Version May 2024
INTRODUCTION — The term stent, derived from the name of the dentist Charles Thomas Stent (1807 to 1885), has been around since the latter part of the 19th century. However, as applied to an intraluminal tube used to maintain patency of a tubular anatomical structure, the term stent has been in use only since the mid-20th century [1].
Having been first used in urology [2], the first use of a stent in a vascular application was not reported until 1983 [3]. The first report of stent placement in association with dialysis vascular access involved four cases reported in 1988 [4]. Since this early report, intravascular stent deployment for the treatment of complications associated with dialysis access has increased at a rapid rate. Not only has the total number of stents being placed increased, but the proposed indications for stent placement have also been expanded.
A large number of stents with different geometric and mechanical features are available for use. Stent behavior depends largely on their mechanical properties [5-7]. Therefore, the mechanical properties of stents influence the choice of a stent for the treatment of a specific lesion [8]. The basic principles of vascular stents are reviewed. Specific clinical uses for vascular stents and their outcomes are discussed in separate topic reviews. (See "Endovascular intervention for the treatment of stenosis in the arteriovenous access" and "Endovascular techniques for lower extremity revascularization".)
The design of stent-grafts to treat aortic aneurysmal disease is reviewed separately. (See "Endovascular devices for abdominal aortic repair" and "Endovascular devices for thoracic aortic repair".)
INDICATIONS FOR STENT PLACEMENT — Vascular stents are placed into the peripheral vasculature for specific indications.
In general, a stent is used when plain old balloon angioplasty is unlikely to provide a durable benefit (lesion recurrence), as demonstrated in clinical studies, for failed angioplasty (elastic recoil of lesion) or for complications (eg, vessel dissection or rupture during intervention).
●Elastic recoil – Lesions of venous stenosis tend to be elastic. As a result, even when obtaining full balloon effacement with angioplasty, a significant residual stenosis may persist (figure 1). This is problematic in that it yields a result that is less than optimal or of only temporary benefit. It has been shown that any residual stenosis following angioplasty results in suboptimal primary patency [9]. A successful angioplasty requires some degree of anatomic disruption of the vessel wall at the target site [10,11]. The indications for placing a stent for an elastic lesion are somewhat different for peripheral and central veins. The elastic lesion in the peripheral vein may respond to an oversized angioplasty balloon or may be more effectively treated with a surgical approach. In the case of a central venous lesion, an oversized balloon may not be possible considering the size (diameter) of these venous structures. In addition, a surgical approach to the lesion may not be practical.
●Recurrence – The goal of angioplasty is to restore patency for the longest possible period of time. Yet, some lesions are resistant to dilatation and some degree of residual stenosis is not unusual. This is an important factor in determining duration of primary patency [9]. In a study that reported the results of 230 stenotic lesions treated with angioplasty, it was found that residual stenosis had a negative correlation with duration of patency [12]. The placing of a stent in these rapidly recurring cases has been recommended [13,14]. A generally accepted definition for rapid recurrence as an indication for stent placement has not been defined.
●Dissection or rupture – The most frequent procedure-related complication seen in association with angioplasty that dictates the need for intervention is venous rupture [15]. The clinical significance of this complication is variable, ranging from none to disaster for the access. The difference lies in the severity of the rupture. While the majority of these cases do not require treatment and are not associated with significant adverse sequelae, there are instances in which the placement of a stent becomes necessary to restore flow, salvage the access, or achieve hemodynamic stability in the case of venous rupture (figure 2) [16].
Relative contraindications — Guidelines advise against placing central venous stents adjacent to cardiovascular implantable electronic devices' (CIEDs; eg, pacemaker, implantable cardioverter-defibrillators) transvenous leads [17]. If a central venous stent is necessary in a patient with transvenous leads, the leads should be extracted prior to stent placement and then replaced, passing through the stent to avoid complications. A better alternative is to avoid stent placement altogether. Angioplasty is generally safe in the presence of transvenous CIED leads [18], and the placement of stents/stent-grafts has been performed to treat central vein stenosis. The presence of a stent/stent-graft does not appear to have any adverse effect on the function of the CIED [19-21]. However, the transvenous leads associated with CIEDs can get infected, which can be quite serious. Infection rates vary, ranging from 0.1 to 19.9 percent; infection rates in patients with chronic kidney disease are reportedly higher compared with those of the general population [22-25]. Treatment of infection generally requires removal of the CIED with extraction of the leads [17]. If a stent/stent graft has been placed, the transvenous leads entrapped by a venous stent/stent-graft can be very difficult to remove using standard endovascular techniques, and open-heart surgery is frequently required.
STENT DESIGN — In its design, the ideal stent should address a broad range of technical issues (table 1). These are discussed in more detail below. (See 'Stent properties' below.)
Available stents all reflect a compromise between competing desirable features, creating subtle differences in their performance characteristics. Unfortunately, efforts to optimize one characteristic of stent design can have detrimental effects on another. For this reason, the interventionalist must have a variety of stents available to address the various applications in which they are required.
Stent structure — Stent flexibility and scaffolding (ability to completely expand to attain complete wall apposition) are derived from stent designs [26,27]. Metallic stents can be made from wire (round or flat), sheet, or tubing. Those used in the treatment of hemodialysis access dysfunction are generally constructed of wire. Each element of the stent's structure is referred to as a strut. The majority of commercially available stents are composed of a series of ring "modules" composed of struts arranged in either a diamond or z-shaped configuration (figure 3) [28]. This represents the expandable portion of the stent. These modules are joined by connecting elements referred to as bridges, hinges, or nodes. Depending on the density of the struts, stents can be classified as those with a closed-cell or an open-cell configuration (picture 1) [26]. In the closed-cell configuration, which is characterized by small free-cell areas between struts, connections involve every inflection point around the circumference of the module. The connections in the open-cell configuration involve only a few of the inflection points alternating with unconnected points in some defined pattern, creating larger uninterrupted free areas between the struts.
Stent composition
Metal — Several metals are used for the construction of stents. The stainless steel used for stent construction is the 316 L form, which has good ductility (the capability of being drawn out into a thin wire), good strength, resistance to corrosion, and a low level of magnetism. Elgiloy, which is a more complex alloy of cobalt, chromium, nickel, iron, molybdenum, and manganese, has been used because it is mechanically strong and corrosion resistant.
The metal most commonly used in the construction of stents for dialysis vascular access interventions is nitinol, which is a shape memory alloy. Nitinol's name is derived from its constituent elements and the Naval Ordnance Laboratory (NOL) where it was discovered. Nitinol has two properties that make it ideal for stent construction: thermal memory and superelasticity [8,28,29].
●The self-expanding property of Nitinol is dependent upon its thermal memory. A change in temperature induces a reversible change in its crystalline structure (martensitic transformation) so that it exists in one form at low temperature (martensite state) and another at higher temperature (austenite form). Adjustments in the alloy can be made, making it possible to predetermine the temperature at which the martensitic transformation occurs. At a low temperature, nitinol is flexible and malleable and so can easily be loaded into a stent introducer system. When released at the higher austenite temperature (body temperature), the stent expands to its predetermined size and becomes more rigid, providing the support (scaffolding) required of a stent [8].
●The second property of Nitinol is superelasticity. This term refers to its enormous elasticity, which can be many times greater than the best stainless steel used in medicine today. In other words, following deformation, the material tends to return to its original configuration. With most materials (ie, like stainless steel), stress increases linearly with strain and decreases along the same path upon unloading. Nitinol behaves differently, following a nonlinear path (figure 4) characterized by a pronounced stress hysteresis (ie, the phenomenon in which the response of a physical system to an external influence depends upon the present magnitude of that influence as well as the previous history of the system). When a nitinol stent is deployed (or following compression), it expands circumferentially until it contacts the vessel wall. After this initial increase in circumferential force, the continuing opening force directed toward the vessel wall, or "chronic outward force," remains very low. However, the force generated by the stent to resist compression, or its radial resistive force, increases rapidly [28,29].
Fabric — The fabric-coated stent, referred to as a stent-graft, is created by adding a fabric layer to a stent skeleton. This layer may line the outer or inner surface of the stent, or the metallic stent may be sandwiched between two layers of fabric.
Two types of fabric covering have been used in the construction of stent-grafts: polyethylene terephthalate (PET, or Dacron) and expanded polytetrafluoroethylene (ePTFE). Of the two, ePTFE is more commonly used. PTFE is also commonly used for vascular grafts and has tremendous tensile strength. Ultrastructurally, ePTFE consists of spindle-shaped condensed nodes interconnected by fine fibrils, which are separated by open spaces that account for 80 to 85 percent of the bulk of the material, giving it a porous, latticework structure.
The design of stent-grafts to treat aortic aneurysmal disease is reviewed separately. (See "Endovascular devices for abdominal aortic repair" and "Endovascular devices for thoracic aortic repair".)
Antiproliferative coatings — Restenosis of the treated lesion, plus the tendency of the stent to stimulate the development of neointimal hyperplasia, is a major problem. A variety of drugs and chemicals have been used to inhibit platelet activation and adhesion to the stent struts during the early phase of stent re-endothelialization and to inhibit the ingrowth of tissue into the stent lumen. Paclitaxel and sirolimus are the drugs most frequently used for this purpose.
STENT NOMENCLATURE — Stents may be categorized by class and by type.
Classes of stents — There are two classes of stents, distinguished by their mechanism of expansion: balloon-expandable (BE) stents and self-expanding (SE) stents (picture 2). The BE stent, which comes mounted on an angioplasty-type balloon, is constructed of material that undergoes plastic deformation through the inflation of the balloon. However, when the balloon is deflated, the stent remains in its expanded shape; it does not recoil significantly. Because BE stents are constructed of material that allows for expansion at a manageable balloon pressure, they can also be compressed by external pressure. This effectively eliminates their application in the peripheral vasculature since once compressed, they cannot recoil (self-expand).
SE stents are manufactured in their expanded form using material that allows them to be compressed and constrained within the delivery system. When released from the delivery system, they expand to their preset diameter. In general, stents used for hemodialysis vascular access applications are of the SE class.
Types of stents — There are three types of stent: the bare-metal stent (BMS), the fabric-covered stent referred to as a covered stent or stent-graft (figure 5), and the drug-coated stent (DCS). The BMS, often referred to simply as a stent. In subsequent discussions, we will use the term stent in a generic sense to refer to all types of stent, and the terms BMS, stent-graft, and DCS in reference to these specific categories.
The first of these devices developed for intravascular use was the BMS. As discussed below, neointimal hyperplasia, which develops in response to the presence of a stent, protrudes between the struts of a BMS to encroach on the lumen of the vessel. The stent-graft was conceived to prevent intimal hyperplasia from occurring [30]. Evidence suggests that the use of the stent-graft does improve primary patency in the treatment of stenosis associated with the dialysis vascular access [31,32].
Because of the high rate of restenosis following plain balloon angioplasty for the treatment of venous stenosis associated with an arteriovenous access, the concept of using a stent coated with an antiproliferative drug was subsequently developed. Although results obtained in early studies were marginal [33], later reports indicate improved primary patency using drug-coated stents [34,35].
STENT PROPERTIES
Biocompatibility — It is important for a stent to be relatively inert and not elicit an inflammatory response. In essence, with metal stents, this requires it to be corrosion resistant. 316 L stainless steel fulfills this requirement relatively well. Nitinol is slightly superior in this regard, although this is unlikely to have any clinical significance [28]. Both metals are biocompatible. Expanded polytetrafluoroethylene (ePTFE), which is used for the construction of stent-grafts, also has a high degree of biocompatibility. However, there are reports that polyethylene terephthalate (ie, Dacron) can cause a "post-implantation syndrome" in some patients [36,37]. For peripheral vascular stents, this is characterized by fever, cutaneous swelling, edema, erythema, leukocytosis, and elevated C-reactive protein.
Both stainless steel and nitinol contain nickel. Nickel allergy is reported to occur in 10 percent of the general population, with a higher rate occurring in women. This creates the possibility for a hypersensitivity reaction in a patient with nickel allergy. Stainless steel contains some nickel but less nickel compared with nitinol (10 to 14 percent versus 54 to 57 percent); however, it is released at a more rapid rate. An allergic reaction to stent placement is rare but can occur [38,39].
Neointimal hyperplasia — Long-term vascular patency is the primary goal of stenting and is closely linked to control of neointimal formation.
When a bare-metal stent (BMS) is placed, a very thin layer of plasma proteins covers its metallic struts within seconds after placement. Its outwardly directed force causes the metal elements of the stent to become embedded in the vessel wall around the circumference of the vessel lumen. The troughs or depressions resulting from this embedding become filled with thrombus that covers the metallic surface; the tissue protruding between the depressions retains its endothelium, which is the source for endothelialization of the stent contact area by multicentric growth. In a few days to a few weeks, the thrombotic layer covering the stent struts is progressively replaced by neointima. Eventually, this replacement is complete, or relatively so. Through this process, the BMS becomes firmly attached and embedded within the vessel wall (figure 6). Studies have shown that there is almost complete neointimal encasement of the stent within one month following implantation [40]. By becoming embedded within the wall of the vessel, the stent in essence becomes become part of the patient's anatomy [28]. Initially, the neointimal covering consists of a relatively thin layer, which can progress with time (see 'In-stent stenosis' below) but can become markedly thickened over time (image 1). (See 'In-stent stenosis' below.)
The in-stent stenosis occurring with the BMS gave rise to the concept of covering the stent with an impermeable material (the stent-graft) to prevent tissue ingrowth. Following the placement of a stent-graft, organizing thrombus forms upon the luminal surface. Within a few weeks, there is evidence of neointima with overlying endothelium at the junction between the stent-graft and vessel. This tissue in-growth continues and can extend for several centimeters over the intimal surface of the device. A foreign-body type of inflammatory reaction develops on the abluminal (the outer surface of a body part or device with an internal cavity) surface of the stent-graft, characterized by monocytes and multinuclear giant cells [30,41]. Some stent-grafts are constructed with bare-metal extensions at either end (figure 7). These serve to help stabilize the device and prevent migration. However, their presence can result in neointimal hyperplasia occurring adjacent to the ends of the stent, creating a "candy-wrapper" appearance (figure 8).
Radiodensity — Radiodensity refers to the capability of a substance or object to hinder the passage of x-rays, allowing it to be seen as a visible radiographic image. Radiopacity is also important to guide safe positioning, adequate deployment, and post-dilatation and to permit assessment of optimal stent expansion. Also, the stent lumen must be sufficiently visible to allow radiographic assessment of flow dynamics and restenosis. Whether or not a stent is radiopaque depends upon the type of metal used in its construction and the amount of metal within the stent skeleton. Stent radiodensity also depends on the fluoroscopy mode that is used for visualization. With lower pulse-per-second rates, detection of a stent is more difficult [42].
The plain radiographic visibility of stainless steel and nitinol is similar; however, the visibility of both is becoming more problematic as stents continue to evolve toward lower mass and finer features (image 2) [28]. To improve their radiographic visibility, markers are often attached to the ends of the stent. These additions are typically made from gold, platinum, or tantalum (figure 9). Corrosion potential is an important consideration when selecting metal components for a stent [28,43]. When two galvanically dissimilar metals are placed in an electrolyte solution (blood), an electrochemical reaction occurs, which results in differing rates of corrosion. For this reason, metals that are electrochemically compatible must be chosen for stent construction. Nitinol and tantalum are galvanically similar, and thus, this combination has no significant corrosion potential. There are corrosion and clinical concerns with the use of gold as a marker [44]. Therefore, if gold is used, an insulating layer is required between the stent and the marker.
Imaging compatibility — Magnetic resonance (MR) imaging compatibility is important for any metallic device that may be inserted into a patient's body. This is true for intravascular stents. Issues such as migration, heating, and image artifacts must all be considered. All of the stents commonly used today are basically compatible with MR imaging. (See "Patient evaluation for metallic or electrical implants, devices, or foreign bodies before magnetic resonance imaging", section on 'Arterial stents, coils, and clips'.)
However, image artifacts can be a problem; the degree to which this occurs depends upon the type of metal used in stent construction. As a result, there is a wide variability in the visibility of different stent models from different manufacturers [45]. Both stainless steel and nitinol can result in a void artifact with MR imaging. Iron-containing alloys cause the greatest problem. Stainless steel interacts strongly with MR imaging, which can create artifacts that make imaging of the stent and nearby areas difficult. However, 316 L stainless steel contains more nickel than other forms of stainless steel, keeping it in an austenitic phase characterized by a low level of magnetism [46]. With appropriate surface treatment, nitinol exhibits a very low magnetic susceptibility, allowing for a relatively clean image [28].
Resistance to external compression — Two physical properties of stents that are somewhat related help the stent resist external compression; these are hoop strength and radial resistive force.
Hoop strength relates to the ability of the stent to withstand a circumferential compressive force. This metric is especially pertinent to the balloon-expandable (BE) stent. Hoop strength relates to the maximum load that can be carried by the stent that is being used to support the vessel before experiencing a plastic deformation. In the case of the BE stent, this deformation is permanent. This is not the case for the self-expanding (SE) stent, which is able to recover from such an effect.
Radial resistive force (also referred to as hoop or radial stiffness) refers to the circumferential force applied to the vessel wall by an SE stent that is being used to scaffold (support) the vessel. This reflects the effectiveness of the stent in resisting diameter loss due to the vessel recoil or internal and external mechanical forces. For dialysis access intervention, SE stents are commonly oversized in relation to target veins to assure optimal wall apposition and prevent migration [47]. A nitinol stent demonstrates dynamic scaffolding of the vessel (eg, if the vessel wall moves outward away from the stent, the stent will expand and continue to apply a force). Over time, this can result in significant changes. Tissue remodeling occurs in response to continuing outward force, and the stent migrates towards the outside of the vessel, a change that can be observed angiographically (figure 6) [28]. This can result in stretch injury to the vessel, a phenomenon especially observed when a stent is oversized for the target vessel. Animal studies have shown that this can result in neointimal proliferation proportional to the severity of the stretch injury to the vessel [48].
In addition to radial resistive force (hoop stiffness), the response of the stent to external compression, or "pinching load," is important in determining the effectiveness of the stent (picture 3). This relates to the stent's ability to regain its original configuration following compression. This becomes especially important in peripheral vein applications. Unlike the BE stent, which does not regain its shape, the SE stent is designed to return to its original configuration following external compression. The ability of the stent to be crush resistant in response to external compression (pinching load) is dependent upon strut thickness, while the radial resistive force (hoop stiffness) is dependent upon strut width. Since both parameters are important for optimal stent function, care must be taken in strut design [28].
Nitinol stents have a unique response to radial resistive force and the force of external compression. The hysteresis (path dependence of nitinol to return to its original configuration when compressed) results in what has been termed as "biased stiffness." When in position, the chronic outward force of the nitinol stent remains very low, while the force generated by the stent to resist compression (radial resistive force) increases rapidly with deflection (figure 4) [28,29].
Resistance to fatigue — Once in place, the stent can be exposed to repetitive distorting forces, which over time can result in failure due to fatigue of its metal elements. Although not a problem in venous applications, an arterial stent is subjected to the pulsatile forces of the vessel, which can result in pulsatile fatigue. It has been estimated that a native artery undergoes approximately 3 to 10 percent diameter change when subjected to a pulse pressure of 100 mmHg [49]. This results in repetitive mechanical movement of the stent. The US Food and Drug Administration requires that a stent be tested for pulsatile fatigue. The ability of nitinol stents to withstand a pulsatile force is much greater than that of other metals [28].
Ease of positioning and deployment — Accuracy in stent placement is important to optimize stent performance. Stent placement introduces some degree of vessel injury, which can lead to inflammatory changes that promote neointimal proliferation. Inadvertently covering the normal endothelium with a stent due to poor placement accuracy can induce the development of neointimal hyperplasia. In addition, inaccurate stent placement can lead to the requirement of an additional stent to augment the effect of the one that was poorly placed (figure 10). This adds to the cost of the procedure. Lastly, inaccuracy with stent placement can promote stent migration.
Foreshortening is a design issue and can be eliminated by stent design. Open-cell stents, in contrast to closed-cell stents, are associated with significantly less foreshortening [50]. This was especially true for the original stainless steel stents, and early SE stents also foreshortened significantly with expansion. The available nitinol stents and stent-grafts demonstrate very little foreshortening.
Depending upon their design, some SE stents tend to spring forward as the leading end begins to emerge from the restraining device. This can lead to inaccuracies in stent placement. Some stent delivery systems consist of an inner catheter upon which the stent is mounted and an outer covering sheath, which is retracted to release the stent. In order to provide flexibility, neither of these two are rigid, and therefore they can either stretch or become compressed. The delivery device can also have some degree of residual memory from being coiled in its packaging. As a result, even though the stent is carefully positioned prior to initiating the release process, it can move forward, requiring repositioning. These undesired effects can be managed by the experienced practitioner if they are anticipated and recognized.
Delivery profile — Stent profile refers to the size of the collapsed stent within its delivery system. The lower the profile, the smaller the sheath that will be needed for its deployment. Using a smaller stent is an obvious advantage. The major determining factor for stent profile is the diameter of the stent. However, the type of stent being used is also important. A stent-graft will generally have a larger profile than a BMS and therefore require a larger sheath for deployment. Stents of comparable diameter produced by different manufacturers may have different profiles and therefore require a different size sheath for introduction. Information related to the sheath size required for stent placement is printed on the label (figure 11).
Flexibility — Flexibility and scaffolding are key characteristics derived from stent design [27]. Flexibility of the stent influences its ability to conform to vessel tortuosity in the deployed state. If a stent that is placed in a tortuous vessel is lacking in flexibility, it will either cause straightening of the vessel with the potential for causing tissue damage or obstruction, or the stent will buckle (become kinked) (figure 12), leading to poor wall apposition and possibly obstruction. Closed-cell stents are more rigid, less flexible, and may develop kinks and deploy incompletely in the tortuous vessel. Conversely, stents with an open-cell configuration, which are flexible and conformable, are preferred in vessels that are angulated or have a tortuous anatomy. The flexibility of different types of stent-grafts is markedly different [28].
In addition to concerns relating to the effect of the inflexible stent on the vessel and buckling leading to obstruction and poor wall apposition, the lack of flexibility can result in stent fracture over time [28]. There are times when a stent needs to be deployed in a mobile area (eg, across a joint). Thus, the stent should be flexible enough to conform to the purposeful movements of the patient. It is important that fatigue of the metal elements of the stent do not predispose it to fracture. A proposed fatigue fracture classification is described above. (See 'Resistance to fatigue' above.)
Stent fracture occurring in the cannulation zone of an arteriovenous (AV) access can perforate the skin, presenting an infection risk for the patient and potential injury for dialysis center staff (picture 4) [51].
Cannulation in stented zone — When dealing with hemodialysis AV vascular access, whether the area being treated is within the cannulation zone is important to determine. In general, placement of either bare-metal stent or stent-grafts within the cannulation zone is discouraged, as they are frequently problematic. As an example, for treatment of a pseudoaneurysm, which is typically in the cannulation zone, using a stent-graft has become common practice [52-60]; however, not all stent-grafts are suitable for this purpose. The spacing of the struts must allow for cannulation through the device. If the struts, together with the connecting elements of the stent, are spaced too closely together, it should not be deployed within the cannulation zone of the access. This stent-graft application has the advantage of avoiding surgical intervention and the possibility for continuous use of the AV graft for hemodialysis without the need for catheter placement. The disadvantage is the cost of prolonging the use of an AV graft that might be more definitively managed surgically. Unfortunately, stent placement in peripheral access has not been without problems, the major ones of which have been infection [61] and stent fracture, which can lead to perforation and possible exsanguination [51,55]. For these reasons, one should be very judicious in the employment of this stent-graft application.
COMPLICATIONS OF STENT PLACEMENT — A number of complications are associated with the usage of stents, including in-stent stenosis, stent fracture, migration, and infection.
In-stent stenosis — While it is part of the normal physiologic response to the presence of a stent, with the passage of time, the neointima tends to continue to proliferate to produce an ever-thickening layer that grows through the openings in the body of the bare-metal stent (BMS). This growth can eventually compromise the lumen of the vessel (in-stent stenosis) and has been a major problem with the use of this type of stent (image 1).
There are factors associated with the stent that increase the likelihood of neointimal hyperplasia. Marked stent oversizing in an arterial application has been shown experimentally to produce an increased degree of neointimal hyperplasia [47]. To what degree this occurs in veins is not clear. However, histologic studies in an animal model of oversized self-expanding (SE) stents have shown a strong positive correlation between stent oversizing and neointimal proliferation [62].
Neointimal hyperplasia has been the major problem with stent usage in the management of dialysis access dysfunction. In many cases, it has made frequent retreatment a necessity, especially with BMS usage.
Stent fracture — The location of placement may expose the stent to repetitive crushing or bending with the potential for fatigue and stent fracture. (See 'Resistance to fatigue' above.)
Stent fracture can range in severity from a single strut to a complete transverse fracture. A system for classifying stent fracture has been proposed, describing fractures as type I to IV. Type I represents a single strut fracture; type II, multiple strut fractures; type III, multiple strut fractures resulting in complete transverse fracture; and type IV, a complete transverse fracture with stent separation [63].
As an example, the injudicious placement of a stent at the costoclavicular junction can result in crushing fatigue and fracture of the stent over time (figure 13) [64]. Stent placement at the level of the elbow can also result in bending fatigue with the same adverse result (image 3) [65]. In addition, obstruction can occur with elbow flexion when a stent is placed above the elbow (image 4).
Stent migration — Migration is the major procedure-related complication generally occurring at the time of stent placement, though its occurrence can be delayed. The possibility of stent migration should always be a concern when performing the procedure.
At a minimum, migration can adversely affect the therapeutic value of the device. When this occurs, it frequently requires the placement of a second stent, which significantly increases the cost of the procedure and exposes a greater length of the normal vessel to the effects of stenting. The most serious degree of stent migration occurs when the device becomes totally free to move with the circulation. Within the venous system, the vessels become larger as one progresses centrally in the direction in which the device is likely to migrate. This means there is nothing to stop its progress until it lodges in the heart or passes through into a pulmonary artery [66]. Although migration of a stent into the heart or pulmonary arteries may be asymptomatic [67-69], serious cardiopulmonary complications, immediate or delayed, can result from stent migration, such as perforation, arrhythmias, tricuspid regurgitation, myocardial infarction, and pulmonary infarction [67-75].
As is always the case when considering any adverse event, the best approach is prevention. This should start with careful size selection and careful adherence to the basic principles of the deployment procedure. Additionally, a "safety guidewire" should always be used. This means that before the deployment procedure begins, a guidewire should be placed so that it extends well into the inferior vena cava. It should be noted that the potential for migration is greater for a stent-graft than for a stent. (See 'Ease of positioning and deployment' above.)
Infection — There is a significant incidence of infection associated with the use of stents. In a retrospective study which examined 235 interventions in 174 patients during a four-year period, infection was noted in 42 percent of cases in which a stent-graft had been used to treat a pseudoaneurysm [76]. This contrasted with an instance of 18 percent for stent-grafts placed at the venous anastomosis or in an outflow vein.
SUMMARY AND RECOMMENDATIONS
●Vascular stents are placed into the peripheral vasculature for specific indications. In general, a stent is used when plain balloon angioplasty to treat a stenotic lesion is unlikely to provide a durable benefit, for failed angioplasty, or to manage angioplasty complications. Stents or stent-grafts are often used to exclude an arterial aneurysm from the circulation. Guidelines advise against placing central venous stents adjacent to cardiovascular implantable electronic devices' (CIEDs; eg, pacemaker, implantable cardioverter-defibrillators) transvenous leads (See 'Indications for stent placement' above.)
●Available stents reflect a compromise between competing desirable features. Stent flexibility and the ability of a stent to completely expand and hold its shape are derived from stent design. Efforts to optimize one characteristic of stent design can have detrimental effects on another. The subtle differences in performance characteristics require the interventionalist to have a variety of stents available to address the various applications for which they are required. (See 'Stent design' above.)
●Complications are associated with the use of stents, including in-stent stenosis, stent fracture, migration, and infection. Neointimal hyperplasia is a normal physiologic response to the presence of a stent, but over time it can compromise the lumen of the vessel (in-stent stenosis). Stent designs including the addition of fabric coatings and antiproliferative drugs aim to reduce the incidence of neointimal hyperplasia. Stent fracture can occur when a stent is placed at anatomic sites that expose the stent to repetitive crushing or bending. Stent placement at such sites is best avoided. The incidence of infection depends on the initial indication for stent placement. When used to treat pseudoaneurysms, the rate of stent infection appears to be increased. (See 'Complications of stent placement' above.)
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