Summary
A ‘stent’ is a tubular meshed endoprosthesis that has contributed to the development of interventional catheterization over the past 30 years. In congenital heart diseases, stents have offered new solutions to the treatment of congenital vessel stenosis or postsurgical lesions, to maintain or close shunt patency, and to allow transcatheter valve replacement. First, stents were made of bare metal. Then, stent frameworks evolved to achieve a better compromise between radial strength and flexibility. However, almost all stents used currently in children have not been approved for vascular lesions in children and are therefore used ‘off-label’. Furthermore, the inability of stents to follow natural vessel growth still limits their use in low-weight children and infants. Recently, bioresorbable stents have been manufactured and may overcome this issue; they are made from materials that may dissolve or be absorbed in the body. In this review, we aim to describe the history of stent development, the technical characteristics of stents used currently, the clinical applications and results, and the latest technological developments and perspectives in paediatric and adult congenital cardiac catheterization.
Résumé
Un stent est une endoprothèse maillée tubulaire qui a contribué à l’essor du cathétérisme interventionnel sur les trente dernières années. En cardiologie pédiatrique et congénitale, les stents ont offert de nouvelles solutions pour traiter des sténoses congénitales des vaisseaux ou des lésions postopératoires, pour maintenir ou occlure la perméabilité d’un shunt et pour permettre le remplacement valvulaire par voie percutanée. À l’origine, les stents étaient formés d’une structure métallique « nue ». Puis plusieurs structures de stents ont été dessinées pour un meilleur compromis entre force radiaire et flexibilité. Néanmoins, la plupart des stents actuellement utilisés n’ont pas été développés pour les lésions congénitales de l’enfant et sont utilisés « off-label ». De plus, l’incapacité des stents à suivre la croissance naturelle des vaisseaux limite leur usage chez les enfants de faible poids et chez les nourrissons. Des stents biorésorbables ont récemment été développés avec la capacité d’être dégradés par l’organisme. Dans cette revue générale, nous avons souhaité décrire l’historique de développement des stents, les caractéristiques techniques des stents actuellement utilisés, les applications cliniques et les résultats, les dernières évolutions technologiques et les perspectives dans le domaine des cardiopathies congénitales.
Background
Since the first report of transcatheter balloon dilation of pulmonary stenosis in 1953 , balloon angioplasty has been widely used for many valvular and vascular congenital lesions . However, ineffective relief of obstruction and vessel damage have been observed . A ‘stent’ is a tubular meshed endoprosthesis that has contributed to overcoming these issues . In this general review, we will first describe the pioneering reports of stents development. Next, we will investigate the technical aspects of the available stents. Concepts sustaining the use of stents in congenital heart disease (CHD) catheterization, as well as clinical applications and complications, will be underlined. Finally, future directions will be discussed.
Historical background
The origins of the word ‘stent’ remain controversial; it may be an old English word derived from the verb ‘stenten’. The Latin root is ‘extendere’, meaning to stretch out. Conversely, the word may have been first used in 1916 by Jan F. Esser, a Dutch plastic surgeon, to describe a medical prosthesis created by an English dentist, Charles Stent (1807–1885) . In 1964, Charles Dotter suggested that an implantable prosthetic device might be used to maintain the luminal integrity of diseased vessels. A precursor stent was successfully positioned in femoral dog arteries, but with secondary dislocations and narrowing . Therefore, it was not until the early 1980s that stents regained interest, with a self-expandable stent with a memory-of-shape property . The first human intracoronary stent (Wallstent™; Boston Scientific, Natick, MA, USA), made of a self-expandable stainless-steel mesh, was successfully deployed in coronary arteries by Jacques Puel in 1986 in CHU Toulouse .
Stents rapidly increased interest in the interventional treatment of CHD. In the mid-1980s, Julio C. Palmaz developed a balloon-expandable stent that was successfully implanted in various locations in 1991 . Later, in the early 2000s, Philipp Bonhoeffer and Younes Boudjemline used a stent as a support to anchor a valve in a right ventricle-to-pulmonary artery (RV-to-PA) prosthetic conduit with valve dysfunction .
The first stent available for use in children was the balloon-expandable closed-cell design Palmaz™ stent (Johnson & Johnson Interventional Systems, Warren, NJ, USA) . Since then, many others stents have been developed, including the Genesis™ stent (Johnson & Johnson Interventional Systems, Warren, NJ, USA) , the Cheatham-Platinum™ (CP) stent (NuMED, Hopkinton, NY, USA) , the IntraStent™ DoubleStrut™ stent (eV3 Inc., Plymouth, MN, USA) and others. Today, the most commonly used stents are the series of eV3 stents (eV3 Inc., Plymouth, MN, USA), the CP stent , the Valeo™ stent (Bard Peripheral Vascular, Tempe, AZ, USA) and valved stents (Melody ® valve, Medtronic Inc., Minneapolis, MN, USA; Edwards-Sapien™ Valve, Edwards Lifesciences LLC, Irvine, CA, USA) . New stents have emerged with promising results in CHD catheterization, such as the Andrastent™ stent (Andramed, Reutlingen, Germany) and the Formula™ stent (Cook Europe, Bjaeverskov, Denmark) . Almost all stents used currently in children have not been approved for vascular lesions in children and are used ‘off-label’.
Stent design
Stents are categorized according to mechanism of delivery, composition, configuration, size and properties. Stent performance depends on material characteristics, form, fabrication mode and geometry. Performances of stents used in CHD are described in eTables 1–3 in the online-only data supplement.
Stent delivery mechanism
Two stent delivery mechanisms are available. Stents are balloon-expandable or self-expandable. In the balloon-expandable system, the stent is crimped manually onto a balloon or premounted by the manufacturer. The balloon catheter stent is moved forward on an intravascular guidewire through a long sheath. The stent is then deployed in the area of interest by inflating the balloon. The stent final diameter is determined by the inflated balloon diameter. The balloon inflation allows a high radial force during stent implantation to relieve vascular obstruction. Postdilation with a larger balloon is possible to further increase stent diameter .
Premounted stents, due to superior nesting to the balloon, have a lower profile and do not require long and large sheaths, enhancing their ‘trackability’ in reaching tortuous destinations and facilitating their use in small children. In PA stenting, the use of premounted stents seems to be associated with fewer complications . The balloon plays a pivotal role in the success of the stent positioning. The Balloon-In-Balloon catheter (BIB ® ; NuMED, Hopkinton, NY, USA) was specifically designed for medium-to-large stent delivery. The inner smaller balloon is inflated first, partially expanding the stent without flaring, and allows repositioning of the stent before final deployment, when the outer balloon is inflated ( Figs. 1 and 2 ; Video 1 ) . Balloon-expandable stents, premounted or not, should always be delivered through a long covering sheath. The covering sheath facilitates stent positioning control and prevents involuntary uncrimping.
The Ensemble™ system (Medtronic Inc., Minneapolis, MN, USA) was specifically developed to facilitate introduction, delivery and deployment of the balloon-expandable Melody valved stent; it incorporates a BIB balloon and a mechanism with a covering sheath to protect the valve as it is advanced through the right ventricular outflow tract (RVOT) ( Figs. 3 and 4 ; Video 2 ). Balloon-expandable stents have a foreshortening phenomenon during inflation. The degree of foreshortening depends on the stent configuration and is maximal at larger diameters. Stent foreshortening must be anticipated before delivery to be certain that it will completely cover the treated lesion.
Self-expandable stents are constrained by a covering sheath. After positioning in the area of interest, the covering sheath is withdrawn and the stent regains its original shape . Self-expandable stents are made of an alloy of nickel and titanium, with a memory-of-shape property, but with less radial strength than most balloon-expandable stents and therefore a higher risk of vessel recoil. Once in place, further expansion of the self-expandable stent is not possible, unlike balloon-expandable stents, which can be further dilated at a later time . This property is essential in small children with growing vessels. Many interventionists have advised against the use of self-expandable stents in growing children .
Composition
Stents are mostly made of metals, allowing high radial strength. Stainless steel is resistant to corrosion and is easily deformable. Cobalt chromium alloys may allow lower crimping profiles with high radial strength. Stents made with metal get incorporated into the vessel wall and do not have the potential to grow or to be degraded by the organism. A variety of degradable materials have been studied for biodegradable stent design, including polyesters, polycarbonates, bacterial-derived polymers and corrodible metals such as magnesium and iron .
Configuration
Two types of stent frameworks have been designed. The ‘slotted-tube’ design means that the stent is formed from a unique stalk cut by laser from a tube. Premounted stents are cut in the crimped position, allowing a low profile. The other stent framework consists of multiple circular stalks repeatedly curved as a crown and linked by connectors. The stent is manufactured from a wire, which is bent and welded to a cylindrical meshwork; this leads to more adjustable flexibility and wide size and length ranges, but the radial strength is usually less than that for stents with slotted tubes.
In an ‘open-cell’ design, geometry does not connect consistently throughout the stent, forming incomplete and non-bridged cells. With expansion, the individual cells merge to form larger open areas. This type of stent has less foreshortening, good conformability and fits well in curved vessels. Depending on the curve of the stent, the cell area may vary. In vessel bifurcation stenting (PA bifurcation, aortic arch), the perfusion of a jailed side branch is unlikely to be altered with an open-cell stent. If it happens, a balloon reopening of the struts (‘kissing’) will be easier with this type of stent and with the low number of connectors ( Figs. 4 and 5 ; Video 2 ).
Alternatively, a stent with many connectors, such as the CP stent, is called a ‘closed-cell’ stent. It has a high radial strength and a high vessel scaffolding area, regardless of the degree of bending. A high scaffolding area allows better protection of a vessel with a parietal tear and prevents parietal tissue hernia between the stent struts.
Covered stents
Some stents are covered by a membrane. Covered stents were originally developed to seal perforated and ruptured arteries. The primary use of covered stents in CHD has been advocated in subatretic coarctation , native coarctation associated with patent ductus arteriosus and coarctation combined with aneurysm . Covered stents should not jail vessels, leading to anterograde flow obstruction.
Stent properties: characteristics of the ideal stent in congenital heart disease
Fifteen particular stent properties are expected for maximal performance and are summarized in Table 1 . Currently available stents do not have all these requirements. As in a rugby team, in which wingers run faster but are weaker than pillars, the choice of stent will be a compromise between priorities and expected properties. The choice integrates the age and size of the patient, the expected adult dimensions of the vascular structure, the morphology of the lesion to treat and expected future surgical procedures.
| 1 | Radiopacity | High radio-opacity facilitates stent positioning before implantation |
| 2 | Low profile | A stent with a low profile decreases the delivery sheath size |
| 3 | Good crimpability | Premounted stents or easy hand crimping with stability of the stent onto the delivery balloon are expected |
| 4 | High flexibility | High flexibility allows delivery in tortuous vessels like pulmonary arteries, gives compliance to the target area and ease of manoeuvrability during deployment |
| 5 | Good conformability | A stent with good conformability will fit well with the vessel geometry and curves, and will protect from vessel distortion |
| 6 | No foreshortening | A predictable expansion diameter with a low degree of stent foreshortening is expected during stent expansion for precision of positioning and to better match the length of lesion to be treated |
| 7 | High radial strength | Stents must have a high radial strength to resist external radial forces of the vessel wall, prevent vessel recoil and keep tight and scarred lesions open |
| 8 | High scaffolding | High scaffolding of the vessel is necessary to prevent parietal tissue protrusion and risk of restenosis |
| 9 | Retrievability | Stent retrievability and repositioning decrease the risk of malpositioning and embolization |
| 10 | Wide struts | Wide struts are expected to maintain blood flow to jailed vessel branches |
| 11 | Soft edges | Rounded and soft edges will prevent vascular tears and balloon rupture during delivery |
| 12 | Potential to grow | An ideal stent implanted in small children would follow the natural growth of the vessel; on the other hand, a stent must be redilatable until the expected adult diameter is reached |
| 13 | Solidity | The framework must be solid enough to resist fracture; loss of integrity decreases the radial strength and increases the risk of restenosis |
| 14 | Imaging compatibility | The stent should be compatible with all imaging modalities without artefacts |
| 15 | Biocompatibility | Biocompatibility must be high, with resistance to thrombus formation, corrosion and unwanted inflammatory or allergic reactions, and avoidance of neointimal proliferation |
Concepts sustaining the use of stents in congenital heart disease
Stent clinical applications in catheterization of CHD are summarized in Table 2 and follow recommendations .
| Stenting localization | Type of stents used most frequently | Refs. |
|---|---|---|
| Stenting to improve efficiency of balloon angioplasty | ||
| Lobar PA stenosis | Formula a , Valeo b , Genesis c , Express Vascular d , coronary stents, IntraStent Max LD | |
| Main branch PA stenosis | eV3 series e , CP8-Zig f , Palmaz c , Genesis, Andrastent XL g | |
| RVOT obstruction in neonates & infants (palliative procedure) | Coronary & small peripheral bare-metal stents, Formula | |
| RVOT obstruction in children & adults | Covered CP8-Zig, CP8-Zig, IntraStent Max LD, Palmaz, Genesis | |
| RV-to-PA conduit obstruction | IntraStent Max LD, CP8-Zig, Palmaz, Genesis, Andrastent XL | |
| Fontan Tunnel obstruction | IntraStent Max LD, CP8-Zig, Palmaz, Genesis, Andrastent XL | |
| PA rehabilitation in infants | Formula, Valeo, Genesis | |
| Vena cava obstruction | IntraStent Max LD, CP8-Zig, Palmaz, Genesis, Advanta V12 h | |
| Postoperative or congenital PV stenosis | Palmaz, Genesis, covered iCAST h , Valeo, coronary stents | |
| Stenting to improve safety of balloon angioplasty | ||
| Aortic coarctation or Aortic recoarctation | Covered CP8-Zig, CP8-Zig, IntraStent Max LD, Andrastent XL, Palmaz, Genesis, covered Advanta V12 | |
| RVOT | Covered CP8-Zig, CP8-Zig, IntraStent Max LD, Palmaz, Genesis, Advanta V12 | |
| PA dissection or rupture | Covered CP8-Zig, CP8-Zig, IntraStent Max LD, Palmaz, Genesis | |
| Mustard/Senning baffle stenosis | CP8-Zig, IntraStent Max LD, Andrastent XL, Palmaz, Genesis | |
| Stenting to close shunt | ||
| Fontan fenestration | Covered CP8-Zig, Advanta V12 | |
| Potts shunt | Covered CP8-Zig | |
| Mustard/Senning baffle leaks | Covered CP8-Zig | |
| Stenting to maintain shunt | ||
| Ductus Arteriosus | Valeo, coronary & peripheral balloon-expandable bare-metal stents, coronary & peripheral self-expandable stents, Palmaz | |
| Atrial septectomy | Coronary balloon-expandable bare-metal stents | |
| Blalock-Taussig shunt | Coronary balloon-expandable bare-metal stents, drug-eluting coronary stents | |
| Valved stent | ||
| Pulmonary regurgitation with RV dilation | Melody valve i , Edwards-Sapien valve j | |
| RVOT obstruction | Melody valve, Edwards-Sapien valve | |
| RV-to-PA conduit | Melody valve, Edwards-Sapien valve | |
| ‘Prestenting’ of the landing zone | CP8-Zig, IntraStent Max LD, Palmaz, Genesis |
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