2003 Stoeckel Self Expanding Nitinol Stents

7/30/2019 2003 Stoeckel Self Expanding Nitinol Stents

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47533 Westinghouse Drive Fremont, California 94539 t 510.683.2000 f510.683.2001

We are Nitinol.

www.nitinol.com

SelfExpandingNitinolStentsMaterialandDesignConsiderations

Stoeckel,Pelton,Duerig

EuropeanRadiology

2003


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Self-Expanding Nitinol Stents -Material and Design ConsiderationsDieter Stoeckel, Alan Pelton, Tom Duerig

Nitinol Devices & Componentsa Johnson & Johnson company47533 Westinghouse DriveFremont, CA 94539 USAPhone 510-623 -6996, Fax 510-623-6995

Abstract:

Nitinol (Nickel-Titanium) alloys exhibit a combination ofproperties which make these alloys particularly suited for selfexpanding stents. Some of these properties cannot be found in engineering materials used for stents today. The paperexplains the fundamental mechanism of shape memory and superelasticity and bow they relate to the characteristicperformance of self-expanding stents. Nitinol stents are manufactured to a size slightly larger than the target vessel sizeand delivered constrained in a delivery system. Afte r deployment they position themselves against the vessel wall with alow, chronic outward force . They resist outside forces with a significantly higher radial resistive force. Despite the highnickel content ofNitinol, its corrosion resistance and biocompatibility is equal to that of other implant materials. Themost common Nitinol stents are listed and described.

IntroductionWhen Charles Dotter experimented with Nitinol wire coilsas intra-arterial scaffolds back in the early nineteeneighties, Nitinol was known only for its unusual shapememory effect [1]. A coil wound to a small diameter anddelivered through a catheter into the vessel, would expandto a larger diameter, e.g. the diameter of the vessel lumen,upon warming with 60C saline solution (Fig. 1) . Although the shape memory effect looked like ideally suitedfor the scaffolding of vessels, it took many more years forNitinol stents to appear in the market. Dotter clearly was

ahead ofhis time. The melting and processing ofNitinol,an intermetallic compound of titanium and nickel, had notbeen fully developed with consistent quality, nor had theproperties of this material been fully understood. Today,twenty years after Dotter s experiments, Nitinol stents areself-expanding without the need for post-deploymentheating. They are superelastic, i.e. crush recoverable. exerta gentle chronic outward force and are generally morephysiologically compatible than balloon-expandablestents. All major medical device companies as well as


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Fig.l Nitinol coil steat used by Dotter [11. coiled fordelivery and heat expanded

many smaller producers now offer Nitinol stents for(mainly peripheral) vascular and non-vascular indications.In the following, after a brief explanation of the mechanisms of shape memory and superelasticity, we willdescribe the unique material properties ofNitinol and howthey relate to the performance characteristics ofNitinolstents.Suoerelastjcjtv and Shape Memory in NjtjnolConventional stent materials, like stainless steel or cobaltbased alloys, exhibit a distinctly differem elastic de formation behavior from that of the structW'a1materials of theliving body. The elastic deformation of these metals andalloys is limited to approx. 1% strain, and elongationtypically increases and decreases linearly (proportionally)with the applied force. In contrast, natural materials, likehair, tendon and bone can be elastically deformed, in somecases, up to 10% strain in a non-linear way [2]. When thedeforming stress is released, the strain is recovered atlower stresses. As shown in Fig. 2 , the loading/unloadingcycle is characterized by a pronounced hysteresis.

Hair

Nitinol

BoneTendon

StrainFig. 2 Biomechanical compatibility ofNitinol: deforma-tion characteristics ofNitinol and living tissues [2]

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A s imilar behavior is found with Nitinol alloys, equiatomicor near-equiatomic intermetallic compounds of titaniumand nickel. Fig. 3 shows a characteristic stress/strain curvefor a Nitinol alloy wire at body temperature (as will besbown later, the properties ofNitinol alloys are stronglytemperature dependent). As with natural materials, theloading and unloading curves show plateaus, along whichlarge deflections (strains) can be accumulated on loading,or recovered on unloading, without significant increase, ordecrease, respectively, in loads (stress). Because deformatiOIl of more than 10% strain can be elastically recovered,this behavior is called superelasticity .

+-- ... .......... .. ..Strain

Fig . 3 Schematic stress-strain diagram for Nitinol andstainless steel

Superelastic Nitinol appears macroscopically to be simplyvery elastic. However, the mechanism of deformation isquite different from conventional elasticity, or simplystretching of atomic bonds. When a stress is applied toNitinol, and after a rather modest elastic deformation, thematerial yields to the applied stress by changing its crystalstructure. TItis stress induced phase transformationallows the material to change shape as a direct response tothe applied stress. When the stresses are removed, thematerial revens to the original structure and recovers itsoriginal shape . While superelasticity is the result of a stressinduced phase transformation, shape memory is the resultof a thermal phase transformation. In fact, whensuperelastic Nitinol is cooled to below a critical temperature (the transformation temperature, which is dependenton alloy composition and processing history), it alsochanges its crystal structure. If no force is applied, thisphase change is not accompanied by a shape change. Thematerial can be plastically deformed in the low temperature phase , but the original shape can be restored bybeating above the transformation temperature (3].Self-expanding Nitinol stents are manufactured with adiameter larger than that of the target vessel. Theirtransfonnation temperature is typically set to 30 degrees C.They can be easily crimped at or be low room temperatureand placed in a delivery system. To prevent premature


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expansion during delivery into the body, the stent isconstrained by a retractable sheath or other means. At thetreatment site it is released from the delivery system andexpands until it hits the vessel wall and confonns to it.Now at body temperature , the stent is superelastic.Material ConsiderationsNitinol is an alloy composed of 55 w.% nickel and balancetitanium. It has found widespread acceptance as a materialof choice for medical implants and devices [4]. It derivesits unique properties from a solid state transfonnation,which can be triggered thermally or mechanically, and isdependent on the composition and processing history ofthe material. This adds another level of complexity to thematerial specification and may explain why ASTMspecifications [5,6,7] describing material composition andtest methods have only recently been issued. In addition to,or even instead of, the commonly known material characteristics like chemical composition, Young s modulus,yield strength, ultimate tensile strength and elongation tofailure, properties like transfonnation temperature, upperand lower plateau stress, recoverable strain and pennanentset have to be taken into account. As mentioned above,these properties are strongly dependent on the processinghistory and play an important role in the design andmanufacturing of self-expanding stents.

Biocompatibility and CorrosionI t is now well understood that Nitinol requires controlledprocessing to achieve optimal shape memory andsuperelastic properties [8]. In the same way, surfaceprocessing is required in order to promote optimal corrosion resistance and biocompatibility. Properly treatedNitinol implants are very corrosion resistant andbiocompatible [9]. Nitinol, like titanium and stainless steela.o., is a self-passivating material, i.e. it fonns a stablesurface oxide layer that protects the base material fromgeneral corrosion [10]. Considering the high nickel contentof the alloy, there are, understandably, concerns that nickelmay dissolve from the material due to corrosion and causeadverse effects. On the other hand, other alloys thatcontain high levels ofnickei, such as MP35N (a Co alloywith 35 weight % Ni), or 300 series stainless steel (approx.10 w.% Ni) exhibit good biocompatibility, and have longbeen used as implants in orthodontics, orthopedics andcardiovascular applications [11]. Several studies havemeasured nickel release during the exposure ofNitinolimplants to body fluids. During an in vitro dissolutionstudy ofNitinol dental archwires in saliva [12], it wasfound that Nitinol appliances released an average of 13.05mg/day nickel, which is significantly below the estimatedaverage dietary intake of200-300 mg/day. In another study[13], orthodontic patients with Nitinol appliances had Niconcentration in their blood measured during a period of 5

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months. Results showed no significant increase in thenickel blood level throughout the study.A comparative in vitro cell culture study [14] measurednickel release from Nitinol and 316L stainless steel i nfibroblast and osteoblast cell culture media. In both media,nickel levels were higher in the Nitinol group the first dayand decreased rapidly with time to achieve similar levelsas 316L after 8 days. I t is important to highlight that eventhough higher levels ofnickel were measured in theNitinol group, nickel did not reach toxic values and cellproliferation or cell growth near the implant surface wasnot affected. Furthennore , in this study, Nitinol was onlymechanically polished while stainless steel waselectropolished. The authors speculated that passivationtreatments, such as electropolishing, would decrease thenickel release from Nitinol. To evaluate the effect ofdifferent surface treatment methods on the Ni-ion release,Trepanier et al [15] immersed mechanically polished andelectropolished samples ofNitinol, MP35N and 316Lstainless steel in Hank s physiological solution at 37degrees C for a period of greater than 1000 hours (Fig 4).It was found that samples that were prepared by mechanical polishing released higher amounts ofNi-ions thanthose prepared by electropolishing. Surface analysis datademonstrate that the electropolishing process removesexcess nickel from the surface and fonns a layer enrichedin titanium (in the fonn ofTi02). In contrast, the mechani-

150; 100&l.soz

...... MPNm

........ EPNiTI_ MP316L_ _ EP316L_ MPMP35N-.- EPMP35N

10 100 1000 10000Time (hours)

Fig 4: Ni ion release from Nitinol, MP35N and stainlesssteel (MP: mechanically polished, EP: electropolished)Material Surface Condidon Ni:TI Ni:CrNitinol Mem. Polished 0.18Nitinol Electro lished 0.04MP35N Mem. P o l i ~ 0.4MP35N sivated 0.08316L 88 Mech. Polished Ol l316LSS EectrQ lished 0.07

Table I: Ratio ofNi to Ti in the surface of mechanically,electropolished or passivated samples ofNitinol, MP35Nand stainless steel


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1500 eiectropolished- - >E 1000 .s -ii

'":"p- "0

~ ."""'"- -o 500c: -t0 . . .Cuftal Deuity (AlaaJ)..,'"O"III 0 6

500 0.01 0.1 1.0 10 100 1000Oxide Th ickness (jim)

Fig. 5 Break-down potential as a function of oxide thickness on Nitinol (oxide created by varying heat treatment timeand temperature); insert: results of potentiostatic corrosion tests of Nitinol samples with elcctropolished and oxidizedsurfaces

cally polished samples have a relatively high concentrationof nickel in the surface (Table I) . Fwtbennore. themechanically polisbed Nitinol and MP35N samples showan increase in Ni ion release after 1000 bours. This maybe due to corrosion activity (Pitting) after the initial 1000bour time period in the non-passivated samples.ASTM standard F2129 provides a quantitative methodrecognized by the FDA for the accelerated assessment ofthe corrosion resistance of implant materials [16] . Themos t relevant data derived from this test is the break-downpotential Ew since most biomaterials corrode locally bypit formation. A high breakdown potential indicates thatthe material is very stable and resists pitting. Although noofficial limits have been established, materials with an ~;/> 500 m V are considered sufficiently corrosion resistantand safe for the use as implants. This value is used byCordis, a Johnson & Johnson company, as the internalstandard for all Nitinol implants. It corresponds with thecorrosion resistance of the stainless steel Palmaz-Schatzstent as a predicative device, the stent with the longestimplanlation history.Anodic polarization tests per ASTM F2129 have been usedto evaluate the influence of surface preparation on thecorrosion susceptibility ofNitinol stents. Trepanier at al.[17] have shown that electropolished Nitinol stents have

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excellent corrosion resistance with breakdown porentials greater than 800 mV, whereas the ~ of 000-electtopolished steats was on the order of200 mY. It wasfurther shown that the breakdown potential ofelectropolished stents was degraded to less than 500 m Vafter thermal treatments in the 400BC to 500J3C range. Thisled to the conclusion that optimal corrosion andbiocompatibility results are obtained with a thin, titaniumoxide (TiOl ) surface layer formed after electropolishing(passivation) treatments. It further appears that uniformity,rather ilian lhickness, of the oxide is most important toprotect the marerial from corrosioD. More recent srudies[ \8 ] correlate ~ wiili the thickness of the oxide layercreated by beat-treating electropolisbed Nitinol samples(Fig.S).To improve the radiopacity ofNitinol stents, markers areoften attached to the stent struts. However, when couplingNitinol with dissimilar materials, galvanic corrosioneffects have to be considered Markers are typically madefrom high density materials like gold, platinum, ortantalum. Nitinol and tantalum are galvanically similar andthus, the combination has no significant effect on theco rrosion resistance. In contrast, gold and platinum aremore noble than Nitinol (or stainless steel) and can causesevere galvanic corrosion of the Nitinol (or stainless steel)stenl. Therefore, the use of the noble metals as markers


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requires either an insulating layer between the stent andthe marker or the assembly has to be coated with aprotective coating.In 1999, the medical community as well as the deviceindustry were alerted to the corrosion issue by repons byRiepe et al [19] on the observation of severely corrodedNitinol graft scaffolds from expl anted Stentor aortic stentgrafts after 5 months implantat ion (Fig. 6). It was preliminarily speculated that cell-induced electrochemicalcorrosion or active cellular destruction of the surfaces(e.g., osteo-clasts-bone) might have been responsible forthe severe corrosion. However, subsequent cell culturetesting with Nitinol test samples performed by Riepe sgroup did not induce any corrosion [20]. Further analysisof the failed components revealed an oxide thickness of0.2-0.3 J.Lm (determined by Auger analysis) and an Ebd of280 mV (from anodic polar ization tests). In contrast, 12month explants of electropolished graft scaffolds examinedby Pelton et al showed no signs of corrosion. The oxidethickness on these devices was approximately 0.01 !lID andthe E..t > 900 mV. This highlights the importance ofoptimized surface preparation. Most Nitinol stentsmarketed today have electropolished surfaces. There havebeen no further reports on corrosion cases.

' t ! '\ 'tit , .

Fig 6: top: heavily corroded Nitinol explant (5 months[19]), bottom: electropolished Nitinol explant (12 months,with Ta marker attached)

Material Specific Device CharacteristicsThe most unusual property ofNitinol alloys is stresshysteresis. While in most engineering materials stressincreases linearly with strain upon loading and decreases

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along the same path upon unloading (as shown in Fig. 3with steel as an example), Nitinol exhibits a distinctlydifferent behaviour. After an initial linear increase in stresswith strain, large strains can be obtained with only a smallfurther stress increase. lhis is called the loading plateau.The end of this plateau is reached at about 8% strain.Unloading from the end of the plateau region, causes thestress to decrease rapidly until a lower plateau ( unloadingplateau) is reached. Strain is recovered in this region withonly a small decrease in stress. The last portion of thedeforming strain is finally recovered in a linear fashion.

@COF

a _ Stent Diameter StrainFig. 7 Schematic stress hysteresis and concept of biasedstiffuess as demonstrated with the cycle insert ion intodelivery system/deployment/compression of a stent

The stress hysteresis or path dependence ofNitinol resultsin a device feature termed biased stiffness [21]. lhisconcept is illustrated in Figure 7, which again shows aschematic superelastic stress-strain curve for Nitinol,illustrating both non-linear response and hysteresis. Usingthis graph, we will follow the cycle of crimping a stentinto a delivery system, deploying it and have it expand andinteract with the vessel. For this purpose, the axes havebeen changed from stress - strain to hoop force - stentdiameter. A stent of a given size larger than the vessel(point a) is crimped into a delivery system (point b),then packaged, sterilized and shipped. After insertion tothe target site, the stent is released into a vessel, expandingfrom b until movement is stopped by impingement withthe vessel (point c). At this point, further expansion ofthe stent is prevented. Because the stent did not expand toits pre-set shape, it continues to exert a low outward force,termed chronic outward orce or COE However, it willresist recoil pressures or any other external compressionforces wit h forces dictated by the loading curve from pointc to d, which is substantially steeper (stif fer) than theunloading line (towards e). These forces are calledradial resistive forces or RRF [22].


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Mylar loop

8 6 4 2Diameter (mm)

Fig. 8 UnJoading curves ofNitinol stents (Cordis SMARn at different deployment temperatures; insert radial forcetest setup, schematic

The unusual elastic hysteresis of Nitinol anows thecontinuing opening force of the stent acting on the vesselwall, COF, to remain very low even through large deflections and oversizing of the stent. Meanwhile the forcesgenerated by the stent to resist compression, RRF, increaserapid1y with deflection until the plateau stress is reached.Although most selfexpanding stent placements arepreuded by a percutaneous transluminal balloonangioplasty, there are indications that the chronic outwardforce of a Nitinol stent placed without previous PTAcauses the vessel to remodel with less intimal hyperplasiathan if PTA is performed prior to stenting (23).Another unusual feature ofNitinol stents is their temperalUre dependent stiffness. Stents with a transition temperature of 30 degrees C feel quite weak when squeezed orcrushed at room or lower temperature. In contrast, theyfeel much stiffer when squeezed at temperatures above 30degrees. Fig. 8 shows acmal unloading curves of a Nitinolstent (Cordis SMART Stent) with a diameter of 10 mm atdifferent temperatures. Th e test setup (insert) is describedin [24]. As can be seen from this graph, the chronicoutward force actually doubles when the temperature isincreased from 20 to 37 degrees C. As mentioned before,the transition temperature of the stent can be adjusted to acertain extent during processing. This gives the designeranother option to increase or decrease the radial forces ofthe stent without changing the design or physical dimensioos. as for each degree that the transition temperature isbelow body temperature, the loading and unloading forcesincrease by approximately 4 N/mml.Kink resistance is an important feature ofN itinol forstents in superficial vessels that could be deformed

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through outside forces. Th e carotid artery is a primeexample. There is a perceived risk for balloon-expandablestents in carotid arteries to be permanently deformedthrough outside pressure resulting in a partially or com-pletely blocked vessel. once the buckling strength of thestent is exceeded. Although Nitinol stents typically don thave the buckling strength of stainless steel stents, theycannot be permanently deformed through outside forces.Nitiool stents can be completely compressed (crushed) flatand will return to their original diameter when the deforming force is removed (Fig. 9). A quantitative analysis of theforces relevant to the performance of superelastic stentscan be found in [22].

Fig. 9 Extrem deformation ofa Nitinol stent (CordisSMART); the stent will recover after the load is removedNitino l is non-ferromagnetic with a lower magneticsusceptibility than stainless steel. MRI compatibility isdirectly related to the susceptibility properties of a


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material, relative to human tissue. Therefore, Nitinolproduces less artifacts than stainless steel, similar to puretitanium. It has to be noted. however, that processing ofthe material can influence the quality of the MR imagesubstantially.Nitinol Stent DesignsIn the following, we will tty to list and describe the selfexpanding Nitinol stents currently being marketed or inevaluation (Table 2). Designs included in this survey havebeen documented in brochures and company websites.Like others, this review is clearly not complete and maydescribe stents that are not yet, no longer, or not worldwide available.

(Microvasive, BSC). Newer designs are the ZA biliaryStent (Cook, Fig. 13), a modified knitted design, and thebraided Expander Stent (Medicorp). The Boston ScientificSymphony Stem is a wire formed design with strutswelded to form hexagonal cells. While wire based stentsgenerally are very flexible, the Symphony Stent is quiterigid (Fig.14).

Wire-based Stent Designs Fig. 11: Deployment and retreaval (far right) of theHorizon stent (EndoCare)The evolution ofNitinol stent designs is clearly linked tothe development of the material itself Early on, Nitinolwas only available in wire form. Consequently, earlyNitinol stents were wire coils, similar to Dotter s experimental device. Today, coil stents made from round or flatNitinol wire are still available. They are mainly used fornon-vascular applications (e.g. Endocare s Horizon Stemfor the reliefof bladder outlet obstruction), with theexception of the IntraCoil Stent (Intratherapeutics, Fig.10), which is indicated for the treatment ofpatients withsuperficial femoral artery and popliteal artery lesions. Oneadvantage of simple wire coils is their retrievability in Fig. 12: Cragg Stentcertain applications. As described earlier, Nit inollooses itsstiffuess when cooled. The EndoCare Horizon or the D&EMemokath prostatic stents can be retrieved from theprostate by chilling the device with cold solution. Thestents become soft and pliable and can be retrieved with agrasping forceps (Fig. II ).

Fig. 10 Intracoil stent (IntraTherapeutics)Other early wire based stent designs are the Cragg Stent(MinTec, Fig. 12), a sinusoidal coil with peak-to-valleysuture connections for vascular and non-vascular applications, and the knitted Ultraflex Esophageal Stent

Fig. 13: Cook ZA knitted stent

Fig. 14: Welded Symphony Stent (BSC)

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Compa ny Name Product Name Fabrication Method ConunentsBani MelIDtherrn Laser cut tube [25]Bani MelIDtherrn-Flexx Laser cut tubeBani Luminexx Laser cut tube Welded Ta markers 2BBraun Vascuflex SE Laser cut tube 2Biotronik Philon Laser cut tube SiC coated 2BSC Radius Laser cut tubeBSC Symphooy Welded wire Sleeve PtIr markers I [29] !BSC Ultraflex Knitted wireBolton Medical Sprinter Braided wire 30Campus Cam"", Laser cut tube I [31]Cook ZA Knitted wire Sleeve Au markO'S 32Cook Zilver Laser cut tube Coined Au markersCordis SMART Laser cut tube I [33 ,,Cordis SMARTeR Laser cut tube Coined Ta markersCordis SMARTControl Laser cut tube Coined Ta markersCordis Precise Laser cut tubeEndoCare Horizon Flat wire co il 34EndoTex NexStent Laser cut tube 3Enlrineers&Doc tors MetrJ)kath Wire Coil 3FlexStent Medical FlexStent Braided wire Au coated 3Guidant I Dynalink Laser cut tube 3lntratherapeutics IntraCoil Wire coil 3Intratheraoeutics Pmtg Laser cut tubeIntratherapeutics Pmtg GPS Laser cut tube Coined Ta markerslntratherapeutics EndoCoil Flat wire coilIntratherapeutics EsornaCoil -SR Flat wire co ilJomed Jostent SeIfX Laser cut tube 40Jotec FlowStem Diamond Laser cut tube DLC coated [41]Medicorp Expander Braided wire 42Medtronik AVE Bridge SE Laser cut tube 43I Optimed Sinus Laser cut tube [44]

I Optimed Sinus-Aorta Laser cut tubeI Ootimed Sinus-Flex Laser cut tube DLC coated (optI Optimed Sinus-TIPPS Laser cut tube Pre-shaoedI Optimed Sinus-REPO Laser cut tube DLC coated (opt)

Vascular Architects Aspire dual rail ladder coil ePTFE covered 451 ,

Table 2: List of popular Nitinol self-expanding stents

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Sheet-based Stent DesignsA perceived disadvantage ofbraided or knitted wire-basedstents is the crossing of the filaments. Ibis increases thewall thickness of the stent and the delivery profile.

Moreover, there are concerns about fretting corrosion orthe wear of the Nitinol at the cross-over points. WhenNitinol sheet became available, Angiomed (Bard) developed the first laser-cut Nitinol stent by cutting a patternfrom sheet. rolling it up and welding at specific strutlocations (Fig. 15).An interesting sheet based Nitinol stent is the experimentalratcheting EndoT ex stent, similar to the design suggestedby Sigwart (Fig. 16) [46]. It is chemically etched from thinNitinol sheet to produce a series ofwindows and a lockingfeature at one edge. It is rolled up to a small diameter rolland placed onto a PTCA balloon. The assembly is thenplaced into the vessel and the diameter of the stent isadjusted by inflating the balloon. As the balloon expands,the stent uncoils to the desired diameter to prop open thevesseL The stent is locked into place by unique tabs thatslide into the stent windows upon baloon deflation. Thisdesign provides a wide range ofdiameters to custom fit foreach treatment. I t combines balloon expandability with thesuperelasticity after deployment However, it has some ofthe perceived disadvantages of the knitted wire stents withnon-uniform cross-section and potential fretting cross-overpoints.

Fig. 15: Sheet-based Memothenn Stent withlap welded struts

...." '"

Fig. 16 Concept ofa sizable superelastic stent [44]

over-

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Vascular Architect s aSpire stent uses a dual-rail laddertype frame that is also etched from Nitinol sheet andcovered with ePFTFE. It is helically coiled onto a deliverysystem that allows deployment with a variable pitch tokeep vessel sidebranches open.Tube-based Stent DesignsIn the mid 1990s, Nitinol seamless tubing appeared in themarket in production quantities. With it came laser cuttingof tubular Nitinol components. Today, by far most selfexpanding Nitinol stents are produced by laser cutting ofNitinol tubing. Early examples are the Angiomed (Bard)Memotherm and the Scimed Radius stents. TheMemotherm was a rigid, closed-cell design with a diamond shaped pattern similar to the original Palmaz balloonexpandable stent. The Radius, on the other hand, is aflexible open-cell design with sequential rings and periodicpeak-to-peak. non-flex bridges. Most laser-cut Nitinolstents employ variations and/or combinations of thesebasic design features (Fig. 17, Fig. 18). There are Nitinolstents in the market that are coated with silicon carbide(SiC) or diamond like carbon (DLC). I t is probably fair tostate that these developments are more driven by productdifferentiation than actual scientific considerations [47].

Fig. 17: Laser-cut tubular Nitinol stents, left: SMARTStent (Cordis), right: Memotherm Stent (Bard)

Fig. 18: Laser-cut tubular Nitinol stents, top: Jostent SelfXStent (lomed), bottom: Dyoalink Stent (Guidant)


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Radiopacity EnhancementsTheoretical calculations as well as experimental srudiesshow that the radiopacity ofNitinol is similar to that ofstainless steel for equivalent dimensions. However, as thestent profiles continue to shrink to accommodate smallerdelivery systems, the cross section decreases with aconcomitant decrease in x-ray visibility. Therefore, toimprove the fluoroscopic visibility of the Nitinol stents,radiopaque markers are often attached or integrated intothe design of the stent. The Optimed Sinus stem family, forexample, features a set of tab markers at the stent ends thatare integral parts of the stem cut out of the tubing (Fig.19). The advantage of this approach is that there are nocompatibility issues, as no dissimilar metals are involved.On the other hand, it allows only moderate visibilityimprovement. Tantalum markers are riveted or coined intoeyelet-shaped tabs at the ends of the Cordis Smarter andSmartControl stents (Fig.20). As mentioned earlier,Tantalum and Nitinol are close together in the galvanicseries of metals, i.e. galvanic corrosion is not a problem.The Cook Zilver stent is of similar design, but uses goldmarkers instead of Tantalum. I t is assumed that the entirestent is coated with a thin polymer layer to protect it fonngalvanic corrosion.

Fig. 19 Nitinol marker of the Sinus stent (Optimed)

Fig. 20 Coined Tantalum markers of the SMARTeR stent(Cordis)

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Tantalum tabs are welded to the ends of the BardLuminexx stents (Fig. 21). Because of the large mass ofthese tabs, the X-ray visibility of this stent is very good.There are concerns, however, that brittle interface layerscan be created during welding ofNitinol and Tantalum.

Fig.2l Welded Tantalum markers of the Luminexx Stent(B",d)

Fig.22 Platinum-Iridium sleeve marker of the Symphonystent (Boston Scientific)Platinum-Iridium sleeves are used as markers for the wirebased BSC Symphony stent (Fig. 22) while the Cook ZAknitted stent uses Gold sleeves. As mentioned abovecompatibility issues have to be considered when usingthese material combinations.


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R sch J (I983) Transluminal expandable nitinolco il stem grafting; preliminary report.. Radio logy14n59

[2] ShabaJovskaya S (1996) On the nature of the (17)biocompatibility and medical applications ofNiTisbape memory and superelastic alloys. BioMedMat Eng 6: 267

[3] Duerig TW, Melton KN, Wayman CM , St ckel D [I S](1990) Engineering Aspects of Shape MemoryAlloys. Butterworth-Heinemann Ltd, London

[4] Stoeckel D (2000) Nitinol medical devices and [19 ]implants. Min Invas Ther & Allied Techno19:81

[5] ASTM F 2063-00 (2002) Standard Specificationfor Wrought Nickel-Titanium Shape MemoryAlloys for Medical Devices and Surgical Im-plants. [20 )

[6} ASTM F 2004-00 (2002) Test Method forTransformation Temperature of Nickel-Titanium [21 ]Alloys by Thermal Analysis.

(7] ASTM F 20S2-01 (2002) Method for the Deter- [22]mination of Transformation Temperature ofNickel-Titanium Shape Memory Alloys by Bendand Free Recovery. [23]

[S) Pelton AR, DiCello J, Miyazaki S (2000)Optimisation of processing and properties ofmedicaJ grade Nitinol wire. Min Invas Ther &Allied Technol9:107

[9] We ver DJ, Ve ldhuizen , AG, Sanders , MM , [24]Schakenraad, 1M (1997) Cytotoxic, allergic andgenotoxic activity of a nickel-titanium alloy.Biomaterials 18:1115.[10] Wever DJ, Veldhuizen, AG, de Vries, J, Busscher, [25]HJ, Uges, DRA, vanHom,JR{I99S) Electrochemi- [26]cal and surface characterization of a nickel-titanium [27]alloy. Biomaterials 19:761. [2S]

[11] Brown SA, Hugbes PJ, Merritt K (1988) In vitro [29]studies of fretting corrosion of orthopaedic materi- [30]als. J. Orthop. Res., 6:572. [31]

[12] Barren RD , Bishara SE , Quinn JK (1993) Biodeg- [32]radation of orthodontic appliances, part I biodegra- {33]dation of nickel and chromium in vitro. Am. J. [34]Orthod Dentofac. Orthop., 103:S. (35]

[13) Bislwa SE . Banett RD . Selim. MI (1993) Biodeg- [36)radation oforthodontic appliances, pann changes (37)in the blood level of nickel. Am. J. Ortbod. Deotofac. [3SJOnhop 103 ,1I5. [39 J

(14) Ryhanen J, Niemi E, Serlo W. Niemel E, Sandvik [40]P, Pernu H, Salo T (1997) Biocompatibility of [41]ruckel-titanium shape memory metal and its corro- [42]sion behavior in human cell cultures. 1. Biomed. [43]Mate r. Res 35:451. [44]

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I I

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2003 Stoeckel Self Expanding Nitinol Stents