1、Designation: F 2606 08Standard Guide forThree-Point Bending of Balloon Expandable Vascular Stentsand Stent Systems1This standard is issued under the fixed designation F 2606; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year
2、 of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide provides guidelines for quantitatively char-acterizing balloon-expandable stent and stent system flexib
3、ilityusing three-point bending procedures. Guidelines are providedfor characterizing deployed stent flexibility, and for character-izing pre-deployment stent system flexibility in the region ofthe stent and balloon.1.2 This guide is not recommended for test articles thatcannot be appropriately evalu
4、ated using a span length to stentouter diameter (as tested) ratio of at least 4:1. Test articles thatdo not meet this requirement are likely to exhibit appreciabledeformation by modes other than bending.1.3 This guide does not provide procedures for characteriz-ing the bending flexibility of self-ex
5、panding stents, self-expanding stent systems, endoprostheses (stent-grafts), or en-doprostheses systems. However, some aspects of this guidemay be useful for developing appropriate three-point bendingcharacterization procedures for these devices. While this guidewas developed with vascular stents an
6、d stent systems in mind,it may be useful for characterizing the bending flexibility ofballoon-expandable stents and stent systems used in non-vascular applications.1.4 The values stated in SI units are to be regarded as thestandard. The values given in parentheses are mathematicalconversions to inch
7、-pound units that are provided for informa-tion only and are not considered standard.2. Terminology2.1 Definitions:2.1.1 bending flexibility, na measure of the ability of a testspecimen to bend.2.1.2 bending stiffness, na measure of the ability of a testspecimen to resist bending.2.1.3 conformabilit
8、y, nthe degree to which a stent or stentsystem matches the native curvature of the vasculature.2.1.4 deflection, ndisplacement of the dynamic support(loading anvil) at any point in the test.2.1.5 delivery system, ncatheter that is used to deliver anddeploy a stent at the target site. A delivery syst
9、em forballoon-expandable stents may be similar to a balloon-dilatation (angioplasty) catheter.2.1.6 midspan bending moment, na linear estimate of thebending moment at the center of the span. See 8.5.2.1.7 midspan curvature, na linear estimate of the curva-ture of the center of the span. See 8.5.2.1.
10、8 span length, ndistance between the centers of thesupports.2.1.9 stent system, ndelivery system with a pre-mountedstent.2.1.10 stent, vascular, nsynthetic structure that is perma-nently implanted in the native or grafted vasculature and that isintended to provide mechanical radial support to enhanc
11、evessel patency.3. Significance and Use3.1 This guide can be used to obtain force versus deflectionor midspan bending moment versus midspan curvature curvesfor stents and stent systems subjected to three-point bendingconditions. Bending flexibility of a stent system may be afactor in its ability to
12、track through the vascular anatomy, andmay be a factor in vascular trauma along the delivery pathwaydistal to the guide catheter. Bending flexibility of a deployedstent may be one measure of its ability to flex with a vessel, orto conform to the natural curvature of a vessel. Bendingflexibility of a
13、 delivery system may also be of interest if it isdesired to assess the separate contributions of the deliverysystem and the mounted stent to the overall flexibility of thestent system.3.2 This guide is not intended to determine material prop-erties, stent system trackability (ability of a stent syst
14、em tofollow a guide wire and/or guide catheter through vasculartortuosity), or stent system deliverability (ability of a stentsystem to deliver a stent to the implantation site(s) or throughparticular level(s) of vascular tortuosity). While this guide doesnot determine stent system trackability or d
15、eliverability, it canprovide quantitative insight into how stent system bending1This guide is under the jurisdiction of ASTM Committee F04 on Medical andSurgical Materials and Devices and is the direct responsibility of SubcommitteeF04.30 on Cardiovascular Standards.Current edition approved Oct. 1,
16、2008. Published October 2008.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.flexibility affects trackability and deliverability. Similarly,while this guide does not determine conformability of adeployed stent, it can provide quantit
17、ative insight into howstent and/or stent system bending flexibility affects deployedstent conformability. Since this guide quantifies bending flex-ibility, it may be useful in determining the magnitude ofbending flexibility effects on bending-related performancedifferences between the test article a
18、nd control devices.3.3 The three-point bending procedures provided in thisguide are intended to be used to characterize balloon-expandable stent and stent system flexibility during productdevelopment. They may not necessarily satisfy any particularrequirements of national or international regulatory
19、 bodies.4. Summary of Guide4.1 The specimen is loaded onto a three-point bend fixture.The specimen is supported from below by two static supportsseparated by a known span and bent by a force applied on thetop and midway between the lower supports. The bendingflexibility of the stent is obtained from
20、 force-versus-deflectionplots and/or midspan bending moment versus midspan curva-ture plots. Bending flexibility evaluations may be made onballoon-expandable stent systems, and/or on deployed balloon-expandable stents. Bending flexibility of a delivery system mayalso be evaluated if it is desired to
21、 assess the separatecontributions of the delivery system and the mounted stent tothe overall flexibility of the stent system.4.2 Bending flexibility assessments may be made using afixed span between the lower supports or by using a span thatincreases with the specimen length.4.2.1 The fixed span len
22、gth method permits force versusdeflection comparisons that are independent of stent length.This method may be useful when comparing the flexibility ofstents with different diameters or structural designs, and it maypermit bending flexibility to be evaluated at multiple longitu-dinal positions along
23、longer test articles.4.2.2 The variable span length method allows the bendingmoment arm length to be maximized for any given stent lengthin order to minimize the potential for non-bending deformationat a given applied load and/or deflection. Bending flexibilitycomparisons may be made at different sp
24、an lengths by com-paring midspan bending moments at given midspan bendingcurvatures. The variable span length approach also permits thestudy of bending load variation with span length, but thebending loads are not solely dependent on the inherent bendingstiffness of the test article.4.2.3 Both the f
25、ixed and variable span length methodspermit the study of bending load variation with test articlecurvature by varying deflection to cause variation in curvature.In the variable span length method, test article curvature mayalso be varied by changing the span length.5. Apparatus5.1 The three-point be
26、nd test apparatus consists of a meansof applying and accurately measuring various loads anddeflections to a specimen mounted in a three-point bendingfixture. Stents can be tested in the deployed state and stentsystems can be tested prior to stent deployment. The three-point loading fixture consists
27、of two lower static supports andone upper load applicator as shown in Fig. 1. As the stent isdeflected, the force and deflection values are recorded. The testsystem should include a computerized data acquisition system.5.1.1 The two lower static supports should be parallelcylinders with a diameter o
28、f 6.35 mm (14 in.), unless anothergeometric shape and/or diameter is more appropriate for thespan and/or to minimize adverse stent/support interaction, forexample, to minimize resistance to stent movement over thesupport during loading. The static supports may containcircumferential grooves or shoul
29、ders to keep the test articleaxis perpendicular to the supports during testing. Alternativelower supports should be designed to ensure that the appliedloads result in bending, not kinking, buckling or crushing, ofthe test article. It is recommended that the lower static supportsbe designed to minimi
30、ze friction by using materials such aspolytetrafluoroethylene (for example, Teflon), acetal poly-oxymethylene (for example, Delrin) or other low-frictionmaterial, or by allowing support rotation with bearings. Theheight of the static supports needs to be sufficient to provideadequate distance for de
31、flection.5.1.2 The upper dynamic load applicator should also be acylinder with a 6.35 mm (14 in.) diameter, unless anothergeometric shape and/or diameter is more appropriate for thespan and/or minimization of adverse stent/support interactions(for example, to minimize the potential local deformation
32、(kinking or crushing), to minimize the resistance to movementof the specimen over the lower supports during specimenbending, to minimize the potential for the upper support toresist specimen bending, and so forth). The upper load appli-cator should be parallel to, and horizontally centered between,t
33、he lower static supports.Alternative designs for the upper loadapplicator should ensure that the applied loads result inbending, not kinking, buckling or crushing, of the test article.5.1.3 The bending fixtures lower (static) support diametersand upper (dynamic) load applicator diameter should beapp
34、ropriate for the span length and test article diameter (that is,span length lower support diameter + upper support diameter+23 (test article diameter).FIG. 1 Schematic of Three-Point Bending Apparatus with StentSystemF26060825.1.4 Table 1 provides span length and maximum deflectionrecommendations fo
35、r the variable span length method. Thedeflection listed in Table 1 is a maximum deflection recom-mendation. Actual test deflections should be lower, especiallyif it is desired to avoid plastic deformation. Alternative spanlengths and deflections for deployed stent flexibility character-izations may
36、be based on stented vessel flexure conditionsanticipated for the intended implant location(s) or other appro-priate considerations. Alternative span lengths and deflectionsfor stent system flexibility characterizations may be based onvessel curvature conditions anticipated along the intendeddelivery
37、 pathway(s) or other appropriate considerations.5.1.5 The span length used for the fixed span method shouldbe small enough to permit evaluation of the minimum stentlength to be tested using the selected maximum deflection. It isrecommended that span length be at least 4 mm smaller thanthe minimum st
38、ent length to be evaluated.5.1.6 The apparatus should have adequate displacement(deflection) rate control and the displacement rate should besufficiently slow to avoid excessive loads due to inertial effects. Consideration should also be given to the displacement rate(s)expected clinically if the te
39、st article is sensitive to strain ratevariations between the quasi-static strain rates used to avoidexcessive inertial loads and the strain rates associated withclinically relevant displacement rates. Use of similar displace-ment rates are recommend for device comparison purposes.5.1.7 The accuracy
40、of the apparatus force and displacementmeters should be within 65 % of the maximum reported forceand displacement values.5.2 An inflation device for deploying the stent.5.3 Means to preheat and maintain the temperature of thetest article at 37 6 2C, if appropriate.5.4 Means to hydrate and maintain t
41、he hydration of the testarticle, if appropriate.6. Test Specimens6.1 A minimum of three like specimens (same labeled stentdiameter and length) should be tested for each test condition(span length, rotational orientation, and so forth) in order toprovide some indication of bending stiffness variation
42、 for agiven device and test condition. More samples may be needed,depending on the claims to be made based on the test results.6.2 If using the variable span method, the shortest stent of agiven diameter may be used to determine the maximumbending load associated with a given deflection or curvature
43、.Conversely, the longest stent of a given diameter may be usedto determine the minimum bending load for a given deflectionor curvature.6.2.1 If using the fixed span method, the variation ofbending load with stent length at a given deflection cannot bedetermined. However, the variation in bending loa
44、d withcurvature may be determined by varying deflection to causevariation in curvature.6.3 If bending stiffness may be sensitive to the rotationalorientation of the test article about its longitudinal axis, themaximum and minimum bending stiffness orientations shouldbe evaluated.7. Procedure7.1 If d
45、eformation of the stent and/or delivery system occursduring a test, the sample should not be re-tested. Even ifdeformation is not observed, retesting specimens is not recom-mended as test results may still be affected by prior tests.7.2 Prepare the fixture as described in Section 5 with the testarti
46、cle positioned perpendicular to the lower static supports.7.3 Mount the upper load applicator parallel to the staticsupports and centered between them. The load applicatorshould be able to move vertically to the maximum testdeflection without crushing the test article.7.4 If the bending properties o
47、f the test article are affectedby temperature variation between ambient and 37C, the testshould be conducted while maintaining the specimen tempera-ture at 37 6 2C.7.5 If the bending properties of the test article are sensitiveto hydration, the test should be conducted while keeping thetest article
48、fully hydrated.7.6 Mark the middle of the stent to indicate the position andorientation of the applied deflection.7.7 If a stent system is to be tested, cut the stent system justproximal of the proximal balloon seal.7.8 If a deployed stent is to be tested, deploy the stent asindicated in the Instruc
49、tions for Use. Deploy at 37 6 2C ifdeployment at temperatures between ambient and 37C mayaffect stent diameter or flexural stiffness.7.9 If testing is to be conducted at 37 6 2C, preheat the testfixture and test article to 37 6 2C.7.10 Place the test article in the center of the three-pointbending fixture and position it such that the dynamic loadapplicator will contact the midpoint of the stent as marked in7.6.7.11 Maintain the test article at 37 6 2C throughout thetest, if appropriate.7.12 Balance (zero) the force transducer.7.13 Decide whether to start the test w
