptimalresults have been salvaged, and long-term patency rates with percutaneoustherapy now approach the success rates obtained with surgical reconstruction.The following is a review of the percutaneous devices available and technicalconsiderations relative to percutaneous iliac artery revascularization.

Endovascular treatment of stenotic or occluded iliac arteries is primarilyindicated only in patients who are symptomatic. The main clinical indicationsfor endovascular treatment of an iliac artery lesion include the following:intermittent claudication which adversely affects the patient's lifestyle,(SVS Category 3); rest pain, (SVS Category 4); tissue loss (SVS Category 5 and6); and to improve inflow into a graft or an extremity (i.e., femoral-poplitealbypass graft, femoral to femoral artery cross-over graft, prior to amputationto lower the level of amputation and/or promote healing at the amputation site,and to improve inflow or outflow in the extremity which has had a previousbypass graft placed).

The best results for iliac PTA usually are obtained in patients withCategory 1 and Category 2-type lesions, as determined by the American HeartAssociation Guidelines (table 1).1 Iliac artery stenting may have its greatestbenefit in the treatment of Category 3 and Category 4 lesions.

A stenosis is considered hemodynamically significant if the cross-sectionalarea of the vessel lumen is reduced by 70%. Occasionally, despite multipleprojections, it is difficult to determine the degree of stenosis significance.In these situations, a trans-lesion pressure gradient should be measured. Apeak-to-peak systolic pressure gradient of 10 mm Hg or greater at rest isconsidered hemodynamically significant. A peak-to-peak systolic pressuregradient of greater than 15 mm Hg following the administration of a vasodilatoralso is considered significant. When measuring a pressure gradient, it isprudent to measure simultaneous pressures above and below the lesion.Intraarterial pressures can vary significantly from heartbeat to heartbeat,rendering a "pullback" pressure measurement difficult to interpret.

Although the goal of any intervention is to have no residual stenosis, thisdoes not always happen. However, a residual stenosis of less than 30% andreduction of the pressure gradient below 10 mm Hg is considered satisfactory.If a larger residual stenosis or a persistent gradient greater than 10 mm Hgremain following PTA, early recurrence is likely.2

The indications for iliac artery stent placement include the treatment ofimmediate elastic recoil following balloon angioplasty, flow-limiting intimaldissections, a residual stenosis of at least 30%, a residual peak-to-peaksystolic pressure gradient of more than 5 mm Hg, or chronic occlusions. In thissubset of patients, placement of intravascular stents has lead to short-termpatency rates of greater than 90% and long-term patency rates between 85 and90%.3 Although the benefits are yet unproven, iliac stents also have beenplaced in patients to treat severely ulcerated plaques.

Contraindications to iliac stenting are not well defined. Obviously,fluoroscopic equipment, which allows adequate guidance to deploy the stentsafely, must be available. In the setting of ongoing bacteremia, insertion ofan iliac artery stent is contraindicated. In addition, placement of anuncovered stent in a ruptured iliac vessel or in the presence of apseudoaneurysm should be done with caution.

About one in three patients undergoing iliac PTA have suboptimal results4and may benefit from the placement of an intravascular stent. The diameter ofthe stent to be deployed is determined by either direct iliac arterymeasurement from a cut film arteriogram, calculated iliac artery measurementsusing internal or external calibration with a digital angiographic system, orfrom vessel diameter measurements obtained using intravascular ultrasound(IVUS). Currently, there are two types of intravascular stents that are FDAapproved: the balloon-expandable Palmaz stent (Cordis, Johnson & Johnson,Inc., Warren, NJ), and the flexible self-expanding Wallstent (Schneider, Inc.,Minneapolis, MN). Several other self-expanding stents (Symphony stent,Meditech, Boston Scientific Corp., Natick, MA, and the Memotherm stent, C.R.Bard, Inc., Covington, GA) are FDA approved for biliary applications, but havebeen used in the vascular system only in Europe.5,6

The Palmaz balloon expandable intravascular stent is made from a single tubeof 316L stainless steel with etched rectangular slots that are arranged instaggered rows around the circumference of the stent. The Palmaz stent comes inseveral sizes (table 2). As with all intravascular stents, foreshorteningoccurs when the stents are expanded. The degree of foreshortening varies withthe individual stent type and diameter. Approximately 10% of foreshorteningoccurs when the larger size stents are expanded to the maximum recommendeddiameter, and about 20% of foreshortening occurs when the medium size stentsare fully expanded. However, due to its method of deployment and minimalforeshortening, the Palmaz stent can be placed with a high degree of precision.

The manufacturer recommends that these stents be placed on ascratch-resistant balloon to avoid balloon rupture during stent deployment. Inaddition, the compliance of the angioplasty balloon used for stent deploymentalso can be important. A semi-compliant balloon such as the Opta-5®(Cordis, Johnson & Johnson Co, Miami, FL) can result in some molding of thestent to the contour of the vessel, and also can provide the opportunity tomildly over-expand the stent at higher inflation pressures (tables 3,4).However, a compliant balloon tends to expand the most outside the lesion. Anoncompliant balloon (table 5) can be useful in expanding the stent in areas ofincreased resistance or at sites of residual stenosis following stentdeployment.

It is also important to mount the Palmaz stent on the middle of mostballoons. Otherwise during balloon inflation, the stent will "squirtoff" the balloon in a partially-expanded state (figure 1). It is best tomount the stent with gentle hand crimping or with the use of a crimping tool,being careful not to use a shearing motion, which might perforate the balloonwith the sharp ends of the stent. Once the stent is tightly crimped on theballoon and the introducer is placed over the balloon and stent, they areadvanced coaxially over a guidewire and into the vascular sheath. The onlyexception to this rule is when using the Olbert balloon (Meditech, BostonScientific, Corp., Natick, MA). The stent should be crimped off-center on theOlbert balloon, closer to the proximal radiopaque marker on the ballooncatheter.

The manufacturer recommends the use of an introducer sheath at least 2 Flarger than the shaft size of the angioplasty balloon catheter. The largerstents (P308) which are FDA approved for iliac stenting require a 5.8 to 7-Fshaft balloon system and an 8 to 10-F introducer. However, manyinterventionalists try to use medium-sized Palmaz stents (which are not FDAapproved for intravascular use) for vessels of 4 to 10 mm in diameter. Themedium size Palmaz stents can be mounted on balloon catheters with a 5-F shaftand can be inserted through 6 to 7-F introducer sheaths. The vascular sheathshould be long enough to allow the balloon and Palmaz stent combination to bedelivered to the lesion in a covered fashion.

Once the stent and balloon are positioned across the lesion, the sheath isretracted to completely uncover the stent and balloon. A radiopaque marker atthe tip of the sheath facilitates visualization of the sheath tip. Confirmationof stent position can be performed by injecting the sheath just prior todeployment. Road mapping also is helpful for precise positioning. Oncepositioning is confirmed, the balloon is then inflated with dilute contrast orsaline. Following inflation, the balloon is deflated and rotated in a clockwisemotion to help fold the wings free of the stent and is then removed from thesheath.

Stent placement can be confirmed by injecting through the sheath (figure 2).For long lesions, place additional stents in a similar manner, remembering tobegin with placement of the stent furthest from the puncture site. The stentshould be overlapped approximately 3 to 5 mm. The use of long stents reducesthe need for multiple, overlapping stent placement. At the end of theprocedure, remove the guidewire and sheath, making sure that the J-tip of theguidewire does not catch on the free end of the newly placed stent.

The Palmaz stent is a rigid stent and usually cannot be advanced across theaortic bifurcation to the contralateral iliac artery. This stent has beenreported to have the highest radial force of the available intravascularstents.7 However, because the Palmaz stent is made of stainless steel, it isnot MRI compatible. The manufacturer suggests that an MRI not be performed forapproximately 6 weeks on patients who have had a Palmaz stent placed.

The Wallstent is a self-expanding metal stent comprised of metal filamentswoven into a tubular, braid-like configuration. The Wallstent is made ofElgiloy®, a superalloy combining cobalt, chromium, nickel, and othermetals. Elgiloy contains a relatively low amount of iron, making the stentnon-ferromagnetic and MRI compatible. The iliac Wallstent ranges in size from 6to 10 mm in diameter and comes in varying lengths (table 6). Because of itsflexibility, the Wallstent is able to be placed within tortuous or angulatedvessels without kinking. In addition, its flexibility allows for placementacross the aortic bifurcation into the contralateral iliac artery.

Because the Wallstent is a self-expanding stent, it is not mounted on aballoon, but rather is constrained in a pre-packaged 7-F delivery catheter.Positioning of the Wallstent is accomplished by placing the stent catheterassembly (figure 1) through a 7-F sheath; the sheath is needed only to provideaccess to the vessel and does not need to be across the lesion. When placingthe Wallstent, close attention should be paid to the constrained stent lengthbecause, upon deployment, the Wallstent foreshortens significantly. Knowing theunconstrained length of the stent at different diameters is critical to aid instent positioning (table 7). As the Wallstent is uncovered and foreshorteningoccurs, the lesion should be kept at the center of the Wallstent by gentlypulling back the partially constrained stent to the appropriate position acrossthe lesion. With the newly designed Wallstent (Unistep Plus System®,Schneider), the stent can be reconstrained if at least a little over 10% of thestent remains covered. Once reconstrained, the stent can be repositioned anddeployed in the appropriate position.

Following deployment of the Wallstent, if there is incomplete expansion ofthe stent, a scratch resistant angioplasty balloon (table 5) can be used tohelp further expand the stent. If multiple stents are being placed, it isimportant to remember that foreshortening may continue over time. Therefore,adequate overlaying of the stents is important in order to prevent theformation of gaps between the stents over time. Otherwise, the shortest stentpossible should be used to minimize the amount of foreign body reaction.

The Symphony stent (Meditech, Boston Scientific Corp.) is a flexible stentmade from a single strand of nitinol wire, arranged in a hexagonal design andwelded at the contact points (figure 1). Nitinol is a nickel and titaniumthermal memory alloy that can accommodate large changes in shape in response tochanges in temperature. The Symphony stent is self-expanding and is mountedwithin a 7-F catheter with a pistol-like delivery system. It is available in 6,7, 8, 10, 12 and 14-mm diameter sizes and varying lengths. The stentforeshortens between 5 and 10% during deployment and expansion (table 8). Whenplacing the Symphony stent, the manufacturer recommends that the nominaldiameter of the unconstrained stent be at least 1 mm larger than the diameterof the target vessel. The lack of radiopacity of this stent makes it difficultto visualize during fluoroscopy (figure 3). Because the Symphony stent is madeof nickel, it is MRI compatible. As noted earlier, this stent is currentlyapproved by the FDA for use in the biliary tract in the United States, and aniliac artery clinical trial is now underway in the United States.

The Memotherm stent (C.R. Bard, Inc.) also is a flexible stent etched from asingle piece of nitinol material, with no braids or crossing filaments (figure1). Because it is made of nitinol, it is flexible and compatible with MRI. Dueto the construction of the stent, minimal foreshortening occurs with stentdeployment. The Memotherm stent is available in 7, 8, 9, 10 and 12-mm diametersizes (table 9). It comes in lengths from 30 through 110 mm (in 10 mmincrements). These stents are delivered via a 7-F preloaded delivery system.The Memotherm stent currently is only approved by the FDA in the United statesfor use in the biliary tract.

The Symphony and Memotherm stents are placed through a 7-F sheath which doesnot need to cross the lesion. Once these stents are positioned across thelesion, repeated firings of the delivery gun trigger slowly move the coveringback, exposing and ultimately deploying the stents. Markers on the catheterdemonstrate the position of the stent and the amount of uncovered stent. TheSymphony stent can be pulled back for minor adjustments if less than 50% of thestent is uncovered; the Memotherm stent cannot be repositioned once deploymentis initiated. Once deployed, a scratch-resistant balloon angioplasty catheter(table 5) can be used to help further expand the stent.

Because of their flexibility, the Wallstent, Symphony stent, and Memothermstent can be placed over the aortic bifurcation into the contralateral iliacarteries (figure 4).

Lesions at the aortic bifurcation that extend into both iliac arteries canbe treated using the "kissing" stents technique. As with"kissing" balloon angioplasty, "kissing" stents preventplaque shifting at the aortic bifurcation and inadvertent compromise of thecontralateral iliac artery lumen. When placing "kissing" stents, itis important to attempt to have the proximal ends of the stent completely overthe ostia of the left and right iliac arteries at the aortic bifurcation.Failure to completely cover the ostia could lead to early recurrence of anatherosclerotic lesion at this site.

Occasionally, disease extends into the aortic bifurcation and stenting ofthe aortoiliac segment becomes necessary. In this situation, the proximal endof the stents are placed further into the aorta, moving the newly constructed"bifurcation" more proximally (figure 5). A disadvantage of thistechnique is that large portions of the iliac stents can remain uncovered atthe bifurcation and become a nidus for thrombus formation. As in"kissing" balloon angioplasty at the aortic bifurcation, it isimportant to note the diameter of the aorta where the two balloons will"kiss" to be sure that the diameter of the lower aorta canaccommodate simultaneous inflation of both balloons and stents (figure 6).

If the origin of one iliac artery is being stented, the other iliac arteryorigin, even if it appears normal, should also be stented to prevent plaquefrom shifting from the diseased iliac artery into the unaffected vessel. Inaddition, the use of bilateral stents will hopefully reduce the unevenhemodynamic stress which may result in early progression of atheroscleroticdisease in the contralateral iliac artery if it remains unstented.

Early recurrences and high failure rates were common with simple balloonangioplasty of iliac occlusions. However, improved long-term patency ofocclusions following placement of a stent is now being seen. Once the occlusionis crossed with a guidewire, permanent stenting of the occlusion withoutpredilating the lesion has been advocated in order to trap any thrombus andplaque between the stent and vessel wall, thereby reducing the risk for distalembolization.8

Although primary stenting of all iliac artery lesions has been advocated,several large studies have shown that there is clearly a large subset ofpatients who are adequately treated with PTA alone.4,9 Given the success ofballoon angioplasty alone in such a large number of patients, it is unclear ifprimary stenting of all iliac lesions is cost-effective.

Stent deployment is usually uneventful, but problems can occasionally occur.Familiarity with these potential problems beforehand makes dealing with themeasier, should they occur. The Palmaz stent may move on the balloon if notproperly crimped, or if the stent is advanced through a tortuous vessel oracross the lesion without a covering. When the stent moves on the balloon, anattempt should be made to pull the Palmaz stent back into the sheath and removethe sheath, stent, and balloon catheter as a unit over the guidewire. A newsheath and stent-balloon combination can then be inserted. If the stent cannotbe pulled back into the sheath, it may be possible to partially inflate theballoon, trapping the stent, and then reposition the partially inflated balloonwith the attached stent across the lesion before deploying it. Should thisfail, the stent can be deployed where it is and a new stent should be insertedappropriately and positioned across the lesion.

Occasionally the Palmaz stent can "squirt off" the end of theballoon during balloon inflation if it is not properly centered on the balloonwhen mounted or if a silicone coating is present on the balloon. If this shouldoccur, it is important to maintain guidewire access through the stent at alltimes. If the Palmaz stent becomes dislodged, the old balloon can be removedand a new balloon with a low profile can occasionally be advanced through thestent. This new balloon can then be gently inflated, trapping the Palmaz stentand allowing repositioning of the stent at the desired location. If the stentcannot be repositioned, deploy it where it is. A new stent is then inserted anddeployed in the appropriate position.

If balloon rupture should occur during the initial inflation, the balloonoften can be inflated enough to deploy the Palmaz stent by using a 5 ccsaline-filled syringe and injecting the syringe as rapidly as possible. Theballoon can then be deflated using a 60 cc syringe, and a new balloon can beinserted in the stent to complete the inflation. Should the balloon becomecaught in the stent, careful rotation of the balloon will usually free it. Ifthis maneuver is unsuccessful, advancement of the sheath to the edge of thestent before pulling back on the balloon may help prevent the stent frommigrating back as the balloon is removed. If a 5-F balloon catheter cannot beadvanced through the unexpanded stent, a 4-F catheter can be advanced over theguidewire, through the stent, and a 0.018'' platinum-tipped wire can beadvanced through the 4-F catheter. After removing the 4-F catheter, a smallvessel balloon catheter (3.5-F Symmetry, Meditech, Boston Scientific Corp.,Natick, MA) can be advanced over the wire through the stent. The stent can thenbe partially opened, which will allow the insertion of the 5-F balloon catheterthrough the stent. The stent can then be deployed.

Because all of the stents mentioned have spaces between the metal struts,guidewires can easily pass through these spaces. The use of a J-wire will helpprevent penetration of the stent spaces and keep the passage of the guidewireintraluminal.

Rarely, a stent will migrate and float freely in the aorta. Should thisoccur, the stent may be entrapped with a loop snare and deployed in a smallervessel. This maneuver is much easier to perform if guidewire access has beenmaintained. Complete removal of a Palmaz stent from the vessel usually requiresa surgical cutdown. Once snared, the Wallstent will fold in half quite easily,and can be removed through an 8-F sheath.

The use of prophylactic antibiotics just prior to stent placement iscontroversial. However, prophylactic antibiotics make sense when placing anintravascular stent, given the frequent antibiotic use before placement ofpermanent surgical prosthetic grafts by vascular surgeons. Broad spectrum firstor second generation cephalosporins have been shown to be adequate forprophylaxis when graft material is placed in vascular surgery patients. Animalresearch has shown that should transient bacteremia occur at the time of stentplacement, implanted stents are more likely to become "seeded" andsubsequently infected when compared to an angioplasty site without stentplacement.10 Recent data also suggests that transient bacteremia may occur morefrequently than once considered.11 There have been several reports of infectedintravascular stents in the literature.12,13

The technical success rate for placement of iliac stents is greater than95%.3 Richter has reported cumulative 5-year patency rates of 93.6% for thePalmaz stent, although other authors have reported somewhat lower patencyrates.3,14 The 36-month patency rate for the Wallstent has been reported to beapproximately 80%.15

Unfortunately, recurrent stenoses can occur in successfully treated lesions.Recurrence usually is due to intimal hyperplasia within the stent orprogression of disease proximal or distal to the stent. When this occurs,balloon angioplasty occasionally produces an adequate result, improving theluminal diameter and abolishing the pressure gradient within the stent.Frequently, an additional stent must be placed within the previously placedstent to restore luminal diameter and abolish the pressure gradient within thestent (figure 7). Occasionally, the Simpson atherectomy catheter (DVI,Mallinckrodt Medical, St. Louis, MO) can be used to remove the hyperplastictissue from within the stent.

Acute stent thrombosis occasionally occurs. Thrombolysis with urokinaseusually is effective in restoring patency of the stent and native artery.Delayed stent thrombosis usually is secondary to intimal hyperplasia within thestent or progression of atherosclerotic disease above or below the stent.Following thrombolysis, these lesions need to be addressed.

In order to minimize the risk for the development of acute stent thrombosisand exuberant intimal hyperplasia, endothelialization of the stent should bepromoted. Ideally, the process is aided by embedding the stent struts into thevessel wall so that the final stent diameter is approximately 10 to 15% largerthan the diameter of the vessel. If under-expansion of the stent has resultedin incomplete stent strut approximation, thrombus will be deposited along thelength of the stent which is not embedded. Multicentric endothelializationthrough the metal lattice of the stent will not occur, and theendothelialization process will end much more slowly. Slow endothelializationis prone to continuance of thrombus formation and smooth muscle proliferation,which ultimately leads to narrowing of the stented lumen and/or stentthrombosis.16 Therefore, it is important to insure good stent-to-vessel wallapproximation. To this end, IWS has been shown to be helpful.17

Following stent placement, daily aspirin use and cessation of smoking toslow the progression of atherosclerotic disease are recommended. However, theefficacy of coumadin following stent placement in the iliac arteries has notbeen demonstrated.

Conclusion

Intravascular stents have been shown to be helpful in improving theimmediate cosmetic and hemodynamic results of iliac angioplasty in a definedsubset of patients. Hopefully, ongoing studies will determine if iliac stentingcan improve upon the long-term patency of successful balloon angioplasty. Inthe meantime, utilizing a strategy of selective iliac artery stenting seems tobe most prudent. In addition, individuals performing iliac artery stentingshould be familiar with the nuances of each device and how to minimize the riskof both technical and clinical complications.AR

Dr. Spinosa, Dr. Angle, Dr. Hagspiel, Mr. Pyle, and Dr. Matsumotoare in the division of Angiography and Interventional Radiology at theUniversity of Virginia Health Sciences Center in Charlottesville, VA.

References

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