ASTM F2942-19

Standard Guide for <emph type="bdit">in vitro</emph> Axial, Bending, and Torsional Durability Testing of Vascular Stents

SIGNIFICANCE AND USE
5.1 It is important to consider the durability of stent designs in deformation modes that are intended to model in vivo conditions. The appropriate amplitude and number of cycles in each of the modes have to be determined independently for the particular clinical use proposed for the stent. These tests do not replicate all varieties and aspects of the deployment process nor the in vivo mechanical environment in its entirety, and as such they cannot be proofs of durability. Instead, the tests provide evidence of durability. The durability tests can also provide a means of assessing design, material or processing changes.  
5.1.1 This guide might be useful for development testing, specification acceptance testing, and regulatory submission testing and filings as it provides a basic assurance that the tests are designed, executed, and reported in a suitable fashion.  
5.1.2 If the tests are conducted using a well defined FTF methodology, they can be useful in:
5.1.2.1 Potential design improvement through identification of better and worse geometries, materials, and manufacturing processes;
5.1.2.2 Understanding product durability by estimating the effects of changes in geometry, materials, or manufacturing processes;
5.1.2.3 Estimating the safety factor relative to the amplitudes and other factors in use conditions; and
5.1.2.4 Validating finite element analysis (FEA) and fatigue life models.  
5.1.3 As stated in the scope, this guide is not intended to provide the in vivo physiologic deformation conditions to which a vascular stent can be subjected. Reliable clinical data characterizing cyclic vascular deformation may be lacking for some indications. The user should develop and justify the boundary conditions (e.g., by literature review, in vivo studies, cadaver studies, or modeling of stent vessel interaction) for the chosen durability bench tests. Additional conditions that may be considered include vessel calcification, vessel taper, eccentric lesions, d...
SCOPE
1.1 This guide includes three separate cyclic deformation durability guides related to vascular stents: bending, axial, and torsional.  
1.2 This guide does not address flat plate, local crush durability, or multi-mode testing. Although this guide does not address multi-mode testing, the information included herein could be applicable to developing this type of test.  
1.3 This guide applies to balloon-expandable and self-expanding stents fabricated from metals and metal alloys. It does not specifically address any attributes unique to coated stents (i.e., stents with a surface layer of an additional material(s)), monolithically polymeric stents, or absorbable stents, although the application of this standard to those products is not precluded.  
1.4 This guide applies to endovascular grafts (“stent-grafts”) and other conduit products commonly used to treat aneurismal disease, peripheral vessel trauma, or to provide vascular access. The information provided herein does not address all issues related to testing of these devices.  
1.5 This guide is applicable to testing of stent(s) (or a representative portion of a stent). While durability testing of coupon samples (e.g., a scaled-up portion of the stent structure) can provide useful information, it is not within the scope of this guide.  
1.6 This guide applies to in vitro modeling of stent durability from non-radial arterial motions. Such motions may arise from musculoskeletal activities, including walking and breathing, and cardiac motion. Test Methods F2477 addresses pulsatile (i.e., radial) durability of vascular stents.  
1.7 This guide does not provide the in vivo physiologic deformation conditions for a vascular stent. It is incumbent upon the user of the standard to develop and justify these boundary conditions (e.g., by literature review, in vivo studies, cadaver studies, or modeling of stent vessel interaction) in these durability bench tests. Additional ...

General Information

• Status: Published
• Publication Date: 31-May-2019
• ICS: 11.040.40 - Implants for surgery, prosthetics and orthotics
• Technical Committee: F04 - Medical and Surgical Materials and Devices
• Drafting Committee: F04.30 - Cardiovascular Standards

Relations

• Refers: ASTM F3211-17 - Standard Guide for Fatigue-to-Fracture (FtF) Methodology for Cardiovascular Medical Devices
• Effective Date: 01-Sep-2017
• Refers: ASTM F2477-07(2013) - Standard Test Methods for <emph type="bdit">in vitro</emph> Pulsatile Durability Testing of Vascular Stents
• Effective Date: 01-Mar-2013
• Refers: ASTM F2477-07 - Standard Test Methods for <i>in vitro</i> Pulsatile Durability Testing of Vascular Stents
• Effective Date: 01-Apr-2007
• Refers: ASTM F2477-06 - Standard Test Methods for <bdit>in vitro</bdit> Pulsatile Durability Testing of Vascular Stents
• Effective Date: 01-Nov-2006

Overview

ASTM F2942-19 is a comprehensive standard guide developed by ASTM International, designed to support the in vitro testing of vascular stents for durability under axial, bending, and torsional loads. The guide is intended for use in simulating non-radial arterial movements, such as those caused by musculoskeletal activity or cardiac motion. This standard facilitates robust and reproducible bench testing of vascular stents and similar endovascular devices to assess their mechanical durability and performance before clinical use.

ASTM F2942-19 serves as a valuable resource for manufacturers, regulatory bodies, and research organizations. Its guidelines increase confidence that durability testing reflects clinically relevant conditions and that results can be used to evaluate design changes, support regulatory submissions, and provide assurance of consistent product quality.

Key Topics

• Types of Durability Testing: Specifies procedures for cyclic axial, bending, and torsional deformation tests on stents.
• Scope of Application: Applies to metallic balloon-expandable and self-expanding stents, endovascular grafts (stent-grafts), and other conduit devices but does not address coated, polymeric, or absorbable stents specifically.
• Testing Configurations: Supports tests on whole stents or representative stent segments but excludes coupon samples and multi-mode (combined loading) testing.
• Simulation Environment: Emphasizes the use of mock vessels and physiologically relevant test conditions, including temperature and solution composition.
• Boundary Condition Justification: Requires users to establish and justify boundary conditions (through literature, modeling, or experimental data) that reflect the intended clinical environment.
• Fatigue Methodologies: Describes both test-to-success (TTS) and fatigue-to-fracture (FTF) approaches for evaluating stent fatigue life.
• Reporting Requirements: Outlines details for comprehensive reporting of specimens, apparatus, test parameters, inspection procedures, and results.

Applications

ASTM F2942-19 is used in various phases of product development and regulatory compliance, with practical applications including:

• Device Development: Supporting design and material selection by comparing durability among stent geometries, materials, and manufacturing processes.
• Regulatory Submissions: Providing data for regulatory filing and ensuring compliance with durability testing requirements for cardiovascular implants.
• Design Changes: Evaluating alterations to stent geometry, material, or manufacturing process by quantifying impacts on fatigue life and robustness.
• Fatigue Analysis and Modeling: Validating finite element analysis (FEA) and other computational fatigue models through physical testing.
• Safety Assessment: Determining safety factors for stents relative to expected deformation amplitudes and in-use mechanical conditions.
• Quality Control: Routine verification of product durability as part of production control and batch release.

This guide addresses the need for standardized test protocols to ensure that stents can endure biomechanical stresses encountered in the body, such as those due to respiratory and musculoskeletal movement, thereby reducing the risk of fractures and failure in clinical settings.

Related Standards

To provide a holistic framework for vascular stent testing, ASTM F2942-19 references several additional standards and guidance documents:

• ASTM F2477: Standard Test Methods for In Vitro Pulsatile Durability Testing of Vascular Stents (radial durability).
• ASTM F3211: Guide for Fatigue-to-Fracture (FTF) Methodology for Cardiovascular Medical Devices.
• ISO 25539-2: Cardiovascular Implants - Endovascular Devices - Part 2: Vascular Stents, an internationally recognized standard for stent testing.
• FDA Guidance #1545: Non-Clinical Engineering Tests and Recommended Labeling for Intravascular Stents and Associated Delivery Systems.
• ASTM STP 588: Manual on Statistical Planning and Analysis relevant for test methodology and data interpretation.

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Keywords: vascular stent, durability testing, axial fatigue, bending fatigue, torsional fatigue, endovascular graft, stent-graft, in vitro testing, cardiovascular implants, medical device durability, ASTM F2942-19, stent fatigue test, stent quality assurance, regulatory submission testing.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: F2942 − 19
Standard Guide for
in vitro Axial, Bending, and Torsional Durability Testing of
Vascular Stents
This standard is issued under the fixed designation F2942; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope cadaver studies, or modeling of stent vessel interaction) in
these durability bench tests.Additional conditions that may be
1.1 This guide includes three separate cyclic deformation
considered include vessel calcification, vessel taper, eccentric
durability guides related to vascular stents: bending, axial, and
lesions, loading excursions (e.g., exercise), and vessel remod-
torsional.
eling.
1.2 This guide does not address flat plate, local crush
1.8 This standard does not purport to address all of the
durability, or multi-mode testing.Although this guide does not
safety concerns, if any, associated with its use. It is the
address multi-mode testing, the information included herein
responsibility of the user of this standard to establish appro-
could be applicable to developing this type of test.
priate safety, health, and environmental practices and deter-
1.3 This guide applies to balloon-expandable and self-
mine the applicability of regulatory limitations prior to use.
expanding stents fabricated from metals and metal alloys. It
1.9 This international standard was developed in accor-
does not specifically address any attributes unique to coated
dance with internationally recognized principles on standard-
stents (i.e., stents with a surface layer of an additional
ization established in the Decision on Principles for the
material(s)), monolithically polymeric stents, or absorbable
Development of International Standards, Guides and Recom-
stents, although the application of this standard to those
mendations issued by the World Trade Organization Technical
products is not precluded.
Barriers to Trade (TBT) Committee.
1.4 This guide applies to endovascular grafts (“stent-
2. Referenced Documents
grafts”) and other conduit products commonly used to treat
aneurismal disease, peripheral vessel trauma, or to provide
2.1 ASTM Standards:
vascular access. The information provided herein does not
F2477 Test Methods for in vitro Pulsatile Durability Testing
address all issues related to testing of these devices.
of Vascular Stents
F3211 Guide for Fatigue-to-Fracture (FtF) Methodology for
1.5 This guide is applicable to testing of stent(s) (or a
Cardiovascular Medical Devices
representative portion of a stent). While durability testing of
couponsamples(e.g.,ascaled-upportionofthestentstructure)
2.2 Other Documents:
canprovideusefulinformation,itisnotwithinthescopeofthis ASTM STP 588 Manual on Statistical Planning and
guide.
Analysis, R.E. Little, 1975
1.6 This guide applies to in vitro modeling of stent durabil-
3. Terminology
ity from non-radial arterial motions. Such motions may arise
3.1 Definitions of Terms Specific to This Standard:
from musculoskeletal activities, including walking and
breathing, and cardiac motion. Test Methods F2477 addresses 3.1.1 axial, adj—compression or tension of a stent and/or
mock vessel along its longitudinal axis.
pulsatile (i.e., radial) durability of vascular stents.
3.1.2 bending, adj—deformation on the longitudinal axis of
1.7 This guide does not provide the in vivo physiologic
a stent and/or mock vessel to achieve a specified stent radius of
deformation conditions for a vascular stent. It is incumbent
curvature.
upon the user of the standard to develop and justify these
boundary conditions (e.g., by literature review, in vivo studies,
3.1.3 fracture, n—complete separation of a stent element or
component (e.g., strut, apex, bridge, marker).
This test method is under the jurisdiction ofASTM Committee F04 on Medical
and Surgical Materials and Devices and is the direct responsibility of Subcommittee
F04.30 on Cardiovascular Standards. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
CurrenteditionapprovedJune1,2019.PublishedJuly2019.Originallyapproved contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
in2013.Lastpreviouseditionapprovedin2013asF2942–13.DOI:10.1520/F2942- Standards volume information, refer to the standard’s Document Summary page on
19. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F2942 − 19
3.1.3.1 Discussion—Fracture of a stent does not necessarily the TTS is based upon the number (if any) and type of stent
constitute failure (i.e., loss of functionality). strut fractures. The FTF methodology entails selection of the
appropriate number of cycles considered runout (i.e., point to
3.1.4 mock vessel, n—a simulated vessel typically manufac-
tured from an elastomeric material. stop testing a specimen), selection of a sufficient number of
specimens, and characterization of the stent fatigue perfor-
3.1.5 radius of curvature, n—the inner, outer or centerline
mance by applying multiple deformation levels (i.e., loading
bend radius of a stent.
amplitudes) and conducting periodic inspections of the stent
3.1.6 specimen, n—article consisting of an implantable de-
during testing to obtain test specimens with and without
vice or a representative portion of an implantable device, that
fracture. For specimens that fracture, the number of cycles
is tested according to this guide.
applied to cause fracture is obtained. The successful comple-
3.1.7 torsional, adj—twisting of a stent and/or mock vessel
tion of a FTF test is based upon a comparison of stent fatigue
about its longitudinal axis.
performance, at the various deformation levels, to the physi-
ologically relevant deformation levels. Selection of deforma-
4. Summary of Test Guides
tion levels to characterize the fatigue behavior of the stent may
use the methodology described in ASTM STP 588.
4.1 This guide covers in vitro durability testing of vascular
stents using modes that represent those that might be observed
5. Significance and Use
in vivo such as bending, axial, or torsional deformation modes.
5.1 It is important to consider the durability of stent designs
Examples include, but are not limited to, the axial and bending
in deformation modes that are intended to model in vivo
deformation that occurs in the superficial femoral artery during
conditions. The appropriate amplitude and number of cycles in
the walking gait, the bending that occurs in the renal artery
each of the modes have to be determined independently for the
during respiration, and the bending that occurs in the coronary
particular clinical use proposed for the stent.These tests do not
artery during the cardiac cycle. This guide provides details and
replicate all varieties and aspects of the deployment process
guidance for separate tests for each deformation mode: axial,
nor the in vivo mechanical environment in its entirety, and as
bending, and torsional. This guide allows the direct fixation of
such they cannot be proofs of durability. Instead, the tests
the ends of the stent or indirect fixation inside a mock vessel.
provide evidence of durability. The durability tests can also
Direct fixation of the ends of the stent allows better control of
provide a means of assessing design, material or processing
stent deformation; however, such fixation might not be repre-
changes.
sentative of in vivo conditions and care should be taken to
5.1.1 This guide might be useful for development testing,
minimize attachment-induced test artifact.
specification acceptance testing, and regulatory submission
4.1.1 Axial Durability Test Guide—The purpose of this test
testing and filings as it provides a basic assurance that the tests
is to subject the stent to a specified amount of cyclic axial
are designed, executed, and reported in a suitable fashion.
deformation. The stent is deployed into a mock vessel, unless
5.1.2 If the tests are conducted using a well defined FTF
a justification is provided. This test guide is described in more
methodology, they can be useful in:
detail in Annex A1.
5.1.2.1 Potential design improvement through identification
4.1.2 Bending Durability Test Guide—The purpose of this
of better and worse geometries, materials, and manufacturing
test is to subject the stent to a specified amount of cyclic
processes;
bending deformation. There are three suggested bending
5.1.2.2 Understanding product durability by estimating the
guides presented in AnnexA2, AnnexA3, and AnnexA4, each
effects of changes in geometry, materials, or manufacturing
involving placement of the stent inside of a mock vessel:
processes;
column buckling, bending on a mandrel, and bending in an arc
5.1.2.3 Estimating the safety factor relative to the ampli-
without a mandrel. In order to avoid test artifacts, these test
tudes and other factors in use conditions; and
guides recommend placement of the stent inside a mock vessel
5.1.2.4 Validating finite element analysis (FEA) and fatigue
(stent ends not fixed) with subsequent cyclic bending of the
life models.
mock vessel. When selecting the guide to conduct bending
5.1.3 As stated in the scope, this guide is not intended to
durability testing, consider the potential for the stent design
provide the in vivo physiologic deformation conditions to
under evaluation to be adversely affected by the methods and
which a vascular stent can be subjected. Reliable clinical data
test apparatus described by a particular guide and the ability of
characterizing cyclic vascular deformation may be lacking for
each test to simulate a particular clinical condition.
some indications. The user should develop and justify the
4.1.3 Torsional Durability Test Guide—The purpose of this
boundary conditions (e.g., by literature review, in vivo studies,
test is to subject the stent to a specified amount of cyclic
cadaver studies, or modeling of stent vessel interaction) for the
torsional deformation.This guide is described in more detail in
chosen durability bench tests. Additional conditions that may
Annex A5.
be considered include vessel calcification, vessel taper, eccen-
4.2 Each test may utilize either ‘test-to-success’ (TTS) or tric lesions, deformation excursions (e.g., exercise), and vessel
the ‘fatigue-to-fracture’ (FTF) methodology (see Guide remodeling.
F3211). The TTS methodology entails selection of a set of 5.1.4 Test methods other than those provided in the annexes
boundary conditions considered physiologically relevant, se- of this document might be appropriate, depending upon stent
lection of a sufficient number of specimens, and application of design. However, these methods are beyond the scope of this
theappropriatenumberofcycles.Thesuccessfulcompletionof guide.
F2942 − 19
6. Specimen Size, Configuration, and Preparation 6.4 The number of specimens tested for each stent size
and/or geometry should be sufficient to support any claims
6.1 Unless otherwise justified, all specimens selected for
made based on the test results. The results of testing according
testing should be taken from fully processed, implant quality
to this guide in combination with other tests, animal and
product. Sterilization should be performed unless it can be
clinical tests, analysis (such as FEA), and/or comparisons to
shown not to influence the durability test results.
predicate devices can be sufficient to enable demonstration of
NOTE 1—Although sterilization may not directly affect the stent itself,
adequate durability. In this guide, one stent or a set of two
it may affect the delivery system and, thus, the condition of the
overlapped stents should be considered one specimen.
as-deployed stent.
6.2 Prior to deployment and durability testing, specimens
7. General Apparatus Requirements
loaded in or on their delivery systems should be tracked
7.1 The axial, bending and torsional dynamic displacements
through a model representative of the vasculature to simulate
of the test equipment should be verified at the selected test
clinical delivery.
frequencies. The dynamic stent deformation verification docu-
6.3 To reduce the number of specimens to be tested, mentation should include justification of the verification means
durability may be evaluated for the worst case device size/
(see 8.6).
model, with specimen selection being justified through appro-
7.2 Dimensional Measurement Devices—Devices such as
priate methods (e.g., finite element analysis). If multiple
linear variable displacement transducers (LVDTs), lasers, and
sizes/models or deployment diameters are to be tested, an
high-speed cameras should be calibrated.
appropriate bracketing scheme should be employed (e.g.,
7.3 Cycle Counting System—Theapparatusshouldincludea
largest and smallest length and/or diameter or models).
cycle counting system for measuring the number of deforma-
6.3.1 Stent Length—The axial and torsional durability test-
tion cycles applied to the stent. The cycle counting system
ing modes act to induce stent deformation normalized with
should be verified at the test frequencies and the verification
length (length change per length, and transverse angle change
should be documented.
per length, respectively). Thus, the fatigue resistance of a stent
7.4 Temperature Control System—The apparatus should in-
design with a repeating unit or cell design would also be
clude a calibrated temperature control and measurement sys-
independent of length and any length may be tested. In cases
tem to maintain the temperature of the stents being tested.
where the stent design is length-dependent (e.g., non-repeating
unit cells), the length predicted or expected to perform worst
8. General Test Parameters
should be justified (e.g., by finite element analysis).
8.1 Completion of the durability test for stents deployed
NOTE 2—Because of the nature of these test methods, it may not be
within a mock vessel, in air alone, or in fluid alone, depends on
possible to test the longest stent length within a family of sizes, especially
the deformation mode (i.e., axial, torsional, or bending), the
in the overlapped configuration. In such cases, other means may need to
be implemented to justify the stent length tested or to allow extrapolation
material used to construct the stent (i.e., self-expanding or
of test conclusions to the lengths not tested (e.g. justification based on
balloon-expandable), as well as the test purpose. For example,
finite element analysis).
cyclic axial tests that are being conducted to predict stent
6.3.2 Stent Diameter—The fatigue resistance of any specific durability under in vivo use conditions are likely to be
stent design might be dependent upon the diameter.Arationale conductedinamockvessel.Forcyclicaxialteststhatarebeing
basedonfiniteelementanalysisoranexplanationastowhythe conducted as part of a development process or as part of a FTF
particular diameter is predicted or expected to perform worst investigation, it may be possible to complete the testing
should be provided. If different labeled diameter stents within without a mock vessel. Regardless of the test configuration, the
a family have significantly different strut patterns, each unique user of the standard should provide justification for the test
conditions. If testing is conducted in air, heating of the stent
pattern should be considered separately.
resulting from applied accelerated cyclic deformation might
6.3.3 Deployment Diameter—For each labeled diameter
occur. In such a case, means (e.g., convection cooling) should
stent tested, the test stent should be deployed to the “worst-
be implemented to minimize heating and evidence provided
case” deployed diameter per the instructions for use (IFU) (see
that any remaining heating does not significantly increase the
8.2 Mock Vessels). The diameter predicted or expected to
fatigue life.
perform the worst should be justified by means such as finite
element analysis.
8.2 Mock Vessels—The mock vessel should be durable,
6.3.4 Stent Overlapping—When stents are expected to be capable of withstanding the test conditions, and able to
overlapped in clinical use, durability testing of overlapped maintain the desired stent deformations. The inner diameter
stentsshouldbeperformed.Anoverlaplengthrepresentativeof (ID) of the mock vessel is important to the outcome of the
clinical use should be selected. The relative position (rotation durability tests in this standard guide. The stented mock vessel
and overlap length) of the overlapped stents should be selected ID should be appropriate for the selected stent deployed
to ensure sufficiently challenging application of strain. Fretting diameter as described in 6.3.3, and should remain essentially
and/or wear might lead to fracture of overlapped stents during constant (i.e., not drift with time) over the duration of the test.
durability testing. Thus, further analysis (e.g., scanning elec- The wall thickness, coefficient of friction, and elasticity of the
tron microscopy (SEM)) of the stents after durability testing mock vessel might influence the testing results. For example,
might be necessary to determine the failure mode. during the bending durability test, undesired kinking may
F2942 − 19
result with an inappropriate mock vessel, or during the axial Stent deformation verification is not required for every test
durability test the stent may not elongate or compress as specimen. The number of stents used for the deformation
intendedifthefrictionbetweenthemockvesselandstentistoo verificationshouldbeadequateandjustified.Theresultsofthis
high or too low. Measures to reduce excessive diameter verification activity should be used to establish the procedure
reduction during axial testing (effect of incompressibility of for controlling the deformation of each test specimen. For
elastomers or conservation of volume), ovalization during example, if it can be shown that the cross head displacement of
bending testing, and localized instability during torsional the axial testing apparatus adequately correlates with the
testing, should be used, where appropriate. For, example, intended deformation of the stent (e.g., with high speed
appropriate mock vessels may or may not need a physiologi- imaging), this may be used to control the deformation during
cally relevant compliance and stiffer and/or thicker walled testing. Using a mathematical relationship between the cross
mock vessels may be used in order to obtain the desired head displacement and the stent deformation might also be
deformation of the stent. appropriate.
8.2.1 It is important for the stent not to migrate in the mock
8.7 Acceptance Criteria—A detailed prospective test proto-
vessel during testing. The mock vessel should be designed to
col that describes all procedures, including those unique to the
minimize stent migration.
stent being evaluated, should be written. The specific failure
8.2.2 Whensimulatingexpected in vivodeformationswitha
modes to be identified, the inspections to be performed during
TTS methodology, it is important that the expected deforma-
and/orafterdurabilitytestingtoidentifythosefailures,andany
tions be simulated as close as reasonably possible.
prospective acceptance/rejection criteria should be included in
8.2.3 Stent Deployment—The test specimens should be
this protocol.
deployed in the mock vessel in such a manner as to minimize
8.8 Fracture Detection—Detection of stent strut fractures
end effects where the vessel is connected to the test apparatus
while the stent is deployed in the mock vessel and mounted on
and at a sufficient distance from other test specimens that may
the testing apparatus can be difficult. Clear or translucent mock
be deployed in the same vessel. In the case of testing
vessels can allow for better visualization of the stent. Also, a
overlapped stents, the length of overlap should be justified.
strobe light can aid in identifying fractures during testing. The
8.3 Temperature—The temperature of the test specimen
use of a bore scope or high resolution x-ray can also be
should be maintained at 37 6 2°C for the duration of the test.
appropriate for detecting stent strut fractures. Care should be
If another temperature is used, a rationale stating why the
taken not to damage the stents during the inspection process.
particular temperature is considered relevant should be pro-
Re-deployment of stents in the mock vessel following removal
vided.
from the mock vessel for fracture inspection is not recom-
mended as the stent configuration might change and the stent
8.4 Solutions—The test solution should be phosphate buff-
might be damaged during this procedure. If the stented mock
ered saline (PBS) or equivalent, unless testing in a different
vessel is removed from the test apparatus for fracture
environment (such as in distilled water or in air) can be
inspection, use some means to ensure stent orientation can be
justified. The pH of the PBS should be adjusted to 7.4 6 0.5
maintained when remounting (especially for bending durabil-
with the appropriate buffering chemicals (e.g., sodium phos-
ity) and consider verifying stent deformations after remount-
phatedibasic(Na HPO )toraisethepHandsodiumphosphate
2 4
ing.
monobasic (NaH PO ) to lower the pH). The pH should be
2 4
verified at the beginning and at the end of the test. Biological
8.9 Test Termination—The choice of the test end point can
growth can affect the post-test evaluation of the stent surface
be varied and is dependent on the purpose of the durability
characteristics. A biological growth inhibitor (such as an
testing. For example, the end of the test could be triggered by
algaecide or chemical agent) may be used unless such use
a prespecified duration or by a certain event like the first
would negatively impact the test by unintended degradation of
fracture.
the specimen or the test setup.
8.10 Post-Test Inspection:
8.5 Test Frequency—The test should be run at a frequency
8.10.1 After the test end point is reached, a thorough
that provides a consistent cyclic deformation (e.g., with mini-
evaluation of all specimens is recommended to determine all
mal secondary harmonics) that enables the application of the
fracture locations. For certain stent designs or configurations,
desired deformation of the stent.
(e.g., braided stents or overlapped stents), fretting wear should
also be evaluated. The test specimen should be removed from
8.6 Stent Deformation Verification—Applied displacement
the test apparatus (keeping track of stent orientation for
is the translation of the motion of the actuation mechanism to
bending durability tests). Carefully remove the stent from the
the mock vessel and/or stent that results in deformation of the
mock vessel (if applicable) and inspect with light microscopy
stent. The gripping technique, slip between the mock vessel
or SEM to identify through-strut fractures. Identify and record
and the stent, or dynamic forces might result in stent deforma-
the location of any through-strut fractures. Also note the
tion (i.e., axial, bending, torsional) that is greater or less than
direction of bending, if applicable. Other anomalies (e.g.,
intended.Thus,theinvestigatorshoulddemonstratethatduring
significant wear, cracks) should be recorded.
the cyclic displacement the test specimen is subjected to the
intended deformation at the frequency used in the durability 8.10.2 SEM images may be taken of fracture surfaces and
test. The verification activity should be performed on a test fracture locations to characterize the nature and origin of the
specimen or a stent similar in structure to the test specimen. fracture. Consideration should be given to whether or not the
F2942 − 19
boundary conditions related to the testing apparatus (e.g., 9.2.4.2 Mock vessels.
gripping method) might have resulted in artifactual strut 9.2.4.3 Measurement devices.
fracture. 9.2.4.4 Inspection equipment.
8.10.3 If testing is continued beyond the first fracture, it 9.2.5 Description of test method, including all justifications
may become difficult to correctly determine the cause of and rationales recommended by this guide.
additional fractures in the same stent. In such cases, the first 9.2.6 Summary of stent deformation verification activity.
fracture and all subsequent fractures should be recorded in the 9.2.7 Description of and justification for protocol devia-
sequence observed, if possible. Where possible, the root tions.
cause(s) of the first fracture and all subsequent fractures should 9.2.8 Storage location of raw data.
beidentified,throughtheprovisionofevidence-basedrationale 9.2.9 Test results:
(e.g., SEM, fractrographic analysis, FEA comparisons). 9.2.9.1 Fracture reporting:
(1) Reportinspectionintervalsforstentfracture.Reportthe
9. Test Report
number of cycles when the first fracture was detected. It may
9.1 The test report should include a complete summary of
be appropriate to select inspection intervals on a log scale to
the materials, methods, and results, including any rationale(s)
capture low cycle fatigue fractures accurately.
for choices within the test guide and deviations from this
(2) Fractures may be described according to various litera-
standard guide and/or the detailed test protocol. The effects of
ture classification schemes or by clear descriptions in the
any such deviations on the significance of the test results
report.
should be reported. All real, artifactual, and anomalous obser-
(3) Include the location of all fractures on a diagram, plus
vations should be reported, including a justification for con-
representative photographs. If multiple fractures occur within a
sidering negative findings as artifacts or discounting their
single stent, the order of fractures should be reported, if
clinical significance.
possible.
(4) Root cause assessment of fractures may be warranted.
9.2 Test reports should include:
This type of analysis may include a comparison of fracture
9.2.1 Purpose/objective statement, such as:
location to FEA predictions and fractography to detect the
9.2.1.1 Design verification.
initiation site.
9.2.1.2 Scope statement regarding stents and implant loca-
(5) For the FTF methodology, data should be presented in
tions to which the testing is considered applicable.
tabular form providing the load level and number of cycles
9.2.2 Testparameters,acceptancecriteria,andjustifications:
whenfracturewasobserved.Thenumberofcyclescorrespond-
9.2.2.1 Test parameters, such as:
ing to the last inspection interval when fractures were not
(1) Mock vessel material and dimensions (as applicable).
observed should also be reported. In addition to the tabular
(2) Test solution including any anti-microbial agents used
presentation of data, data may be presented in a figure showing
and temperature requirements.
the load level and number of cycles when fracture was
(3) Test specimen gauge length.
observed.
(4) Average minimum and maximum test specimen axial
(6) For the TTS methodology, data should be presented in
deformation as a percentage of gauge length.
tabular form identifying specimens with and without fractures
(5) Average minimum and maximum test specimen radii of
and the corresponding number of cycles when the test was
curvature (inner or centerline).
terminated or when fractures were observed. The number of
(6) Average minimum and maximum test specimen torsion
cycles corresponding to the last inspection interval when
angles per gauge length.
fractures were not observed should also be reported.
(7) Justification for applied deformation and acceptable
9.2.9.2 Fretting wear reporting:
deformation limit.
(1) The evaluation of fretting wear should be reported for
(8) Test monitoring intervals to verify stent deformations.
braided stents and overlapped stents.
9.2.2.2 Acceptance criteria, when applicable.
9.2.10 Conclusions.
9.2.3 Test specimen information:
9.2.3.1 Number of test specimens.
10. Precision and Bias
9.2.3.2 Size (diameter, length, or other relevant dimensions)
10.1 Intra-laboratory and inter-laboratory reproducibility
of all test specimens.
has not been systematically determined.
9.2.3.3 Rationale for the number of test specimens and the
sizes used.
11. Keywords
9.2.3.4 Statement regarding how representative the speci-
mens are of the finished product. 11.1 axial fatigue; bending fatigue; coronary stent; durabil-
9.2.3.5 Sterilization condition of specimens. ity test; endovascular cardiology; endovascular graft; endovas-
9.2.3.6 Traceability information. cular prostheses; fatigue test; interventional cardiology; intra-
9.2.4 Equipment used: vascular device test; peripheral stent; stent durability; stent
9.2.4.1 Test equipment. fatigue; stent-graft; stent test; torsional fatigue; vascular stent
F2942 − 19
ANNEXES
(Mandatory Information)
A1. AXIAL DURABILITY OF VASCULAR STENTS
A1.1 Summary of Test Guide FEAto adjust the test to account for the additional stress/strain
that would occur due to the constraint of a vessel.
A1.1.1 This test guide describes an axial durability test
where the purpose is to subject the stent to a specified amount
A1.3.4 For direct fixation of the stent to the apparatus, an
of cyclic axial deformation. The stent is deployed into a mock
appropriate method of attachment of the ends of the stent
vessel,unlesssuitablejustificationisprovidedforthetestingof
should be selected. The attachment should not induce artificial
the stent without a mock vessel as described above in 8.2. This
stresses such that fracture occurs prematurely or at artificial
guidedescribesapproachesfordirectfixationoftheendsofthe
locations due to the fixation method.Also, note that the gauge
stent and mock vessel to the testing apparatus or placement of
length should not be the length of the stent.
the stent inside a mock vessel (stent ends not fixed) with
subsequent cyclic stretching of the mock vessel. A minimum
A1.4 Procedure
and a maximum cyclic stent length are justified and applied.
A1.4.1 If applicable, perform simulated delivery as indi-
During the test each test specimen is monitored for the
cated in 6.2.
occurrence of strut fracture.
A1.4.2 Specimen Deployment:
A1.2 Significance and Use
A1.4.2.1 Stent Ends Directly Secured in Test Apparatus:
A1.2.1 Thistestisusedtodeterminethedurabilityofastent
(1) The stent should be deployed into the mock vessel in a
exposed to cyclic axial deformation. The test may be used to
temperature environment of 37 6 2°C according to instruc-
assess conformance to product specifications and/or guidance
tions for use, as appropriate. The stent should be deployed into
documents, to support regulatory submissions, and for quality
a mock vessel that is not mounted on the testing apparatus.The
and/or manufacturing control purposes.
amount of stretch of the mock vessel during deployment is
A1.2.2 The success of this test depends on the ability of the
critically important to the test. For example, if the intent of the
apparatus and mock vessel to consistently induce the desired
axial durability test is to impose an axial shortening on the test
length change on the stent at the applied and verified test
specimen, then the mock vessel should be sufficiently stretched
frequency for the entire duration of the test.
upon deployment of the specimen. Thus, when the stretch in
the mock vessel is released, the stent will undergo axial
A1.3 Apparatus and Mock Vessel Selection
shortening and by stretching the mock vessel an appropriate
A1.3.1 The apparatus should provide secure attachment
amount, the stent will not be compressed axially by the mock
points for the mock vessel (and/or stent, if applicable) and be
vessel.
capable of delivering quantifiable cyclic axial deformation to
(2) After stent deployment into the mock vessel, the stent’s
the stent.
deployed length should be measured and recorded. The rela-
tionshipbetweenthestent’sunconstraineddeployedlengthand
A1.3.2 Select an appropriately sized mock vessel (e.g.,
its deployed length in the mock vessel might have a significant
silicone tube) for the stent being tested as discussed in 8.2.The
impact on the durability testing results. Stent length during
mock vessel chosen for this test should be durable and able to
withstand the test conditions without significant creep or deployment may be affected by the stent geometric changes
change in compliance. If the stent will not be directly secured that occur during the expansion process and the deployment
to the apparatus, care should be taken, and experimentation is procedure (e.g., inadvertent movement of the delivery system
recommended, when selecting the mock vessel material, wall duringdeployment).Therefore,themanufacturershouldhavea
thickness, and the relationship between the cyclic mock vessel
criterion for the deployed stent length; for example, the stent’s
length and the test specimen length. Due to the difference in
deployed length should be within 6 10% of the labeled length.
axial compliance between the stented and unstented tubing, the
Observe and note the uniformity of deployment. The state of
displacement of the apparatus may not be equal to the
the stent after deployment into the stretched mock vessel prior
deformationofthestent.Ifthestentwillnotbedirectlysecured
to mounting on the test apparatus is considered the stent
to the apparatus, mock vessel selection and the stent deforma-
deployed condition.
tion verification (8.6) are critical.
(3) While maintaining the stretched condition of the mock
vessel, move the stented mock vessel to the testing apparatus
A1.3.3 With appropriate justification, testing by securing
and secure the ends of the stent to the testing apparatus. Ensure
the stent ends to the test fixture may be done without a mock
that the stented mock vessel does not twist during mounting.
vessel. Examples of appropriate justification include experi-
Fill the mock vessel with the test solution as specified in 8.4.
mental deformation measurements, experimental fatigue
Once mounted, the unsecured stented mock vessel length (i.e.,
results, or finite element analysis indicating that the stent peak
between secured ends) should be adjusted so that axial com-
strains/stresses are in the same location and of the same or
greater magnitude at its unconstrained diameter, or the use of pression or tension is not applied to the stent relative to its
F2942 − 19
deployed condition. Verify that the unsecured stent length in may need to be shortened.As applicable, the wall thickness of
the testing apparatus is equivalent to the deployed stent length the mock vessel may also need to be adjusted.
minus the length of the secured portion of the stent.
(2) Measure the stent length between the grips.
(4) Whentestingwithoutamockvessel,thestentshouldbe (3) Adjust the apparatus to yield the desired axial stent
deployed according to the instructions for use to the deployed
deformation. The setup procedure established during the stent
diameter as discussed in 6.3.3, and carefully mounted so as not deformation verification activity (8.6) should be followed to
to twist the stent.
ensure that the intended axial deformation will be applied to
A1.4.2.2 StentEndsNotDirectlySecuredinTestApparatus: the stent. For example, if the static axial deformation of the
(1) The stent should be deployed into the mock vessel in a
stent was deemed to correlate well with the dynamic
temperature environment of 37 6 2°C according to instruc-
deformation, move the tester through a fatigue cycle and
tions for use, as appropriate. The stent may be deployed into a
measure the stent length at the appropriate positions to verify
mock vessel that is not mounted on the testing apparatus.
that the relationship between the deployed stent length, mini-
Alternately, if the test apparatus is designed to allow deploy-
mumstentlength,andmaximumstentlengthareappropriateto
ment of the stent, the mock vessel may be mounted on the
yield the desired axial stent deformation.
system prior to deployment.The amount of stretch of the mock
(4) When calculating percent axial shortening, the follow-
vessel during deployment is critically important to the test. For
ing equation should be used:
example, if the intent of the axial durability test is to impose an
L 2 L
~ !
Free Min
axial shortening on the test specimen, then the mock vessel %AS 5 3100 (A1.1)
F G
L
Free
should be in a sufficiently stretched position upon deployment
where:
of the specimen.
(2) After stent deployment, the stent’s deployed length
%AS = percent axial shortening,
should be measured and recorded. The relationship between L = unsecured stent length (as measured in the mock
Free
the stent’s unconstrained deployed length and its deployed
vessel when mounted on system), and
L = minimum unsecured stent length throughout fatigue
length in the mock vessel might have a significant impact on
Min
the durability testing results. Stent length during deployment cycle.
may be affected by the stent geometric changes that occur (5) When calculating percent axial lengthening, the follow-
during the expansion process and the deployment procedure ing equation should be used:
(e.g., inadvertent movement of the delivery system during
~L 2 L !
Max Free
deployment). Therefore, the manufacturer should have a crite-
%AL 5 3100 (A1.2)
F G
L
Free
rion for the deployed stent length; for example, the stent’s
deployed length should be within 6 10% of the labeled length. where:
Observe and note the uniformity of deployment.
%AL = percent axial lengthening,
(3) Fill the mock vessel with the test solution as specified
L = maximum stent length between the grips throughout
Max
in 8.4.
the fatigue cycle, and
(4) If the stent is deployed into a mock vessel that is not
L = unsecured stent length (as measured in the mock
Free
mounted on the testing apparatus, mount the stented mock
vessel when mounted on the system).
vessel on the testing apparatus. Ensure that the stented mock
A1.4.4.2 Stent Ends Not Directly Secured to the Test Appa-
vessel does not twist during mounting. The deployed stent
ratus:
lengthshouldbemaintainedandverifiedoncethestentedmock
(1) Secure the mock vessel to the apparatus. Alignment of
vessel is mounted on the testing system.
thetestapparatusforattachmentoftheendsofthemockvessel
(5) Record the stent’s position within the mock vessel or
is critical. Small amounts of misalignment can lead to artifac-
mark the exterior of the tubing in order to monitor for
tual buckling of the stent at large compression percentages. If
migration throughout the test.
adequate alignment is attained and buckling of the stent is still
A1.4.3 Stent Diameter Measurement:
occurring,thelengthofthestentmayneedtobeshortened.The
A1.4.3.1 Measure or determine the stent diameter in the
wallthicknessofthemockvesselmayalsoneedtobeadjusted.
mock vessel. The methods provided in appendix X2 of Test
(2) Adjust the apparatus to yield the desired axial stent
Methods F2477 may be used to calculate the inner diameter of
deformation. Commonly, axial durability testing is designed to
the stented mock vessel (assumed to be equivalent to stent
impose axial deformation in only one direction from the
outer diameter) from the outer diameter of the stented mock
deployedlength(eitheronlyshorteningoronlylengthening).If
vessel.
this is the case, then at one extreme of the axial deformation,
A1.4.4 Test Setup: the stent’s length should match the previously recorded de-
A1.4.4.1 Stent Ends Directly Secured in Test Apparatus: ployed stent length. For example, if the goal of the test is to
simulate axial shortening of the stent, the maximum stent
(1) Secure the stent and mock vessel to the apparatus.
Alignment of the test apparatus for attachment of the ends of length throughout one fatigue cycle should be equal to the
deployed length. The setup procedure established during the
the mock vessel and/or stent is critical. Misalignment can lead
to artifactual kinking of the stent or unintentional bending stent deformation verification activity (8.6) should be followed
to ensure that the intended axial deformation will be applied to
deformation. If adequate alignment is attained and kinking of
the stent is still occurring, the unsecured length of the stent the stent at the testing frequency. For example, if the static
F2942 − 19
axial deformation of the stent was found to correlate well with where:
the dynamic deformation, move the tester throughout a fatigue
%AL = percent axial lengthening,
cycle and measure the stent length at the appropriate positions
L = maximum stent length throughout fatigue cycle,
Max
to verify that the relationship between deployed stent length,
and
minimum stent length, and maximum stent length is appropri-
L = deployed stent length (as measured in the mock
Deployed
ate to yield the desired axial stent deformation. vessel when mounted on the system).
(3) When calculating percent axial shortening, the follow-
A1.4.5 Running the Axial Durability Test:
ing equation should be used:
A1.4.5.1 Zero the counter.
~L 2 L !
Deployed Min
%AS 5 3100 (A1.3) A1.4.5.2 Start the durability test instrument and adjust the
F G
L
Deployed
frequency to the verified rate.
where:
A1.4.5.3 Perform periodic monitoring of the test at prede-
%AS = percent axial shortening,
termined intervals in order to verify that the desired axial stent
L = deployed stent length (as measured in the mock
Deployed deformation and the position of the stent within the mock
vessel when mounted on the system), and
vesselaremaintainedthroughoutthetest.Thenumberofstents
L = minimum stent length throughout fatigue cycle.
Min
to be monitored for deformation control should be justified
(4) When calculating percent axial lengthening, the follow-
(unless each stent is monitored). At predetermined intervals,
ing equation should be used:
stents should be examined for fractures as described in 8.8.
L 2 L A1.4.5.4 Terminate the test and perform a post-test inspec-
~ !
Max Deployed
%AL 5 3100 (A1.4)
F G
L tion as detailed in sections 8.9 and 8.10.
Deployed
A2. BENDING DURABILITY OF VASCULAR STENTS: COLUMN BUCKLING GUIDE
A2.1 Summary of Test Guide ance documents, to support regulatory submissions, and for
quality and/or manufacturing control purposes.
A2.1.1 The stent is deployed into a mock vessel after which
the mock vessel is fixed to a set of bending fixtures. These
A2.2.2 The success of this test depends on the ability of the
bending fixtures enable the rotation of the ends of the mock
apparatus and mock vessel to consistently induce the desired
vessel. The bending fixtures are mounted on an apparatus
bending on the stent at the applied and verified test frequency
capable of imparting consistent cyclic axial motion. When the
for the entire duration of the test.
distance between the two bending fixtures is reduced, the ends
of the mock vessel rotate and the mock vessel containing the
A2.3 Test Apparatus and Mock Vessel Selection
stent bends to a radius of curvature. A minimum and a
A2.3.1 This test requires a durability testing apparatus
maximum stent bending radii are justified and applied. Each
capable of producing a linear displacement with adequate
specimen is monitored during the test for the occurrence of
accuracyandresolutiontorepeatablyobtainthedesiredchange
strut fracture.
in stent bending radius of curvature. An example fixture that
A2.2 Significance and Use
enables column buckling, bending durability testing is shown
in Figs. A2.1-A2.3. These drawings are provided to show an
A2.2.1 This test is performed to determine the durability of
example of a fatigue apparatus. Alternative fixtures may be
a stent exposed to cyclic bending deformation. It may be used
to assess conformance to product specifications and/or guid- used.
FIG. A2.1 Example of a Bending Fixture Housing (quantity =2)
F2942 − 19
FIG. A2.2 Example Bending Axle Rod and Attachment Rod (quantity =4)
its deployed length in the mock vessel might have a significant
impact on the durability testing results. Stent length during
deployment may be affected by the stent geometric changes
that occur during the expansion process and the deployment
procedure (e.g., inadvertent movement of the delivery system
duringdeployment).Therefore,
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: F2942 − 13 F2942 − 19
Standard Guide for
in vitro Axial, Bending, and Torsional Durability Testing of
Vascular Stents
This standard is issued under the fixed designation F2942; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This guide includes three separate cyclic deformation durability guides related to vascular stents: bending, axial, and
torsional.
1.2 This guide does not address flat plate, local crush durability, or multi-mode testing. Although this guide does not address
multi-mode testing, the information included herein could be applicable to developing this type of test.
1.3 This guide applies to balloon-expandable and self-expanding stents fabricated from metals and metal alloys. It does not
specifically address any attributes unique to coated stents (i.e., stentstents with a surface layer of an additional material(s)),
monolithically polymeric stents, or absorbable stents, although the application of this standard to those products is not precluded.
1.4 This guide applies to endovascular grafts (“stent-grafts”) and other conduit products commonly used to treat aneurismal
disease, peripheral vessel trauma, or to provide vascular access. The information provided herein does not address all issues related
to testing of these devices.
1.5 This guide is applicable to testing of stent(s) (or a representative portion of a stent). While durability testing of coupon
samples (e.g., a scaled-up portion of the stent structure) can provide useful information, it is not within the scope of this guide.
1.5 This guide applies to endovascular grafts (“stent-grafts”) and other conduit products commonly used to treat aneurismal
disease, peripheral vessel trauma, or to provide vascular access. The information provided herein does not address all issues related
to testing of these devices.
1.6 This guide applies to in vitro modeling of stent durability from non-radial arterial motions. Such motions may arise from
musculoskeletal activities, including walking and breathing, and cardiac motion. ASTM Test Methods F2477 addresses pulsatile
(i.e., radial) durability of vascular stents.
1.7 This guide does not provide the in vivo physiologic deformation conditions for a vascular stent. It is incumbent upon the
user of the standard to develop and justify these boundary conditions (e.g., by literature review, in vivo studies, cadaver studies,
or modeling of stent vessel interaction) in these durability bench tests. Additional conditions that may be considered include vessel
calcification, vessel taper, eccentric lesions, loading excursions (e.g., exercise), and vessel remodeling.
1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.9 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
F2477 Test Methods forin vitro Pulsatile Durability Testing of Vascular Stents
F3211 Guide for Fatigue-to-Fracture (FtF) Methodology for Cardiovascular Medical Devices
This test method is under the jurisdiction of ASTM Committee F04 on Medical and Surgical Materials and Devices and is the direct responsibility of Subcommittee
F04.30 on Cardiovascular Standards.
Current edition approved Aug. 15, 2013June 1, 2019. Published September 2013July 2019. Originally approved in 2013. Last previous edition approved in 2013 as
F2942–13. DOI: 10.1520/F2942-13.10.1520/F2942-19.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F2942 − 19
2.2 Other Documents:
ASTM STP 588 Manual on Statistical Planning and Analysis, R.E. Little, 1975
ISO 25539 Cardiovascular Implants–Endovascular Devices–Part 2: Vascular Stents
FDA Guidance Document #1545 “Non-Clinical Engineering Tests and Recommended Labeling for Intravascular Stents and
Associated Delivery Systems” (Issued April 18, 2010)
3. Terminology
3.1 Definitions of Terms Specific to This Standard:
3.1.1 axial, adj—compression or tension of a stent and/or mock vessel along its longitudinal axis.
3.1.2 bending, adj—deformation on the longitudinal axis of a stent and/or mock vessel to achieve a specified stent radius of
curvature.
3.1.3 fracture, n—the complete separation of a stent structural feature.element or component (e.g., strut, apex, bridge, marker).
3.1.3.1 Discussion—
Fracture of a stent does not necessarily constitute failure (i.e., loss of functionality).
3.1.4 mock vessel, n—a simulated vessel typically manufactured from an elastomeric material.
3.1.5 radius of curvature, n—the inner, outer or centerline bend radius of a stent.
3.1.6 specimen, n—article consisting of an implantable device or a representative portion of an implantable device, that is tested
according to this guide.
3.1.7 torsional, adj—twisting of a stent and/or mock vessel about its longitudinal axis.
4. Summary of Test Guides
4.1 This guide covers in vitro durability testing of vascular stents using modes that represent those that might be observed in
vivo such as bending, axial, or torsional deformation modes. Examples include, but are not limited to, the axial and bending
deformation that occurs in the superficial femoral artery during the walking gait, the bending that occurs in the renal artery during
respiration, and the bending that occurs in the coronary artery during the cardiac cycle. This guide provides details and guidance
for separate tests for each deformation mode: axial, bending, and torsional. This guide allows the direct fixation of the ends of the
stent or indirect fixation inside a mock vessel. Direct fixation of the ends of the stent allows better control of stent deformation;
however, this can result such fixation might not be representative of in vivo conditions and care should be taken to minimize
attachment-induced test artifact.
4.1.1 Axial Durability Test Guide—The purpose of this test is to subject the stent to a specified amount of cyclic axial
deformation. The stent is deployed into a mock vessel, unless a justification is provided. This test guide is described in more detail
in Annex A1.
4.1.2 Bending Durability Test Guide—The purpose of this test is to subject the stent to a specified amount of cyclic bending
deformation. There are three suggested bending guides presented in Annex A2, Annex A3, and Annex A4, each involving
placement of the stent inside of a mock vessel: column buckling, bending on a mandrel, and bending in an arc without a mandrel.
In order to avoid test artifacts, these test guides recommend placement of the stent inside a mock vessel (stent ends not fixed) with
subsequent cyclic bending of the mock vessel. When selecting the guide to conduct bending durability testing, consider the
potential for the stent design under evaluation to be adversely affected by the methods and test apparatus described by a particular
guide and the ability of each test to simulate a particular clinical condition.
4.1.3 Torsional Durability Test Guide—The purpose of this test is to subject the stent to a specified amount of cyclic torsional
deformation. This guide is described in more detail in Annex A5.
4.2 Each test may utilize either ‘test-to-success’ (TTS) or the ‘fatigue-to-fracture’ (FTF) methodology.methodology (see Guide
F3211). The TTS methodology entails selection of a set of boundary conditions considered physiologically relevant, selection of
a sufficient number of specimens, and application of the appropriate number of cycles. The successful completion of the TTS is
based upon the number (if any) and type of stent strut fractures. The FTF methodology entails selection of the appropriate number
of cycles considered runout (i.e., point to stop testing a specimen), selection of a sufficient number of specimens, and
characterization of the stent fatigue performance by applying multiple deformation levels (i.e., loading amplitude)amplitudes) and
conducting periodic inspections of the stent during testing to obtain some test specimens with fractures and some without.without
fracture. For specimens that fracture, the number of cycles applied to cause fracture is obtained. The successful completion of a
FTF test is based upon a comparison of stent fatigue performance, at the various deformation levels, to the physiologically relevant
deformation levels. Selection of deformation levels to characterize the fatigue behavior of the stent may use the methodology
described in ASTM STP 588.
F2942 − 19
5. Significance and Use
5.1 It is important to consider the durability of stent designs in deformation modes that are intended to model in vivo conditions.
The appropriate amplitude and number of cycles in each of the modes hashave to be determined independently for the particular
clinical use proposed for the stent. These tests do not replicate all varieties and aspects of the deployment process andnor the in
vivo mechanical environment so in its entirety, and as such they cannot be proofs of durability. Instead, the tests provide evidence
of durability. The durability tests can also provide a means of assessing design, material or processing changes.
5.1.1 This guide might be useful for development testing, specification acceptance testing, and regulatory submission testing
and filings as it provides a basic assurance that the tests are designed, executed, and reported in a suitable fashion.
5.1.2 If the tests are conducted using a well-defined well defined FTF methodology, they can be useful in:
5.1.2.1 Potential design improvement through identification of better and worse geometries, materials, and manufacturing
processes;
5.1.2.2 Understanding product durability by estimating the effects of changes in geometry, materials, or manufacturing
processes;
5.1.2.3 Estimating the safety factor relative to the amplitudes and other factors in use conditions; and
5.1.2.4 Validating finite element analysis (FEA) and fatigue life models.
5.1.3 As stated in the scope, this guide is not intended to provide the in vivo physiologic deformation conditions that to which
a vascular stent can be subjected. Reliable clinical data characterizing cyclic vascular deformation may be lacking for some
indications. The user should develop and justify the boundary conditions (e.g., by literature review, in vivo studies, cadaver studies,
or modeling of stent vessel interaction) for the chosen durability bench tests. Additional conditions that may be considered include
vessel calcification, vessel taper, eccentric lesions, deformation excursions (e.g., exercise), and vessel remodeling.
5.1.4 Test methods other than those provided in the annexes of this document might be appropriate, depending upon stent
design. However, these methods are beyond the scope of this guide.
6. Specimen Size, Configuration, and Preparation
6.1 Unless otherwise justified, all specimens selected for testing should be taken from fully processed, implant quality product.
Sterilization should be performed unless it can be shown not to influence the durability test results.
NOTE 1—Although sterilization may not directly affect the stent itself, it may affect the delivery system and, thus, the condition of the as-deployed stent.
6.2 Prior to deployment and durability testing, specimens loaded in or on their delivery systems should be tracked through a
model representative of the vasculature to simulate clinical delivery.
6.3 To reduce the number of specimens to be tested, durability may be evaluated for the worst case justified device size/model.
Alternatively, multiple sizes (length and/or diameter) at potentially multiple deployment diameters would need to be tested with
device size/model, with specimen selection being justified through appropriate methods (e.g., finite element analysis). If multiple
sizes/models or deployment diameters are to be tested, an appropriate bracketing scheme should be employed (e.g., largest and
smallest length and/or diameter or models).
6.3.1 Stent Length—The axial and torsional durability testing modes act to induce stent deformation normalized with length
(length change per length, and transverse angle change per length, respectively). Thus, the fatigue resistance of a stent design with
a repeating unit or cell design would also be independent of length and any length may be tested. In cases where the stent design
is length-dependent (e.g., non-repeating unit cells), the length predicted or expected to perform worst should be justified (e.g., by
finite element analysis or description of stent design). analysis).
NOTE 2—Because of the nature of these test methods, it may not be possible to test the longest stent length within a family of sizes, especially in the
overlapped configuration. In such cases, other means may need to be implemented to justify the stent length tested or to allow extrapolation of test
conclusions to the lengths not tested (e.g. justification based on finite element analysis).
6.3.2 Stent Diameter—The fatigue resistance of any specific stent design might be dependent upon the diameter. A rationale
based on finite element analysis or an explanation as to why the particular diameter is predicted or expected to perform worst
should be provided. If different labeled diameter stents within a family have significantly different strut patterns, each unique
pattern should be considered separately.
6.3.3 Deployment Diameter—For each labeled diameter stent tested, the test stent should be deployed to the “worst-case”
deployed diameter per the instructions for use (IFU) (see section 8.2 Mock Vessels). The diameter predicted or expected to perform
the worst should be justified by means such as finite element analysis.
6.3.4 Stent Overlapping—When stents are expected to be overlapped in clinical use, durability testing of overlapped stents
should be performed. An overlap length representative of clinical use should be selected. The relative position (rotation and overlap
length) of the overlapped stents should be selected to ensure sufficiently challenging application of strain. Fretting and/or wear
might lead to fracture of overlapped stents during durability testing. Thus, further analysis (e.g., scanning electron microscopy
(SEM)) of the stents after durability testing might be necessary to determine the failure mode.
6.4 The number of specimens tested for each stent size and/or geometry should be sufficient to support any claims made based
on the test results. The results of testing according to this guide in combination with other tests, animal and clinical tests, analysis
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(such as FEA), and/or comparisons to predicate devices can be sufficient to enable demonstration of an adequate durability. In this
guide, one stent or a set of two overlapped stents should be considered one specimen.
7. General Apparatus Requirements
7.1 The axial, bending and torsional dynamic displacements of the test equipment should be verified at the selected test
frequencies. The dynamic stent deformation verification documentation should include justification of the verification means (see
section 8.6).
7.2 Dimensional Measurement Devices—Devices such as linear variable displacement transducers (LVDTs), lasers, and
high-speed cameras should be calibrated.
7.3 Cycle Counting System—The apparatus should include a cycle counting system for measuring the number of deformation
cycles applied to the stent. The cycle counting system should be verified at the test frequencies and the verification should be
documented.
7.4 Temperature Control System—The apparatus should include a calibrated temperature control and measurement system to
maintain the temperature of the stents being tested.
8. General Test Parameters
8.1 Completion of the durability test for stents deployed within a mock vessel, in air alone, or in fluid alone, depends on the
deformation mode (i.e., axial, torsional, or bending), the material used to construct the stent (i.e., self-expanding or
balloon-expandable), as well as the test purpose. For example, cyclic axial tests that are being conducted to predict stent durability
under in vivo use conditions are likely to be conducted in a mock vessel. For cyclic axial tests that are being conducted as part
of a development process or as part of a FTF investigation, it may be possible to complete the testing without a mock vessel.
Regardless of the test configuration, the user of the standard should provide justification for the test conditions. If testing is
conducted in air, heating of the stent resulting from applied accelerated cyclic deformation might occur. In such a case, means (e.g.,
convection cooling) should be implemented to minimize heating and evidence provided that any remaining heating does not
significantly increase the fatigue life.
8.2 Mock Vessels—The mock vessel should be durable, capable of withstanding the test conditions, and able to maintain the
desired stent deformations. The inner diameter (ID) of the mock vessel is important to the outcome of the durability tests in this
standard guide. The stented mock vessel ID should be appropriate for the selected stent deployed diameter as described in section
6.3.3 above, , and should remain essentially constant (i.e.(i.e., not drift with time) over the duration of the test. The wall thickness,
coefficient of friction, and elasticity of the mock vessel might influence the testing results. For example, during the bending
durability test, undesired kinking may result with an inappropriate mock vessel, or during the axial durability test the stent may
not elongate or compress as intended if the friction between the mock vessel and stent is too high or too low. Measures to reduce
excessive diameter reduction during axial testing (effect of incompressibility of elastomers or conservation of volume), ovalization
during bending testing, and localized instability during torsional testing, should be used, where appropriate. For, example,
appropriate mock vessels may or may not need a physiologically relevant compliance and stiffer and/or thicker walled mock
vessels may be used in order to obtain the desired deformation of the stent.
8.2.1 It is important for the stent not to migrate in the mock vessel during testing. The mock vessel should be designed to
minimize stent migration.
8.2.2 It is important for the stent not to migrate in the mock vessel during testing. The mock vessel should be designed and/or
modified to minimize stent migration. When simulating expected in vivo deformations with a TTS methodology, it is important
that the expected deformations be simulated as close as reasonably possible.
8.2.3 Stent Deployment—The test specimens should be deployed in the mock vessel in such a manner as to minimize end effects
where the vessel is connected to the test apparatus and at a sufficient distance from other test specimens that may be deployed in
the same vessel. In the case of testing overlapped stents, the length of overlap should be justified.
8.3 Temperature—The temperature of the test specimen should be maintained at 37 6 2°C for the duration of the test. If another
temperature is used, a rationale stating why the particular temperature is considered relevant should be provided.
8.4 Solutions—The test solution should be phosphate buffered saline (PBS) or equivalent, unless testing in a different
environment (such as in distilled water or in air) can be justified. The pH of the PBS should be adjusted to 7.4 6 0.5 with the
appropriate buffering chemicals (e.g., sodium phosphate dibasic (Na HPO ) to raise the pH and sodium phosphate monobasic
2 4
(NaH PO ) to lower the pH). The pH should be verified at the beginning and at the end of the test. Biological growth can affect
2 4
the post-test evaluation of the stent surface characteristics. A biological growth inhibitor (such as an algaecide or chemical agent)
may be used unless such use would negatively impact the test by unintended degradation of the specimen or the test setup.
8.5 Test Frequency—The test should be run at a frequency that provides a consistent cyclic deformation (e.g., with minimal
secondary harmonics) that enables the application of the desired deformation of the stent.
8.6 Stent Deformation Verification—Applied displacement is the translation of the motion of the actuation mechanism to the
mock vessel and/or stent that results in the deformation of the stent. The gripping technique, slip between the mock vessel and the
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stent, or dynamic forces might result in stent deformation (i.e., axial, bending, torsional) that is greater or less than intended. Thus,
the investigator should demonstrate that during the cyclic displacement the test specimen is subjected to the intended deformation
at the frequency used in the durability test. The verification activity should be performed on a test specimen or a stent similar in
structure to the test specimen. Stent deformation verification is not required for every test specimen. The number of stents used
for the deformation verification should be adequate and justified. The results of this verification activity should be used to establish
the procedure for controlling the deformation of each test specimen. For example, if it can be shown that the cross head
displacement of the axial testing apparatus adequately correlates with the intended deformation of the stent, stent (e.g., with high
speed imaging), this may be used to control the deformation during testing. Using a mathematical relationship between the cross
head displacement and the stent deformation might also be appropriate.
8.7 Acceptance Criteria—A detailed prospective test protocol that describes all procedures, including those unique to the stent
being evaluated, should be written. The specific failure modes to be identified, the inspections to be performed during and/or after
durability testing to identify those failures, and any prospective acceptance/rejection criteria should be included in this protocol.
8.8 Fracture Detection—Detection of stent strut fractures while the stent is deployed in the mock vessel and mounted on the
testing apparatus can be difficult. Clear or translucent mock vessels can allow for better visualization of the stent. Also, a strobe
light can aid in identifying fractures during testing. The use of a bore scope or high resolution x-ray can also be appropriate for
detecting stent strut fractures. Care should be taken not to damage the stents during the inspection process. Re-deployment of stents
in the mock vessel following removal from the mock vessel for fracture inspection is not recommended as the stent configuration
might change and the stent might be damaged during this procedure. If the stented mock vessel is removed from the test apparatus
for fracture inspection, use some means to ensure stent orientation can be maintained when remounting (especially for bending
durability) and consider verifying stent deformations after remounting.
8.9 Test Termination—The choice of the test end point can be varied and is dependent on the purpose of the durability testing.
For example, the end of the test could be triggered by a prespecified duration or by a certain event like the first fracture.
8.10 Post-Test Inspection:
8.10.1 After the test end point is reached, a thorough evaluation of all specimens is recommended to determine all fracture
locations. For certain stent designs or configurations, (e.g., braided stents or overlapped stents), fretting wear should also be
evaluated. The test specimen should be removed from the test apparatus (keeping track of stent orientation for bending durability
tests). Carefully remove the stent from the mock vessel (if applicable) and inspect with light microscopy or SEM to identify
through-strut fractures. Identify and record the location of any through-strut fractures. Also note the direction of bending, if
applicable. Other anomalies (e.g., significant wear, cracks) should be recorded.
8.10.2 SEM images may be taken of fracture surfaces and fracture locations to characterize the nature and origin of the fracture.
Consideration should be given to whether or not the boundary conditions related to the testing apparatus (e.g., gripping method)
might have resulted in artifactual strut fracture.
8.10.3 If testing is continued beyond the first fracture, it may become difficult to correctly determine the cause of additional
fractures in the same stent. In such cases, the first fracture and all subsequent fractures should be recorded in the sequence
observed, if possible. Where possible, the root cause(s) of the first fracture and all subsequent fractures should be identified,
through the provision of evidence-based rationale (e.g., SEM, fractrographic analysis, FEA comparisons).
9. Test Report
9.1 The test report should include a complete summary of the materials, methods, and results, including any rationale(s) for
choices within the test guide and deviations from this standard guide and/or the detailed test protocol. The effects of any such
deviations on the significance of the test results should be reported. All real, artifactual, and anomalous observations should be
reported, including a justification for considering negative findings as artifacts or discounting their clinical significance.
9.2 Test reports should include:
9.2.1 Purpose/objective statement, such as:
9.2.1.1 Design verification.
9.2.1.2 Scope statement regarding stents and implant locations to which the testing is considered applicable.
9.2.2 Test parameters, acceptance criteria, and justifications:
9.2.2.1 Test parameters, such as:
(1) Mock vessel material and dimensions (as applicable).
(2) Test solution including any anti-microbial agents used and temperature requirements.
(3) Test specimen gauge length.
(4) Average minimum and maximum test specimen axial deformation as a percentage of gauge length.
(5) Average minimum and maximum test specimen radii of curvature (inner or centerline).
(6) Average minimum and maximum test specimen torsion angles per gauge length.
(7) Justification for applied deformation and acceptable deformation limit.
(8) Test monitoring intervals to verify stent deformations.
9.2.2.2 Acceptance criteria, when applicable.
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9.2.3 Test specimen information:
9.2.3.1 Number of test specimens.
9.2.3.2 Size (diameter, length, or other relevant dimensions) of all test specimens.
9.2.3.3 Rationale for the number of test specimens and the sizes used.
9.2.3.4 Statement regarding how representative the specimens are of the finished product.
9.2.3.5 Sterilization condition of specimens.
9.2.3.6 Traceability information.
9.2.4 Equipment used:
9.2.4.1 Test equipment.
9.2.4.2 Mock vessels.
9.2.4.3 Measurement devices.
9.2.4.4 Inspection equipment.
9.2.5 Description of test method, including all justifications and rationales recommended by this guide.
9.2.6 Summary of stent deformation verification activity.
9.2.7 Description of and justification for protocol deviations.
9.2.8 Storage location of raw data.
9.2.9 Test results:
9.2.9.1 Fracture reporting:
(1) Report inspection intervals for stent fracture. Report the number of cycles when the first fracture was detected. It may be
appropriate to select inspection intervals on a log scale to capture low cycle fatigue fractures accurately.
(2) Fractures may be described according to various literature classification schemes or by clear descriptions in the report.
(3) Include the location of all fractures on a diagram, plus representative photographs. If multiple fractures occur within a
single stent, the order of fractures should be reported, if possible.
(4) Root cause assessment of fractures may be warranted. This type of analysis may include a comparison of fracture location
to FEA predictions and fractography to detect the initiation site.
(5) For the FTF methodology, data should be presented in tabular form providing the load level and number of cycles when
fracture was observed. The number of cycles corresponding to the last inspection interval when fractures were not observed should
also be reported. In addition to the tabular presentation of data, data may be presented in a figure showing the load level and
number of cycles when fracture was observed.
(6) For the TTS methodology, data should be presented in tabular form identifying specimens with and without fractures and
the corresponding number of cycles when the test was terminated or when fractures were observed. The number of cycles
corresponding to the last inspection interval when fractures were not observed should also be reported.
9.2.9.2 Fretting wear reporting:
(1) The evaluation of fretting wear should be reported for braided stents and overlapped stents.
9.2.10 Conclusions.
10. Precision and Bias
10.1 Intra-laboratory and inter-laboratory reproducibility has not been systematically determined.
11. Keywords
11.1 axial fatigue; bending fatigue; coronary stent; durability test; endovascular cardiology; endovascular graft; endovascular
prostheses; fatigue test; interventional cardiology; intravascular device test; peripheral stent; stent durability; stent fatigue;
stent-graft; stent test; torsional fatigue; vascular stent
ANNEXES
(Mandatory Information)
A1. AXIAL DURABILITY OF VASCULAR STENTS
A1.1 Summary of Test Guide
A1.1.1 This test guide describes an axial durability test where the purpose is to subject the stent to a specified amount of cyclic
axial deformation. The stent is deployed into a mock vessel, unless suitable justification is provided for the testing of the stent
without a mock vessel as described above in section 8.2. This guide describes approaches for direct fixation of the ends of the stent
and mock vessel to the testing apparatus or placement of the stent inside a mock vessel (stent ends not fixed) with subsequent cyclic
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stretching of the mock vessel. A minimum and a maximum cyclic stent length are justified and applied. During the test each test
specimen is monitored for the occurrence of strut fracture.
A1.2 Significance and Use
A1.2.1 This test is used to determine the durability of a stent exposed to cyclic axial deformation. The test may be used to assess
conformance to product specifications and/or guidance documents, to support regulatory submissions, and for quality and/or
manufacturing control purposes.
A1.2.2 The success of this test depends on the ability of the apparatus and mock vessel to consistently induce the desired length
change on the stent at the applied and verified test frequency for the entire duration of the test.
A1.3 Apparatus and Mock Vessel Selection
A1.3.1 The apparatus should provide secure attachment points for the mock vessel (and/or stent, if applicable) and be capable of
delivering quantifiable cyclic axial deformation to the stent.
A1.3.2 Select an appropriately sized mock vessel (e.g., silicone tube) for the stent being tested as discussed in section 8.2. The
mock vessel chosen for this test should be durable and able to withstand the test conditions without significant creep or change
in compliance. If the stent will not be directly secured to the apparatus, care should be taken, and experimentation is recommended,
when selecting the mock vessel material, wall thickness, and the relationship between the cyclic mock vessel length and the test
specimen length. Due to the difference in axial compliance between the stented and unstented tubing, the displacement of the
apparatus may not be equal to the deformation of the stent. If the stent will not be directly secured to the apparatus, mock vessel
selection and the stent deformation verification (section (8.6) are critical.
A1.3.3 With appropriate justification, testing by securing the stent ends to the test fixture may be done without a mock vessel.
Examples of appropriate justification include experimental deformation measurements, experimental fatigue results, or finite
element analysis indicating that the stent peak strains/stresses are in the same location and of the same or greater magnitude at its
unconstrained diameter, or the use of FEA to adjust the test to account for the additional stress/strain that would occur due to the
constraint of a vessel.
A1.3.4 For direct fixation of the stent to the apparatus, an appropriate method of attachment of the ends of the stent should be
selected. The attachment should not induce artificial stresses such that fracture occurs prematurely or at artificial locations due to
the fixation method. Also, note that the gauge length should not be the length of the stent.
A1.4 Procedure
A1.4.1 If applicable, perform simulated delivery as indicated in section 6.2.
A1.4.2 Specimen Deployment:
A1.4.2.1 Stent Ends Directly Secured in Test Apparatus:
(1) The stent should be deployed into the mock vessel in a temperature environment of 37 6 2°C according to manufacturer’s
instructions for use, as appropriate. The stent should be deployed into a mock vessel that is not mounted on the testing apparatus.
The amount of stretch of the mock vessel during deployment is critically important to the test. For example, if the intent of the
axial durability test is to impose an axial shortening on the test specimen, then the mock vessel should be sufficiently stretched
upon deployment of the specimen. Thus, when the stretch in the mock vessel is released, the stent will undergo axial shortening
and by stretching the mock vessel an appropriate amount, the stent will not be compressed axially by the mock vessel.
(2) After stent deployment into the mock vessel, the stent’s deployed length should be measured and recorded. The relationship
between the stent’s unconstrained deployed length and its deployed length in the mock vessel might have a significant impact on
the durability testing results. Stent length during deployment may be affected by the stent geometric changes that occur during the
expansion process and the deployment procedure (e.g., inadvertent movement of the delivery system during deployment).
Therefore, the manufacturer should have a criterion for the deployed stent length; for example, the stent’s deployed length should
F2942 − 19
be within 6 10% of the labeled length. Observe and note the uniformity of deployment. The state of the stent after deployment
into the stretched mock vessel prior to mounting on the test apparatus is considered the stent deployed condition.
(3) While maintaining the stretched condition of the mock vessel, move the stented mock vessel to the testing apparatus and
secure the ends of the stent to the testing apparatus. Ensure that the stented mock vessel does not twist during mounting. Fill the
mock vessel with the test solution as specified in section 8.4. Once mounted, the unsecured stented mock vessel length (i.e.,
between secured ends) should be adjusted so that axial compression or tension is not applied to the stent relative to its deployed
condition. Verify that the unsecured stent length in the testing apparatus is equivalent to the deployed stent length minus the length
of the secured portion of the stent.
(4) When testing without a mock vessel, the stent should be deployed according to the manufacturer’s instructions for use to
the deployed diameter as discussed in section 6.3.3, and carefully mounted so as not to twist the stent.
A1.4.2.2 Stent Ends Not Directly Secured in Test Apparatus:
(1) The stent should be deployed into the mock vessel in a temperature environment of 37 6 2°C according to manufacturer’s
instructions for use, as appropriate. The stent may be deployed into a mock vessel that is not mounted on the testing apparatus.
Alternately, if the test apparatus is designed to allow deployment of the stent, the mock vessel may be mounted on the system prior
to deployment. The amount of stretch of the mock vessel during deployment is critically important to the test. For example, if the
intent of the axial durability test is to impose an axial shortening on the test specimen, then the mock vessel should be in a
sufficiently stretched position upon deployment of the specimen.
(2) After stent deployment, the stent’s deployed length should be measured and recorded. The relationship between the stent’s
unconstrained deployed length and its deployed length in the mock vessel might have a significant impact on the durability testing
results. Stent length during deployment may be affected by the stent geometric changes that occur during the expansion process
and the deployment procedure (e.g., inadvertent movement of the delivery system during deployment). Therefore, the manufacturer
should have a criterion for the deployed stent length; for example, the stent’s deployed length should be within 6 10% of the
labeled length. Observe and note the uniformity of deployment.
(3) Fill the mock vessel with the test solution as specified in section 8.4.
(4) If the stent is deployed into a mock vessel that is not mounted on the testing apparatus, mount the stented mock vessel on
the testing apparatus. Ensure that the stented mock vessel does not twist during mounting. The deployed stent length should be
maintained and verified once the stented mock vessel is mounted on the testing system.
(5) Record the stent’s position within the mock vessel or mark the exterior of the tubing in order to monitor for migration
throughout the test.
A1.4.3 Stent Diameter Measurement:
A1.4.3.1 Measure or determine the stent diameter in the mock vessel. The methods provided in appendix X2 of ASTM Test
Methods F2477 may be used to calculate the inner diameter of the stented mock vessel (assumed to be equivalent to stent outer
diameter) from the outer diameter of the stented mock vessel.
A1.4.4 Test Setup:
A1.4.4.1 Stent Ends Directly Secured in Test Apparatus:
(1) Secure the stent and mock vessel to the apparatus. Alignment of the test apparatus for attachment of the ends of the mock
vessel and/or stent is critical. Misalignment can lead to artifactual kinking of the stent or unintentional bending deformation. If
adequate alignment is attained and kinking of the stent is still occurring, the unsecured length of the stent may need to be shortened.
As applicable, the wall thickness of the mock vessel may also need to be adjusted.
(2) Measure the stent length between the grips.
(3) Adjust the apparatus to yield the desired axial stent deformation. The setup procedure established during the stent
deformation verification activity (section (8.6) should be followed to ensure that the intended axial deformation will be applied to
the stent. For example, if the static axial deformation of the stent was deemed to correlate well with the dynamic deformation, move
the tester through a fatigue cycle and measure the stent length at the appropriate positions to verify that the relationship between
the deployed stent length, minimum stent length, and maximum stent length are appropriate to yield the desired axial stent
deformation.
(4) When calculating percent axial shortening, the following equation should be used:
L 2 L
~ !
Free Min
%AS 5 3100 (A1.1)
F G
L
Free
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where:
%AS = percent axial shortening,
L = unsecured stent length (as measured in the mock vessel when mounted on system), and
Free
L = minimum unsecured stent length throughout fatigue cycle.
Min
(5) When calculating percent axial lengthening, the following equation should be used:
L 2 L
~ !
Max Free
%AL 5 3100 (A1.2)
F G
L
Free
where:
%AL = percent axial lengthening,
L = maximum stent length between the grips throughout the fatigue cycle, and
Max
L = unsecured stent length (as measured in the mock vessel when mounted on the system).
Free
A1.4.4.2 Stent Ends Not Directly Secured to the Test Apparatus:
(1) Secure the mock vessel to the apparatus. Alignment of the test apparatus for attachment of the ends of the mock vessel is
critical. Small amounts of misalignment can lead to artifactual buckling of the stent at large compression percentages. If adequate
alignment is attained and buckling of the stent is still occurring, the length of the stent may need to be shortened. The wall thickness
of the mock vessel may also need to be adjusted.
(2) Adjust the apparatus to yield the desired axial stent deformation. Commonly, axial durability testing is designed to impose
axial deformation in only one direction from the deployed length (either only shortening or only lengthening). If this is the case,
then at one extreme of the axial deformation, the stent’s length should match the previously recorded deployed stent length. For
example, if the goal of the test is to simulate axial shortening of the stent, the maximum stent length throughout one fatigue cycle
should be equal to the deployed length. The setup procedure established during the stent deformation verification activity (section
(8.6) should be followed to ensure that the intended axial deformation will be applied to the stent at the testing frequency. For
example, if the static axial deformation of the stent was found to correlate well with the dynamic deformation, move the tester
throughout a fatigue cycle and measure the stent length at the appropriate positions to verify that the relationship between deployed
stent length, minimum stent length, and maximum stent length is appropriate to yield the desired axial stent deformation.
(3) When calculating percent axial shortening, the following equation should be used:
L 2 L
~ !
Deployed Min
%AS 5 3100 (A1.3)
F G
L
Deployed
where:
%AS = percent axial shortening,
L = deployed stent length (as measured in the mock vessel when mounted on the system), and
Deployed
L = minimum stent length throughout fatigue cycle.
Min
(4) When calculating percent axial lengthening, the following equation should be used:
~L 2 L !
Max Deployed
%AL 5 3100 (A1.4)
F G
L
Deployed
where:
%AL = percent axial lengthening,
L = maximum stent length throughout fatigue cycle, and
Max
L = deployed stent length (as measured in the mock vessel when mounted on the system).
Deployed
A1.4.5 Running the Axial Durability Test:
A1.4.5.1 Zero the counter.
A1.4.5.2 Start the durability test instrument and adjust the frequency to the verified rate.
A1.4.5.3 Perform periodic monitoring of the test at predetermined intervals in order to verify that the desired axial stent
deformation and the position of the stent within the mock vessel are maintained throughout the test. The number of stents to be
monitored for deformation control should be justified (unless each stent is monitored). At predetermined intervals, stents should
be examined for fractures as described in section 8.8.
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A1.4.5.4 Terminate the test and perform a post-test inspection as detailed in sections 8.9 and 8.10.
A2. BENDING DURABILITY OF VASCULAR STENTS: COLUMN BUCKLING GUIDE
A2.1 Summary of Test Guide
A2.1.1 The stent is deployed into a mock vessel after which the mock vessel is fixed to a set of bending fixtures. These bending
fixtures enable the rotation of the ends of the mock vessel. The bending fixtures are mounted on an apparatus capable of imparting
consistent cyclic axial motion. When the distance between the two bending fixtures is reduced, the ends of the mock vessel rotate
and the mock vessel containing the stent b
...