ASTM F2606-08

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Designation: F2606 − 08
StandardGuide for
Three-Point Bending of Balloon Expandable Vascular Stents
and Stent Systems
This standard is issued under the fixed designation F2606; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2.1.4 deflection, n—displacement of the dynamic support
(loading anvil) at any point in the test.
1.1 This guide provides guidelines for quantitatively char-
2.1.5 delivery system, n—catheter that is used to deliver and
acterizingballoon-expandablestentandstentsystemflexibility
deploy a stent at the target site. A delivery system for
using three-point bending procedures. Guidelines are provided
balloon-expandable stents may be similar to a balloon-
for characterizing deployed stent flexibility, and for character-
dilatation (angioplasty) catheter.
izing pre-deployment stent system flexibility in the region of
the stent and balloon.
2.1.6 midspan bending moment, n—a linear estimate of the
bending moment at the center of the span. See 8.5.
1.2 This guide is not recommended for test articles that
cannot be appropriately evaluated using a span length to stent
2.1.7 midspan curvature, n—a linear estimate of the curva-
outer diameter (as tested) ratio of at least 4:1. Test articles that
ture of the center of the span. See 8.5.
do not meet this requirement are likely to exhibit appreciable
2.1.8 span length, n—distance between the centers of the
deformation by modes other than bending.
supports.
1.3 This guide does not provide procedures for characteriz-
2.1.9 stent system, n—delivery system with a pre-mounted
ing the bending flexibility of self-expanding stents, self-
stent.
expanding stent systems, endoprostheses (stent-grafts), or en-
2.1.10 stent, vascular, n—synthetic structure that is perma-
doprostheses systems. However, some aspects of this guide
nentlyimplantedinthenativeorgraftedvasculatureandthatis
may be useful for developing appropriate three-point bending
intended to provide mechanical radial support to enhance
characterization procedures for these devices.While this guide
vessel patency.
was developed with vascular stents and stent systems in mind,
it may be useful for characterizing the bending flexibility of
3. Significance and Use
balloon-expandable stents and stent systems used in non-
3.1 This guide can be used to obtain force versus deflection
vascular applications.
or midspan bending moment versus midspan curvature curves
1.4 The values stated in SI units are to be regarded as the
for stents and stent systems subjected to three-point bending
standard. The values given in parentheses are mathematical
conditions. Bending flexibility of a stent system may be a
conversions to inch-pound units that are provided for informa-
factor in its ability to track through the vascular anatomy, and
tion only and are not considered standard.
may be a factor in vascular trauma along the delivery pathway
distal to the guide catheter. Bending flexibility of a deployed
2. Terminology
stent may be one measure of its ability to flex with a vessel, or
2.1 Definitions:
to conform to the natural curvature of a vessel. Bending
2.1.1 bending flexibility, n—ameasureoftheabilityofatest flexibility of a delivery system may also be of interest if it is
specimen to bend.
desired to assess the separate contributions of the delivery
system and the mounted stent to the overall flexibility of the
2.1.2 bending stiffness, n—a measure of the ability of a test
stent system.
specimen to resist bending.
3.2 This guide is not intended to determine material
2.1.3 conformability, n—the degree to which a stent or stent
properties,stentsystemtrackability(abilityofastentsystemto
system matches the native curvature of the vasculature.
follow a guide wire and/or guide catheter through vascular
tortuosity), or stent system deliverability (ability of a stent
This guide is under the jurisdiction ofASTM Committee F04 on Medical and
system to deliver a stent to the implantation site(s) or through
Surgical Materials and Devices and is the direct responsibility of Subcommittee
particularlevel(s)ofvasculartortuosity).Whilethisguidedoes
F04.30 on Cardiovascular Standards.
not determine stent system trackability or deliverability, it can
Current edition approved Oct. 1, 2008. Published October 2008. DOI: 10.1520/
F2606-08. provide quantitative insight into how stent system bending
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F2606 − 08
flexibility affects trackability and deliverability. Similarly, point loading fixture consists of two lower static supports and
while this guide does not determine conformability of a one upper load applicator as shown in Fig. 1. As the stent is
deployed stent, it can provide quantitative insight into how deflected,theforceanddeflectionvaluesarerecorded.Thetest
stent and/or stent system bending flexibility affects deployed system should include a computerized data acquisition system.
stent conformability. Since this guide quantifies bending 5.1.1 The two lower static supports should be parallel
flexibility, it may be useful in determining the magnitude of cylinders with a diameter of 6.35 mm ( ⁄4 in.), unless another
bending flexibility effects on bending-related performance geometric shape and/or diameter is more appropriate for the
differences between the test article and control devices. span and/or to minimize adverse stent/support interaction, for
example, to minimize resistance to stent movement over the
3.3 The three-point bending procedures provided in this
support during loading. The static supports may contain
guide are intended to be used to characterize balloon-
circumferential grooves or shoulders to keep the test article
expandable stent and stent system flexibility during product
axis perpendicular to the supports during testing. Alternative
development. They may not necessarily satisfy any particular
lower supports should be designed to ensure that the applied
requirements of national or international regulatory bodies.
loads result in bending, not kinking, buckling or crushing, of
thetestarticle.Itisrecommendedthatthelowerstaticsupports
4. Summary of Guide
be designed to minimize friction by using materials such as
4.1 The specimen is loaded onto a three-point bend fixture.
polytetrafluoroethylene (for example, Teflon), acetal poly-
The specimen is supported from below by two static supports
oxymethylene (for example, Delrin) or other low-friction
separated by a known span and bent by a force applied on the
material, or by allowing support rotation with bearings. The
top and midway between the lower supports. The bending
height of the static supports needs to be sufficient to provide
flexibility of the stent is obtained from force-versus-deflection
adequate distance for deflection.
plots and/or midspan bending moment versus midspan curva-
5.1.2 The upper dynamic load applicator should also be a
ture plots. Bending flexibility evaluations may be made on
cylinder with a 6.35 mm ( ⁄4 in.) diameter, unless another
balloon-expandablestentsystems,and/orondeployedballoon-
geometric shape and/or diameter is more appropriate for the
expandablestents.Bendingflexibilityofadeliverysystemmay
span and/or minimization of adverse stent/support interactions
also be evaluated if it is desired to assess the separate
(for example, to minimize the potential local deformation
contributions of the delivery system and the mounted stent to
(kinking or crushing), to minimize the resistance to movement
the overall flexibility of the stent system.
of the specimen over the lower supports during specimen
4.2 Bending flexibility assessments may be made using a
bending, to minimize the potential for the upper support to
fixed span between the lower supports or by using a span that
resist specimen bending, and so forth). The upper load appli-
increases with the specimen length.
cator should be parallel to, and horizontally centered between,
4.2.1 The fixed span length method permits force versus
thelowerstaticsupports.Alternativedesignsfortheupperload
deflection comparisons that are independent of stent length.
applicator should ensure that the applied loads result in
This method may be useful when comparing the flexibility of
bending, not kinking, buckling or crushing, of the test article.
stentswithdifferentdiametersorstructuraldesigns,anditmay
5.1.3 The bending fixture’s lower (static) support diameters
permit bending flexibility to be evaluated at multiple longitu-
and upper (dynamic) load applicator diameter should be
dinal positions along longer test articles.
appropriateforthespanlengthandtestarticlediameter(thatis,
4.2.2 The variable span length method allows the bending
momentarmlengthtobemaximizedforanygivenstentlength
inordertominimizethepotentialfornon-bendingdeformation
at a given applied load and/or deflection. Bending flexibility
comparisons may be made at different span lengths by com-
paring midspan bending moments at given midspan bending
curvatures. The variable span length approach also permits the
study of bending load variation with span length, but the
bendingloadsarenotsolelydependentontheinherentbending
stiffness of the test article.
4.2.3 Both the fixed and variable span length methods
permit the study of bending load variation with test article
curvature by varying deflection to cause variation in curvature.
In the variable span length method, test article curvature may
also be varied by changing the span length.
5. Apparatus
5.1 The three-point bend test apparatus consists of a means
of applying and accurately measuring various loads and
deflections to a specimen mounted in a three-point bending
fixture. Stents can be tested in the deployed state and stent
FIG. 1 Schematic of Three-Point Bending Apparatus with Stent
systems can be tested prior to stent deployment. The three- System
F2606 − 08
spanlength>lowersupportdiameter+uppersupportdiameter provide some indication of bending stiffness variation for a
+ 2 × (test article diameter)). given device and test condition. More samples may be needed,
5.1.4 Table 1 provides span length and maximum deflection depending on the claims to be made based on the test results.
recommendations for the variable span length method. The
6.2 If using the variable span method, the shortest stent of a
deflection listed in Table 1 is a maximum deflection recom-
given diameter may be used to determine the maximum
mendation. Actual test deflections should be lower, especially
bending load associated with a given deflection or curvature.
if it is desired to avoid plastic deformation. Alternative span
Conversely, the longest stent of a given diameter may be used
lengths and deflections for deployed stent flexibility character-
to determine the minimum bending load for a given deflection
izations may be based on stented vessel flexure conditions
or curvature.
anticipated for the intended implant location(s) or other appro-
6.2.1 If using the fixed span method, the variation of
priate considerations. Alternative span lengths and deflections
bending load with stent length at a given deflection cannot be
for stent system flexibility characterizations may be based on
determined. However, the variation in bending load with
vessel curvature conditions anticipated along the intended
curvature may be determined by varying deflection to cause
delivery pathway(s) or other appropriate considerations.
variation in curvature.
5.1.5 Thespanlengthusedforthefixedspanmethodshould
6.3 If bending stiffness may be sensitive to the rotational
be small enough to permit evaluation of the minimum stent
orientation of the test article about its longitudinal axis, the
lengthtobetestedusingtheselectedmaximumdeflection.Itis
maximum and minimum bending stiffness orientations should
recommended that span length be at least 4 mm smaller than
be evaluated.
the minimum stent length to be evaluated.
5.1.6 The apparatus should have adequate displacement
7. Procedure
(deflection) rate control and the displacement rate should be
sufficientlyslowtoavoidexcessiveloadsduetoinertialeffects 7.1 Ifdeformationofthestentand/ordeliverysystemoccurs
.Considerationshouldalsobegiventothedisplacementrate(s) during a test, the sample should not be re-tested. Even if
expected clinically if the test article is sensitive to strain rate deformation is not observed, retesting specimens is not recom-
variations between the quasi-static strain rates used to avoid mended as test results may still be affected by prior tests.
excessive inertial loads and the strain rates associated with
7.2 PreparethefixtureasdescribedinSection5withthetest
clinically relevant displacement rates. Use of similar displace-
article positioned perpendicular to the lower static supports.
ment rates are recommend for device comparison purposes.
7.3 Mount the upper load applicator parallel to the static
5.1.7 The accuracy of the apparatus force and displacement
supports and centered between them. The load applicator
meters should be within 65% of the maximum reported force
should be able to move vertically to the maximum test
and displacement values.
deflection without crushing the test article.
5.2 An inflation device for deploying the stent.
7.4 If the bending properties of the test article are affected
5.3 Means to preheat and maintain the temperature of the
by temperature variation between ambient and 37°C, the test
test article at 37 6 2°C, if appropriate.
should be conducted while maintaining the specimen tempera-
5.4 Means to hydrate and maintain the hydration of the test
ture at 37 6 2°C.
article, if appropriate.
7.5 If the bending properties of the test article are sensitive
to hydration, the test should be conducted while keeping the
6. Test Specimens
test article fully hydrated.
6.1 Aminimum of three like specimens (same labeled stent
7.6 Markthemiddleofthestenttoindicatethepositionand
diameter and length) should be tested for each test condition
orientation of the applied deflection.
(span length, rotational orientation, and so forth) in order to
7.7 If a stent system is to be tested, cut the stent system just
proximal of the proximal balloon seal.
TABLE 1 Recommended Span Length and Maximum Deflection
7.8 If a deployed stent is to be tested, deploy the stent as
for the Variable Span Length Method
C
indicated in the Instructions for Use. Deploy at 37 6 2°C if
Stent Length Span Length Maximum Deflection
A B
(mm) (mm)
deployment at temperatures between ambient and 37°C may
D
10–14 6 1.2
affect stent d
...