ASTM F3067-14(2021)

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: F3067 − 14 (Reapproved 2021)
Standard Guide for
Radial Loading of Balloon-Expandable and Self-Expanding
Vascular Stents
This standard is issued under the fixed designation F3067; 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 2. Referenced Documents
2.1 ASTM Standards:
1.1 This document provides guidance for developing in
E4 Practices for Force Verification of Testing Machines
vitro test methods for measuring the radial strength or collapse
E177 Practice for Use of the Terms Precision and Bias in
pressure of balloon-expandable vascular stents and chronic
ASTM Test Methods
outward force of self-expanding vascular stents.
F2079 Test Method for Measuring Intrinsic Elastic Recoil of
1.2 This guide is applicable to balloon-expandable and
Balloon-Expandable Stents
self-expanding stents of tubular geometry. It covers both stent
F2081 Guide for Characterization and Presentation of the
and stent grafts. It does not cover bifurcated stents. It does not
Dimensional Attributes of Vascular Stents
cover stents with non-circular cross sections or tapered stents.
F2477 Test Methods for in vitro Pulsatile Durability Testing
of Vascular Stents
1.3 Units—The values stated in SI units are to be regarded
as standard. No other units of measurement are included in this
3. Terminology
standard.
3.1 Definitions:
1.4 Thisguidedoesnotrecommendanyspecifictestmethod
3.1.1 balloon-expandable stent—a stent that is expanded at
or apparatus for measuring the radial strength, collapse
the treatment site by a balloon catheter. The stent material is
pressure, or chronic outward force. Instead, this guide provides
plastically deformed by the balloon expansion such that the
examples of test methodologies and equipment that could be
stent remains expanded after deflation of the balloon.
used and recommends the format for presenting test results.
3.1.2 chronic outward force—the minimum continued open-
ing force of a self-expanding stent acting on the vessel wall at
1.5 This guide covers only in vitro bench testing methods.
a specified diameter. The range of chronic outward force is
In vivo behavior might be different.
defined by the unloading curve at the maximum and minimum
1.6 This standard does not purport to address all of the
indicated use diameters. Additional loading force consider-
safety concerns, if any, associated with its use. It is the
ations for self-expanding stents are evaluated as load excur-
responsibility of the user of this standard to establish appro-
sions and described in Appendix X2. Chronic outward force is
priate safety, health, and environmental practices and deter-
not defined for balloon-expandable stents.
mine the applicability of regulatory limitations prior to use.
3.1.3 collapse pressure—the uniform radial load during
1.7 This international standard was developed in accor-
testing with a hydraulic or pneumatic apparatus in which a
dance with internationally recognized principles on standard-
balloon-expandable stent undergoes buckling over a specific
ization established in the Decision on Principles for the
region or the entire stent length.
Development of International Standards, Guides and Recom-
3.1.4 load—a normalized, scalar value of force applied by
mendations issued by the World Trade Organization Technical
the stent to the vessel and, at equilibrium, the vessel upon the
Barriers to Trade (TBT) Committee.
stent. Load should be normalized by length (newton or
millinewton per millimeter length) or by area (pascal or
kilopascal).
This guide 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. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Aug. 1, 2021. Published August 2021. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 2014. Last previous edition approved in 2014 as F3067 – 14. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/F3067-14R21. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F3067 − 14 (2021)
3.1.5 loading line—for balloon-expandable stents, the line shape that is close to the desired final size and shape, when
derived from the substantially linear portion of the radial released from the delivery system.
loading curve during initial compression. The term is not
3.1.13 stent graft—transluminally placed tubular vascular
defined for balloon-expandable stents tested using collapse
prosthesis, with one or more integral stent components to
pressure apparatus.
provide fixation or radial support, or both, residing partially or
completelywithinavascularconduittoformaninternalbypass
3.1.6 radial force—output of radial loading that equals the
or shunt between sections of the vascular system.
radialpressuretimesthestentcylindricalarea.Therelationship
betweenradialforce(F )andradialpressure(P)isgiveninthe
R 3.1.14 stent length—unstressed length of the stent after
equation:
deployment. If the stent has marker bands on non-radial
force-producing components, the length is measured from the
F
R
P 5 (1)
ends of the radial force-producing sections. The measured
A
length of mounted or expanded stents should be measured by
where:
non-contacting instruments (profile projection, laser
P = radial pressure,
micrometer, and so forth) with a resolution of 0.1 mm or better
F = radial force, and
R (see Guide F2081).
A = instantaneous stent cylindrical area:
3.1.15 unloading line—for balloon-expandable stents, the
line derived from the substantially linear portion of the radial
A 5 πDL
unloading curve. The term does not apply for balloon-
where:
expandable stents tested using collapse pressure apparatus.
D = instantaneous stent expanded outer diameter, and
3.1.16 vascular patency—a measure of the extent to which
L = L for length change less than 10 % and L = L(D) for
thevesselisopen(unrestricted).Typicallyreportedasapercent
length change greater than 10 %. L:L is the expanded
of the reference (unrestricted, adjacent) vessel diameter or
stent length for balloon-expandable stents and uncon-
cross-sectional area.
strained length for self-expanding stents. L(D)isthe
3.1.17 vascular stent—a tubular synthetic structure that is
instantaneous length of the stent as a function of the
implanted in the native or grafted vasculature and that is
current instantaneous diameter. L(D) may be either
intended to provide mechanical radial support to enhance
experimentally determined or computationally derived.
vessel patency. For the purpose of this guide, a stent might be
3.1.7 radial loading—a mechanical loading mode in which
metallic or non-metallic. It might be durable or absorbable.
the load is directed perpendicular to the longitudinal axis of a
3.1.18 zero compression diameter—the diameter reference
cylinder and applied to the outer cylindrical surface of the
pointrequiredforthetestingapparatustofullyengagethestent
stent. The load is applied to the entire outer surface or to at
outer surface. Stent compression is calculated in comparison to
least three areas that are equally distributed around the outer
this diameter.
circumference and extend over the entire cylinder length. Load
might be expressed as radial force or radial pressure.
4. Significance and Use
3.1.8 radial loading curve—the graph of radial loading
4.1 Upon deployment, at the site of the vascular stenosis,
output on the y-axis versus diametric deformation of a stent on
the stent establishes the patency of the lumen until vascular
the x-axis.
remodeling occurs. The radial load acting upon the stent is
3.1.9 radial pressure—the area normalized output of radial
imparted by vessel and lesion stretch. Additionally, the vessel
loading equaling the average pressure applied to the stent by
might be affected by excursions due to pulsation (systolic and
the loading fixture in the radial direction toward the stent diastolic variation), muscle-skeletal interactions due to patient
cylindrical axis.
movement, as well as external sources (e.g., patient is struck in
the neck during a car accident). The excursions vary in
3.1.10 radial resistive load—the peak load during a com-
magnitude and type based on the location of the vessel.
pression excursion of a self-expanding stent. The excursion
might be a single event or a cycle. A typical example is 4.2 In order to maintain vessel patency, the stent has to
pulsatile cycling of an implanted self-expanding stent (refer to
withstand the forces acting on it without experiencing exces-
Appendix X2). sive deformation, migration, or sustained collapse; therefore, it
is required that the stent possess adequate resistance to these
3.1.11 radial strength—a specific load on the radial loading
loads.
curve that corresponds with a specific and clinically (practi-
cally) relevant amount of inward plastic deformation from the
4.3 Depending on the type of device and the clinical
unloaded state. The term is defined only for balloon- concern, the resistance to these loads can be presented through
expandablestentstestedwhosesolemechanismofexpansionis
multiple test outputs: radial strength, collapse pressure, or
byaballoon.Additionally,thetermappliesonlytostentstested chronic outward force.
using a segmented head or sling type apparatus and not using
4.4 The guidelines presented here can be used in the
a hydraulic or pneumatic pressure apparatus.
development of test methods to determine the radial loading
3.1.12 self-expanding stent—a stent that expands without properties of stents. This guide provides examples of different
the application of external forces or pressure, to a size and test apparatus (equipment and tooling), radial loading curves,
F3067 − 14 (2021)
and calculations. Although the apparatus and methods pre- 5.4 In order to distinguish between different stent types as
sented can be used as a reasonable simulation of actual clinical well as the apparatus used, separate test outputs are defined in
use, they have not been demonstrated to predict the actual in ordertoclarify,andlimit,thecomparisonsbetweentestresults.
vivo clinical performance of any stent. For example, a distal ring (edge, local) collapse of a balloon-
expandable stent as measured (collapse pressure) using a
5. Summary of Guide
hydraulic/pneumatic test apparatus might not directly convert
or correlate to the radial strength output of the same device
5.1 As defined, radial loading is applied uniformly over the
tested using a segmented head apparatus. Further, because the
entire stent surface at a minimum of three evenly distributed
loading behavior of balloon-expandable and self-expanding
circumferential locations over the full length of the stent.
stents are very different, the self-expanding stent test output
Testing in which a portion of the stent extends outside aperture
terminologyischronicoutwardforceratherthanradialstrength
is not specifically discussed within the guide because it does
or collapse pressure. Different test output terms are utilized in
not result in uniform radial loading since a portion of the stent
order to clarify the differences and limit comparisons.
outside the aperture might also contribute to the load. Further,
the direction of loading is radially inward as shown in Fig. 1.
5.5 The clinical effects listed in Fig. 2 are separate from the
The uniform radial loading is applied to at least three areas that
device effects. The device effects are directly observed stent
are equally spaced around the outer circumference and extend
events, while the clinical effects are the anticipated concerns
over the entire cylinder length.
associated with the event. The clinical concerns presented are
examples; other clinical concerns might be identified from the
5.2 Some stents are designed to have significantly different
mechanical properties along their length. In these cases, it same list of device effects.
might be preferable to test specific regions of the device. This
5.6 Since the test outputs of load are normalized (either by
might require apparatus (equipment or tooling) or changes to
length or area), it is important to realize that the interpretation
accommodate local application of loading (e.g., inserts, ma-
of output has inherent limitations for stents that are designed to
chined gaps, or having the test article extend past the edge of
have significantly stronger (more resistive) and weaker (less
the fixture). In addition, the assessment of the loading is
resistive) portions. This is truer for the sling and segmented
complicated because the loading of the tested region will be
head apparatus than the hydraulic/pneumatic collapse pressure
affected by the portion of the stent which is not being tested.
apparatus. The hydraulic/pneumatic tester can visually detect a
The treatment of these modifications and the normalization of
localized region of weakness that collapses during pressuriza-
loading are not specifically covered in this guide and should be
tion.
mentioned within the test report.
5.3 Radialtestingofstentswilldifferdependingonthestent
6. Apparatus
type (balloon-expandable versus self-expanding) as well as the
6.1 The key element of radial testing is the selection, or
apparatus used (segmented head, sling, or hydraulic/
development, of an apparatus (equipment and tooling) that
pneumatic). The apparatus is selected based on clinical effects
radially loads the stent.
(concerns) and limited by the stent type. For example, the
hydraulic/pneumatic apparatus cannot typically be used for 6.2 The type of radial loading, as described in Fig. 1,isa
theoretical construct and each type of loading apparatus has
testingofself-expandingstentsfromthesheathtotheunloaded
diameterbecausethetubingislikelytoflattenorrupturewithin some degree of deviation from the perfectly distributed radial
the large test range. The following summary outlines different loading. There are multiple types of apparatuses that are
apparatus, based on stent type, and the associated clinical capable of applying a radial load to a cylindrical stent with
effects that can be evaluated (see Fig. 2). adequate uniformity.
FIG. 1 Radial Loading
F3067 − 14 (2021)
FIG. 2 Summary Guide
6.3 This guide describes three specific types of apparatus for precision and bias may be found in Practice E177.
that might be considered for radial testing: segmented head, Consistent use of the terms between different laboratories aids
sling, and hydraulic (or pneumatic) chamber. in clarity when comparing method assessments as well as test
method validation results.
6.4 This guide does not provide detailed descriptions or
guidelines for test apparatus design; thus, specific and unique
6.10 The loading rate affects test output. The loading may
interpretations of the apparatus are expected by the test either be displacement-controlled (linear motion on the load
laboratory or equipment developer. Additional tooling, not
tensile test machine for a sling apparatus) or pressure-
described, might be valuable in improving the test method controlled (pressurization rate for a hydraulic collapse appara-
consistency (precision and robustness) and accuracy. tus). The rate of compression (or expansion) or pressurization
should be slow enough to minimize inertial effects of moving
6.5 It is expected that other apparatus (i.e., significant
parts but quick enough to minimize binding caused by static
deviations from the equipment design concept) not described
friction. The testing, however, does not require continuous
within this standard might also be adequate to radially load
movement. Pausing at intervals might be useful to allow the
vascular stents. If another apparatus is utilized, rationale to
system to equilibrate. The testing does not need to match
justify the suitability of that apparatus should be provided. For
physiologic rates of change, but rather should try to increase
example, a data correlation to test results from one of the test
test result precision and robustness and minimize variation
equipment listed in this guide might be used. It is expected that
between equipment and laboratories.
each apparatus will have specific limitations or requirements
when testing specific groups of specimens or testing specific 6.11 Force and diameter calibration through the entire load
ranges of diameters. Test method development should map the pathforthesegmentedheadandslingtestequipmentshouldbe
use and limitations of the equipment for test articles. completed. The hydraulic/pneumatic head test equipment re-
quires pressure calibration and also requires calibration of the
6.6 It is recognized that the choice of test apparatus is likely
diameter measurement if so equipped.
toinfluencethecharacteristicshapeoftheradialloadingcurves
andthusthetestoutput.Therefore,directcomparisonsbetween 6.12 Segmented Head Apparatus:
results obtained using different equipment is discouraged
6.12.1 Fig. 3 describes the operation of a segmented head
unless data correlations are completed. type radial loading apparatus. This fixture employs wedge-
shaped elements that are simultaneously activated through an
6.7 Because the specimens are often either destroyed (e.g.,
arc. This motion changes the effective diameter of the opening
balloon-expandable stent testing) or change with repetition
in the center of the head, thus compressing the stent or
(e.g., self-expanding stents), creating a correlation between
allowing it to expand. Note that the specimen is shown as
different test apparatus might require comparing test groups
partially inserted into the head, simulating the loading of the
rather than direct specimen correlation (i.e., paired test data).
stent; however, during testing the unit should be fully inserted.
6.8 The apparatus for testing self-expanding and balloon-
6.12.2 Segmented head equipment measures mean load
expandable stents that are sensitive to temperature in the
resistance for an entire stent at a given diameter due to the fact
approximate range of 20 to 40 °C (the range from laboratory to
that the wedge segments are rigid. Because of this, the
bodytemperature)shouldbedesignedtomaintainthetempera-
apparatus cannot discriminate between weaker and stronger
ture at 37 6 2 °C. It should have a temperature control system
regions of a stent. In addition, the apparatus maintains a
as well as monitoring gauges.
circular (inscribed) cross section. Therefore, if a stent deforms
6.9 It is expected that all individual apparatuses and applied in a non-circular shape, forces might change due to local areas
methods should have precision evaluated for the intended test of non-contact with the segments. In this situation the forces
articlesevaluated.Biasevaluationisnotrequiredasthereisnot might not fully characterize the non-circular stent (which are
an accepted reference value or standard. Terms and concepts not in scope of this guide). Preferential deformation along the
F3067 − 14 (2021)
FIG. 3 Segmented Head Fixturing (stent is partially inserted for illustrative purposes)
axial length, as well as edge effects due to vessel collapse, should be considered. It is recommended that the apparatus
might be better evaluated using a hydraulic or pneumatic friction be monitored during long-term testing to track wear or
apparatus. debris buildup within the segments that limit their smooth
6.12.3 The segmented head equipment typically measures motion and cause misalignment to the actuating mechanism.
the applied load from an actuator that causes a subsequent
6.12.7 In addition to the internal friction, the equipment
force application to the test specimen.Thus, the applied load is
developer and test engineer should consider the friction forces
converted to an actual load (or pressure) on the stent, and a
between the head contact surface and the stent outer surface.
conversion is established. The conversion of measured force/
Significant normal forces might be generated; thus the fric-
pressure to the applied force/pressure might be done theoreti-
tional drag between the stent outer surface and the head
cally (e.g., using a free body diagram and force balance) or by
segments. These loads will falsely add to the measured radial
measuring both the input and output forces/pressures and
load. Thus, the design of the fixture, selection of a segment
creating data conversion curves.
material (or surface finish), and rate of loading (or unloading)
6.12.4 Segmented head type equipment has the ability to
should be considered. These loads can vary greatly and might
measure radial load over a very large range of diameter.
be appreciable at high loads. Unfortunately, these loads cannot
Therefore, for self-expanding stents the radial load can be
be evaluated by operating the apparatus head empty. Indica-
measured from sheathed to fully unloaded. The equipment
tions of high frictional loads might be device twisting (seen
might be well suited for investigating the radial forces associ-
post testing) or uneven loading/unloading curves.
ated with loading the stent into the deployment device,
6.12.8 The diameter of the segmented head is defined by the
evaluating chronic outward forces at the minimum and maxi-
inscribed circle defined within the contact segments (see Fig.
mum indicated use diameters, and to conduct excursion testing
4).
(e.g., pulsatile simulation testing) for self-expanding stents
6.12.9 Force applied to the stent might affect the apparent
(refer to Appendix X2).
diameter of the aperture if the aperture size is measured
6.12.5 Apparatus friction reduction and monitoring is
indirectly. If the error is deemed significant, a force correction
important, especially for small diameter as well as short length
curve or table might be used to adjust the diameter measure-
test specimens. The zero load friction effect of the apparatus
ments.
can be evaluated by running it without a stent in order to
6.13 Sling Apparatus:
capture a baseline friction curve. The baseline friction curve
6.13.1 Fig.5showstheoperationofasling-typeradialforce
includes both the loading and unloading in the range of
tester. This fixture employs a low-friction sling which when
diameters and at the same speed as the device is tested. The
pulled through a restriction tightens around the stent test
curve then can be subtracted from all tested device curves or,
article, thus radially compressing the stent. For self-expanding
if negligible, ignored.
stents the fixture might be used to evaluate forces during the
6.12.6 The baseline friction associated with the apparatus
unloading of the stent through the operating range of compres-
(noise) should not significantly affect the result (signal) com-
sion (minimum and maximum indicated use).
pared to the specification or should be subtracted from the
radial loading curve. If the loading result is low (less than 5:1) 6.13.2 As the sling aperture is reduced the sling material
in comparison to baseline friction (signal to noise ratio), willstretch.Thus,thecomplianceofthematerialwillaffectthe
apparatus modifications (shorter head length or segment modi- length of sling material. Because the sling material stretches,
fication) to reduce friction or other testing techniques (e.g., Eq 3 is used to determine the “effective” diameter. Eq 4 is used
evaluating longer stent lengths or testing multiple stents) to determine the radial force from the linear force.
F3067 − 14 (2021)
FIG. 4 Equivalent Diameter for Segmented Head (Example of a Hexagon)
FIG. 5 Sling-Type Radial Loading Fixturing
2 F
L x = position of crosshead (up direction is positive as
D 5 D 2 ∆ x 1 (3)
S D
π K
shown),
F = linear force measured by tensile machine, and
L
where:
K = spring constant associated with sling:
D = diameter as a function of crosshead position
(computed), 2EA
K 5
D = initial diameter (directly measured), and L
∆x = changeinlineardisplacementofthecrosshead(maybe
where:
less than zero depending on x and x ):
E = elastic modulus of sling in tensile direction,
∆x 5 x 2 x
A = cross-sectional area of the sling material, and
L = relaxed length of sling material from attachment
where: 0
location.
x = initial position of crosshead,
F3067 − 14 (2021)
F 5 πF (4) 6.13.7 Repeated calibration or verification of the initial
R L
diameter of the sling should be established to adjust for
where:
slippage within the fixture or plastic deformation of the sling.
F = radial force (computed), and
R
6.13.8 The force testing equipment, which is connected to
F = linear force measured by tensile machine.
L
the sling apparatus, should be calibrated in accordance with
6.13.3 Because of these considerations, the sling should be
Practices E4. In practice, the error of the force test equipment
made of a material that has a low bending stiffness but high
should be significantly less than the artifactual loads associated
tensile stiffness. This is more critical for small diameter test
with the sling apparatus.
specimens.
6.13.9 The displacement range needed for the sling com-
6.13.4 Loads due to sling bending and apparatus friction
puted diameter range should be within the verified range of
associated with the sling, rods, and plates can be partially
displacement for the force test equipment.
evaluated by running a load and unload cycle (for self-
6.14 Hydraulic or Pneumatic Chamber Apparatus:
expanding stents in the elastic region of the device) with the
6.14.1 If one wishes to test one specimen over a large range
loop empty. This is useful for both developing the apparatus
of diameters, the hydraulic (or pneumatic) chamber, as
design (e.g., gaps between rods, sling material, sling thickness)
described, is not considered suitable. Since self-expanding
and for monitoring the apparatus over time. However, it should
stenttestingoftenrequiresrelativelylargediameterranges,itis
be realized that sling bending and friction loads might be a
often not suitable for use for these devices. However, for a
function of stent radial load and stent diameter. This might not
narrow range it is an acceptable apparatus.
be linear and might be appreciable at high radial loads and/or
6.14.2 Fig. 6 describes the operation of a pressurized
small stent diameters. Low and consistent friction is critical.
hydraulic or pneumatic radial force tester.
6.13.5 If the stent result to friction obtained by running the
6.14.3 This fixture employs a pressurization system that
loopempty(signaltonoiseratio)islow(perhapslessthan5:1),
applies a load to a stent deployed in a thin elastic tube.
apparatusmodifications(e.g.,differentslingmaterialorchange
Optionally, an optical system can be used to measure the stent
in fixture gap) to reduce friction, or other methods (e.g., test
or elastic tubing diameter in order to determine the onset of
longer stent length, test multiple stents, or pause to allow
collapse. The system generally measures the deployed stent
friction dissipation), should be considered.
through a clear glass or plastic window within the chamber.
6.13.6 In addition to the internal friction, the equipment
The elastic tubing is sealed to the chamber wall, ensuring that
developer as well as the test engineer should consider the
there is no fluid or air leakage during the pressurization. It is
friction forces between the sling and the stent outer surface.
acceptable to either: (a) deploy directly into the tubing,
Significant normal forces might be generated and thus fric-
measure the stent or stented tube diameter, and then install into
tional drag between the stent outer surface and the sling
the apparatus; or (b) to install the elastic tubing in the
increases. These loads will falsely add to the measured radial
apparatus, deploy the stent, and then measure the diameter. In
load. Thus, the design of the fixture, selection of a sling
either method, the diameter measurement system should be
material (or at least contact surface) with high lubricity, and
calibrated. If an introducer is used, it remains open during the
rate of loading or unloading should be considered. Friction
test to ensure that the stent inner diameter remains at zero gage
forces also tend to build up during compression and expansion.
pressure (laboratory nominal pressure).
Multiple stop points might be used in order to allow friction
forces to be released and measured force values to stabilize. 6.14.4 If it is desired to estimate the stent diameter during
Indications of high frictional loads might be device twisting pressurization, the stent outer diameter might be indirectly
(seen post testing) or uneven loading/unloading curves. approximatedbysubtractingtwicethetubewallthicknessfrom
FIG. 6 Typical Hydraulic or Pneumatic Radial Loading Apparatus
F3067 − 14 (2021)
the measured elastic tube outer diameter. For small changes in of collapse. Either a root mean square calculation for equiva-
tubing diameter, or axial stretch, the unloaded tube wall lent diameter based on the measurement or, for simplicity, the
thickness is adequate; however, for large changes the tube wall arithmetic mean of the measurements can be used as the
thickness should be evaluated as a function of the tube “equivalent” diameter.
condition (stretch, material, and wall thickness; reference Test 6.14.13 For devices with large gaps (for example, gaps to
Methods F2477, Appendix X2, for calculations). allow for vessel side branch access), there might be localized
tubing collapse at high pressures. This should be evaluated in
6.14.5 The tubing should generally be as thin and elastic as
terms of the clinical relevance of the test to determine if any
practical so as to not shield the device from the applied
localized collapse is of concern for the tested device.
pressure load.
6.14.6 Tubing wall thickness and material should be con-
7. Radial Loading Measures and Interpretations
sidered for durability and resistance to pinhole leaks. Pressure
monitoring or visual inspection, if water is used, is recom-
7.1 Categories of Characteristic Radial Loading Curves:
mended to detect leaks.
7.1.1 There are characteristic loading curves (load versus
6.14.7 Tubing wall thickness variation might affect tubing
diameter) based on the type of test apparatus (segmented head,
compliance and therefore the loading applied to the stent. The
sling, or hydraulic/pneumatic) and the type of stent (balloon-
test method should provide for the evaluation (e.g., pre-test
expandable or self-expanding) evaluated. The following sec-
characterization testing) or controls (e.g., storage conditions
tions illustrate the characteristic curves (radial load versus
and shelf life) to ensure tubing elastic and dimensional
diameter) that are generated during testing and the data
consistency.
interpretation that might be used. The curves and interpreta-
6.14.8 Changes in tube elasticity due to aging might affect tions are divided into the following three categories (see Fig.
2):
the test results. If the tubing is to be stored for an extended
period of time, the tube material should be evaluated for its 7.1.1.1 Radial strength testing of balloon-expandable stents
compliance change with aging. If applicable, the test method using segmented head or sling apparatus.
should provide controls to limit tube aging effects or otherwise 7.1.1.2 Chronic outward load testing of self-expanding stent
account for the effects during data analysis.
testing using a segmented head apparatus or sling apparatus.
7.1.1.3 Collapse pressure testing of balloon-expandable
6.14.9 The length of unsupported mock vessel between the
stents using hydraulic/pneumatic apparatus.
inner edge of rigid chamber tube and the stent will contribute
7.1.2 It is best to describe the mechanical action of the
to an edge effect by either aiding or resisting stent compression
apparatus and the resultant load-versus-diameter curves simul-
asthetestprogresses.Somegapmightbenecessaryforclinical
taneously because it clearly and graphically describes both the
representative loading at the stent ends.The desired gap should
testing and results.The following sections are presented for the
be determined based upon elastic tubing characteristics (outer
three categories identified.
diameter, wall thickness, and compliance), stent and chamber
tube outer diameters, and the expected reduction in stent outer
7.2 Balloon-Expandable Stent Loading Curve for Seg-
diameter prior to the initiation of collapse. Generally, the gap
mented Head or Sling Apparatus:
should be less than one diameter in length. It is recommended
7.2.1 A plot for a balloon-expandable stent using a seg-
that the test method or apparatus provide for controlled
mented head or sling apparatus establishes the radial loading
positioning of the deployed stent to achieve the desired gap.
curve. An example is shown in Fig. 7.
6.14.10 If the stent diameter is measured and if the chamber
7.2.1.1 The load is shown as one complete cycle and
is hydraulic (rather than pneumatic), the effect of water
separated into four segments: (1) initial loading, (2) loading
refraction must be included in vision system calibration. The
with increased plasticity, (3) unloading, and (4) return.
compressibility of the fluid (air, water, etc.) will affect the
7.2.1.2 The radial load (y-axis) should be expressed either
ability to pressurize the system quickly. A gaseous fluid (e.g.,
as a force normalized by the initial stent deployed length
air) will require a slower rate of pressurization in order to
(N/mm) or as a force normalized by area (kPa). If using area
precisely measure the system response. An appropriate maxi-
normalized radial load, use the instantaneous stent diameter,
mum pressurization rate should be established. Additionally,
instead of the starting diameter, multiplied by the initial
pausing at stepped intervals might limit the variability associ-
deployed length (see Eq 1). The fixture diameter (x-axis) is
ated with the pressurization rate and fluid compressibility.
expressed in millimeters (mm). When the stent is in complete
6.14.11 An advantage of the hydraulic/pneumatic apparatus
circumferential contact with the fixture then the stent outer
isthatitmightbeabletoidentifyradialresistivecharacteristics
diameter equals the fixture diameter.
associatedwitharegionalstentdesigndifference.Forexample,
7.2.1.3 As shown in Fig. 7, Detail A, there might be a
an increased ring spacing or reduced strut thickness in one
substantially nonlinear portion of the initial load curve as the
region of the stent could be evaluated. Additionally, the
fixture begins to engage the stent. This portion of the curve is
transition between the stented and un-stented region of the
notconsideredtorepresenttheresponseofthestentasmanyof
vessel might be evaluated for edge-loading effects and injury
the struts have not yet fully contacted the fixture loading
potential.
surfaces. A marked increase in radial loading can be observed
6.14.12 Ifstentdiameterisbeingmeasured,takingmeasure- once the fixture starts to fully engage the test specimen.
ments in at least two orientations 90° apart is recommended to 7.2.1.4 After engagement of the stent, the initial portion of
account for non-circularity that might impact the identification the loading cycle is approximately linear (see Fig. 8). The
F3067 − 14 (2021)
FIG. 7 Typical Radial Loading Plot of Balloon-Expandable Stent Using a Segmented Head or Sling Apparatus
FIG. 8 Typical Radial Loading Determination for a Balloon-Expandable Stent Using a Segmented Head or a Sling-Type Apparatus
loading line is created by the steepest, substantially linear techniques for determining the zero compression diameter
portion of the loading curve. One technique of establishing the include using a specified pre-load or a slope (or rate of change
zero compression diameters is to use the intercept of the of slope) criterion. This reference diameter marks the start of
loading line with the x-axis (as shown in Fig. 7). Other valid stent compression. It should be similar to or slightly smaller
F3067 − 14 (2021)
thanthemeasurementoftheintrinsicrecoiledouterdiameterof offsetlineandtheloadingcurveisthemaximumload,andthus
the stent (see Test Method F2079). the radial strength. In some designs, however, the peak load
7.2.1.5 Theinitiallinearpartofthecurvemightbeelasticor might occur prior to the intercept. When this occurs, the radial
a combination of elastic and plastic deformation. Eventually strength is the peak load prior to the intercept and not the load
the stent undergoes increased plastic deformation as the curve associated with the intercept of the offset line and the radial
starts to flatten (the load increases at a decreasing rate) with loading curve (see Fig. 9). Thus, the radial strength is the
greater compression. corresponding maximum load for a given, clinically important,
7.2.1.6 The stent must be compressed to a greater deforma- permanent deformation.
tion than is expected clinically in order to obtain the loading
7.2.1.11 The test noise needs to be minimized adequately,
datarequiredtocomputetheradialstrengthoutputresult.Once
based on the selected offset, to avoid erroneously low radial
this maximum compression is reached the stent is unloaded. It
strength measurement.
has been observed, in a limited number of stent designs, that
7.2.1.12 This approach is based on a loading curve charac-
the unloading lines are approximately parallel for a relatively
teristic in which the unloading line is approximately parallel
widerangeofmaximumcompressiondiameters(seeAppendix
for a relatively wide variety of compressions, including the
X1). This observation allows for the use of an offset line
specificcompressionthatwouldhaveresultedinafinal,critical
parallel to the unloading line and is a key premise to the
plastic deformation. Treatment of the condition where this is
measurement of radial strength.
not true is illustrated in Appendix X1. The methodology of
7.2.1.7 In a similar manner to the loading line, the steepest
cyclic stepped loading in Appendix X1 may always be used
portion of the substantially linear unloading curve is used to
instead of the above approach because it does not assume the
create the unloading line. The difference between the loading
unloading curves to be approximately parallel. If a cyclic
line x-axis intercept and unloading line x-axis is the diametric
steppedloadapproachcannotbeperformedduetoinstabilityin
plastic deformation of the stent due to compression.
the unloading (e.g., buckling), an alternate approach should be
7.2.1.8 The measured unloading line can be offset parallel
developed.
so that its intercept with the x-axis is at a specific amount of
7.3 Self-Expanding Stent Characteristic Loading Curve for
plastic deformation from the initial diameter. The intersection
Segmented Head or Sling Apparatus:
of this offset line with the loading curve establishes a specific
7.3.1 Fig. 10 shows a typical radial unloading and loading
point (load) that is expected to produce the specified amount of
curve for a self-expanding stent using a segmented head or
plastic deformation created by the offset value. This plasticity
sling-type apparatus.
might have clinical significance.
7.3.2 It is a key decision to begin the testing by either: (1)
7.2.1.9 Clinically relevant compressions might be based on
directly deploying into the test apparatus, or (2) compressing
the following possible criteria:
from the free state (fully unloaded).
(1) Stent migration (fixation effectiveness).
7.3.3 For the direct deployment technique, the aperture is
(2) Inadequate acute vessel patency.
7.2.1.10 For stent designs with load deformation curves adjusted to larger than the sheathed diameter (2.5 mm as
similar to that of Fig. 8, the intersection of the offset unloading shown) but smaller than the minimum diameter of use (5.0 mm
line with the loading curve establishes the maximum compres- as shown). There are three squares shown in Fig. 10,as
sion (x) for radial strength determination. The radial strength examples, indicating that there is a range of deployment
for the chosen offset is the maximum radial load which occurs diameters available. The effect of deployment diameter should
atorbeforetheinterceptoftheoffsetlineandtheradialloading not have a significant impact on the resultant chronic outward
curve. Fig. 8 illustrates the condition where the intercept of the force if selected near enough to the sheathed diameter.
FIG. 9 Typical Radial Loading Determination for a Balloon-Expandable Stent Using a Segmented Head or a Sling-Type Apparatus with
the Peak Load Greater Than the Intercept
F3067 − 14 (2021)
FIG. 10 Typical Loading Determination for a Self-Expanding Stent
However, if there is a practically significant difference in diameter associated with the indicated usage. The length
chronic outward force, stent deployment should be performed normalized force at this diameter is the maximum chronic
at a smaller aperture (closer to the sheathed diameter). outward force.
7.3.4 The direct deployment from the delivery system
7.3.8 The opening continues to increase until the maximum
provides the most clinically similar load history for the test
indicated use diameter (7.0 mm in Fig. 10) is reached. The
device; however, it might be very difficult to control the
normalized force at this diameter is the minimum chronic
deployed length and difficult to visually detect an incomplete,
outward force. All deployed chronic outward force is interme-
compressed, or elongated deployment. Because results are
diatebetweenthesevalues;thus,theseresultsbrackettherange
normalized per stent length, and the stretched/compressed
of clinically expected chronic outward force.
shape might affect the overall stent resistive force, the vari-
7.3.9 Indicated use diameters, rather than equilibrium
ability in deployed length and expansion evenness might result
diameters, are used. These are selected because they are
in high test variability (worse precision). The more clinically
independent of vessel compliance and are sufficiently accurate
similar load history, though, is expected to provide more
to obtain reasonable estimates of the chronic conditions.
accurate test output. Controls and verification of proper de-
7.3.10 The rate of aperture opening (rate of diameter in-
ployment and length are important test considerations.
crease) should be slow enough that the chronic outward force
7.3.5 For testing that starts from the freely deployed
isnotsignificantlyaffected.Testingatmultipleopeningratesin
condition,theevaluatorfirstdeploysthestentfromthefinished
order to determine the selected speed should be done as part of
product, or mock deployment system, if justified, per the
the method development. Test rate does not reflect clinical
instructions for use (IFU). Because the behavior of a self-
expansion rates but rather the one used to obtain consistent test
expanding stent is dependent on its deformation path, and
output.
because these stents loaded in the delivery system remain
7.3.11 Lastly, there are a variety of load excursions (e.g.,
constantly under radial compression, it is desirable to establish
pulsatile motion or muscle-skeletal forces associated with
that the radial loading and unloading curves are similar
walking) which the stent might be subjected to after implan-
between free-state and direct deployment.
tation and which might be tested. These cyclic and non-cyclic
7.3.6 If testing from the free-state deployment, the test
load excursions are not discussed within this guide as they are
begins by reducing the device to a diameter well below the
unique to the device and its usage and thus are outside of the
minimum indicated use diameter but greater than the sheathed
scope of the standard. Refer to Appendix X2 for an illustrative
diameter. The evaluator might need to control the rate of
exam
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