27 11 2006 005 jpg Origin The question

27 11 2006 005. jpg

Origin The question remains Who first used (published) the word stent for the vascular prosthesis?

Transluminal expandable nitinol coil stent grafting: preliminary History report. Radiology 147: 259 Dotter CT, (1983) Nitinol coil stent used by Dotter A coil wound to a small diameter and delivered through a catheter into the vessel would expand to a larger diameter, upon warming with 60°C saline solution

HISTORY 1964: Dotter and Judkins proposed the use of percutaneously introduced prosthetic devices to maintain the luminal integrity of diseased blood vessels 1977: Grüntzig, coronary angioplasty 1985. Palmaz et al. introduced the use of balloon mounted stents in peripheral arteries

HISTORY 1986 Schatz et al. modified the Palmaz stent, which led to the development of the first commercially successful stent, the Palmaz– Schatz stent. 1986 Puel & Sigwart were the first to implant a stent in humans they used a self expanding mesh device. 1987 Puel & Sigwart They were also the first to describe the use of this stent for emergency vessel closure during balloon angioplasty, on the basis of the ability of the device to act as a scaffold to move intimal and medial flaps away from the lumen and maintain radial support to offset elastic recoil

Stent characteristics Method of deployment (έκπτυξη) Flexibility (ευκαμπτότητα) Crush (not crush) on external pressure (Σύνθλιψη) Radial strength, Hoop stress (ακτινική δύναμη) Radiopacity (ακτινοσκιερότητα) Predictability of final deployment location and length (Ακρίβεια στην τοποθέτηση , διατήρηση μήκους) • Ability to be repositioned prior to complete deployment (Επανατοποθέτηση) • Material of construction (υλικό κατασκευής) • • •

Stent characteristics Most pronounced characteristic Method of deployment • Self expanding • Balloon expanding

Stent characteristics Balloon expanding stents • Non mounted • Pre-mounted (προφορτωμένα)

Hoop stress The strength and elasticity of the diseased arterial wall may vary considerably the hoop stress (residual circumferential elasticity) that follows balloon dilatation may result in the collapse of a relatively weak stent after its placement. Η ικανότητα του stent να αντιστέκεται στην υπολειπόμενη κυκλοτερή ελαστικότητα της αρτηρίας

Ideal stent • High radiopacity for clear visualization, which facilitates accurate placement • High hoop strength to resist arterial recoil • Minimal or no foreshortening in deployment, for precise placement • Simple and easy-to-use delivery system • Longitudinal flexibility to cross tortuous vessels and aortic bifurcation with the contralateral approach • Radial elasticity to resist external compression without permanent deformation, especially at flexion sites • High expansion ratio and low profile for passage through small introducers or guiding catheters or through tight stenoses • Retrievability in case of faulty deployment • Side branch accessibility • Minimal induction of intimal hyperplasia • Resistance to thrombosis and corrosion • Durability

Development of an optimized electrochemical process for subsequent coating of 316 stainless steel for stents J Mater Sci: Mater Med (2006) 17: 647– 657 AFM micrographs– 316 stainless steel surfaces; (a) as-received; (b) electropolished for 1. 5 min at room temperature; (c) electropolished for 5 min at room temperature.

Corrosion

Balloon expanded stents Palmaz stent Non mounted High expandability Only few diameters (4 -9 mm, 8 -12 mm) Final diameter depends on the size of balloon used to deploy it • Ability to use in areas of vessel size changes e. g. common to external iliac artery? • •

Balloon expanded stents Palmaz stent Risk of dislodgment (undeployed stent from the balloon) WHEN? Turns in the vascular system, calcification Solution Always advancement within a sheath or Guide Catheter Technique for secure fixation in the balloon?

Balloon expanded stents Palmaz stent If partial or complete dislodgment occurs • Inside sheath • Outside sheath SOLUTION If possible deploy the stent in other vessel

Balloon expanded stents Palmaz stent Risk to open forward or backward of the balloon Solution Ensure that the stent remains centered on the balloon before initiating deployment

Balloon expanded stents Palmaz stent Risk of balloon rupture during deployment Fill the balloon faster than the fluid can leak out Use of 2 nd Balloon

Balloon expanded stents Palmaz stent • Risk of delivery fail at aortic bifurcation • Flexibility related • Use only short length balloons Palmaz Schatz (articulated stent flexibility)

Balloon expanded stents Corinthian stent (pre-mounted)

Self expanding stents • Best conformability (προσαρμογή)to changing vessel diameters • Best flexibility (ευκαμπτότητα) in undeployed and deployed state to pass or stent tortuous vessels • Smaller sheath requirements

Self expanding stents Construction • Coil wire (ελικοειδές σύρμα) • Braided wires (δίκτυο συρμάτων) • Thermal memory metals

Self expanding stents Wallstent • 1 st self-expandable stent Braided wires πλέξη συρμάτων • Made of a cobalt (Co)–chromium (Cr) alloy (Elgiloy), Mo was clinically applied in 1986 • Low strength at end-points • Corrosion resistance • Narrow range of expansion (large inventory of stents) • Maximal foreshortening , Accuracy only distally • Can be recaptured during deployment

Self expanding stents Wallstent • Good flexibility but • Apposition (επαφή) to vessel wall fair, may not make adequate wall contact • Avoid its use in tortous vessels • Modular designs (Laser cut Nitinol ) offer better apposition

Nitinol Properties Kink Resistance Extreme deformation of a Nitinol stent (Cordis SMART). The stent will recover after the load is removed Suitable in superficial vessels

Nitinol stents Temperature - dependent stiffness. Nitinol stents with a transition temperature of 30°C Weak when squeezed at low temperature. Much stiffer when squeezed at T above 30°.

Nitinol stents Evolution of Nitinol stent designs : development of the material Early on, Nitinol was only available in wire form. Early Nitinol stents were wire coils. Coil stents made from Nitinol wire are still available. • Non-vascular applications: Endocare’s Horizon Stent for the relief of bladder outlet obstruction), • Vascular applications: Intra. Coil Stent (Intratherapeutic) SFA and popliteal artery lesions. Advantage of simple wire coils: retrievability in certain applications. (Nitinol loses its stiffness when cooled. By chilling the stent it will become soft and pliable and can be retrieved from the prostate with a grasping forceps)

Wire-based stent designs Horizon stent (Endocare) implantation and retrieval

Wire-based stent designs

Wire-based stent designs ZA stent (Cook) Symphony stent (Boston Scientific) Συγκόλληση wire formed design with struts welded to form hexagonal cells Braided or weld wire struts: larger catheters

Sheet-based stent designs When Nitinol sheet (φύλλο) became available, Angiomed (Bard) developed the first laser-cut Nitinol stent by cutting a pattern from sheet, rolling it up, and welding at specific strut locations Memotherm stent

Sheet-based stent designs Endo. Tex stent Nitinol sheet to produce a series of windows and a locking feature at one edge. It is rolled up to a small diameter roll and placed onto a PTCA balloon. The assembly is then placed into the vessel and the diameter of the stent is adjusted by inflating the balloon. As the balloon expands, the stent uncoils to the desired diameter to prop open the vessel. The stent is locked into place by unique tabs that slide into the stent “windows” upon balloon deflation. This design provides a wide range of diameters to custom fit for each treatment. It combines balloon expandability with the superelasticity after deployment

Sheet-based stent designs Aspire Vascular architect’s stent “dual-rail ladder type” frame from Nitinol sheet and covered with e. PTFE. Proposed to keep vessel side branches open. Suitable in distal SFA, popliteal territory

Tube-based stent designs Presently Most self-expanding Nitinol stents are produced by laser cutting of Nitinol tubing.

Tube-based stent designs Laser-cut Nitinol tubular stents. Top: Memotherm (Bard Angiomed) bottom: SMART (Cordis) Laser-cut Nitinol tubular stents. Top: Jostent Self. X (Jomed) bottom: Dyna. Link (Guidant)

Radiopacity enhancements (χρήση ακτινοσκιερών δεικτών) To improve the fluoroscopic visibility of the Nitinol stents, radiopaque markers are often attached or integrated into the design of the stent. Compatibility issues have to be considered when using these material combinations

Radiopacity enhancements Coined Tantalum markers of the SMARTe. R stent (Cordis)

Radiopacity enhancements Nitinol marker of the Sinus stent (Optimed)

ΕΠΙΛΟΓΗ STENT Carotid angioplasty and stenting

ΕΠΙΛΟΓΗ STENT • • Carotid (self-expandable, braided or nitinol) Vertebral (balloon expandable) Subclavian (balloon expandable) Inominate (balloon expandable) Renal (balloon expandable) Aorta (nitinol, balloon expandable) Common Iliac (balloon expandable, nitinol) Ext. Iliac (nitinol)

ΕΠΙΛΟΓΗ STENT • • • Internal Iliac (balloon expandable) Common Femoral (nitinol) Deep Femoral (balloon expandable) SFA (nitinol, PTA only) Popliteal (sheet, coil nitinol, PTA only) Infrapopliteal (unknown)

DES • Currently investigational for non coronary applications • Results from ZILVER PTX and STRIDES non randomized controlled trials are anticipated • DES in SFA provide no additional benefit over current nitinol BMS (COMPLEX LESIONS? ) • Preliminary results in the BK arteries are encouraging (Siamblis 2005, Bosiesr 2006, Commeau 2006)

Bioabsorbable stents The bioabsorbable magnesium-alloy stent (BIOTRONIK AG) was first implanted safely in infrapopliteal and human coronary arteries

Bioabsorbable stents POLYMERS Polylactic, polyglycolic acid Limitation Lower strength Greater bulk Atrophic changes of the arterial wall may lead to aneurysmal dilatation METALS Mg Metals Limitation: Inflammatory response elicited after implantation

Bioabsorbable stents AMS INSIGHT—Absorbable Metal Stent Implantation for Treatment of Below-the-Knee Critical Limb Ischemia: 6 -Month Analysis Marc Bosiers Ζ On Behalf of the AMS INSIGHT Investigators

Bioabsorbable stents Second generation AMS is under development • Optimization of the Mg alloy resulting in a longer degradation time • Stent design with increased radial force • Stent sizes should be better adapted to peripheral lesions.

Carotid angioplasty and stenting

• As • can be seen from this graph, the chronic outward force • actually doubles when the temperature is increased from • 20 to 37°C.

However, the positive electrical potential of metals and alloys may not be detrimental, because it is one of the reasons for the initial attraction of plasma proteins that cover the artificial surface with a layer of fibninogen 5 -20 nm thick in a few seconds. This proteinaceous layer passivates the metal surface, decreasing its thrombogenicity before the arrival of platelets and WBCs.

• • • • • Guidant Dynalink Laser cut tube – Intratherapeutics Intra. Coil Wire coil – Bard Memotherm Laser cut tube – Intratherapeutics Protege Laser cut tube Bard Memotherm-Flexx Laser cut tube – – Bard Luminexx Laser cut tube Intratherapeutics Protege GPS Laser cut Braun Vascuflex SE Laser cut tube – tube Coined Ta markers Biotronik Philon Laser cut tube Intratherapeutics Endo. Coil Flat wire coil – BSC Radius Laser cut tube – BSC Symphony Welded wire Sleeve Intratherapeutics Esopha. Coil-SR Flat wire BSC Ultraflex Knitted wire – coil – Bolton Medical Sprinter Braided wire – Jomed Jostent Self. X Laser cut tube – Campus Laser cut tube – Jotec Flow. Stent Diamond Laser cut tube Cook ZA Knitted wire Sleeve Au markers Cook Zilver Laser cut tube Coined Au markers DLC coated Medicorp Expander Braided wire – Cordis SMART Laser cut tube – Medtronik AVE Bridge SE Laser cut tube Cordis SMARTe. R Laser cut tube Coined Ta markers – Cordis SMARTControl Laser cut tube Coined Ta Optimed Sinus Laser cut tube – markers Optimed Sinus-Aorta Laser cut tube Cordis Precise Laser cut tube – Optimed Sinus-Flex Laser cut tube DLC Endo. Care Horizon Flat wire coil – coated (opt) Endo. Tex Nex. Stent Laser cut tube – Engineers and Doctors Memokath Wire Coil – Optimed Sinus-TIPPS Laser cut tube Pre. Flex. Stent Medical Flex. Stent Braided wire Au shaped coated Optimed Sinus-REPO Laser cut tube DLC coated (opt)

Why Stent? Intravascular stents are generally intended to a) oppose recoil of elastic vascular stenoses after balloon angioplasty has failed to do so b) They are also intended to provide a framework or scaffold for arterial lesions that are likely to dissect or embolize after balloon angioplasty.

Expansibility ratio (Δείκτης Διατασιμότητας) The mesh configuration of the stent dictates: the degree of shortening that occurs with a given stent diameter the proportion of open space to metal surface the ratio of unexpanded to expanded diameter This ratio determines how small a non expanded stent can be to achieve a given expanded diameter. A stent with an expansibility ratio of 4 and an optimal expanded diameter of 5 mm will have an unexpanded diameter of 1. 25 mm, or 3. 75 French. large expansibility ratio → small the diam. of the introducer catheter

Initial Events After Stent Implantation Net electrical charge or potential of the surface is intuitively relevant, as metals and alloys used for most intravascular devices are electropositive in electrolytic solutions, whereas all biological intravascular surfaces are negatively changed.

Initial Events After Stent Implantation The positive electrical potential of metals and alloys it is one of the reasons for the initial attraction of plasma proteins that cover the artificial surface with a layer of fibninogen 5 -20 nm thick in a few seconds. This proteinaceous layer passivates the metal surface, decreasing its thrombogenicity before the arrival of platelets and WBCs. (passivation is the –spontaneous- formation of a hard nonreactive surface film that inhibits further corrosion)

J Appl Physiol 97: 424– 430, 2004. Stent design properties and deployment ratio influence indexes of wall shear stress: a three-dimensional computational fluid dynamics investigation within a normal artery The results suggest that stent designs that reduce strut number and thickness are less likely to subject the vessel to distributions of WSS associated with intimal hyperplasia

Stent coating Prosthetic 1. Passive (pyrolytic carbon, urethanes with favorable surface characteristics to reduce thrombogenicity 2. Active (drugs, heparin, DES) Biological (Cellular adhesion molecules to promote endothelization)