ASTM F2789-10(2020)

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: F2789 − 10 (Reapproved 2020)
Standard Guide for
Mechanical and Functional Characterization of Nucleus
Devices
This standard is issued under the fixed designation F2789; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 1.6 Thisguideisnotintendedtobeaperformancestandard.
It is the responsibility of the user of this guide to characterize
1.1 This guide describes various forms of nucleus replace-
the safety and effectiveness of the nucleus device under
ment and nucleus augmentation devices. It further outlines the
evaluation.
types of testing that are recommended in evaluating the
performance of these devices. 1.7 The values stated in SI units are to be regarded as
standard. No other units of measurement are included in this
1.2 Biocompatibility of the materials used in a nucleus
standard. Angular measurements may be reported in either
replacement device is not addressed in this guide. However,
degrees or radians.
users should investigate the biocompatibility of their device
1.8 This standard does not purport to address all of the
separately (see X1.1).
safety concerns, if any, associated with its use. It is the
1.3 While it is understood that expulsion and endplate
responsibility of the user of this standard to establish appro-
fractures represent documented clinical failures, this guide
priate safety, health, and environmental practices and deter-
does not specifically address them, although some of the
mine the applicability of regulatory limitations prior to use.
factors that relate to expulsion have been included (see X1.3).
1.9 This international standard was developed in accor-
1.4 Multiple tests are described in this guide; however, the
dance with internationally recognized principles on standard-
userneednotusethemall.Itistheresponsibilityoftheuserof
ization established in the Decision on Principles for the
this guide to determine which tests are appropriate for the
Development of International Standards, Guides and Recom-
devices being tested and their potential application. Some tests
mendations issued by the World Trade Organization Technical
maynotbeapplicableforalltypesofdevices.Moreover,some
Barriers to Trade (TBT) Committee.
nucleus devices may not be stable in all test configurations.
However, this does not necessarily mean that the test methods
2. Referenced Documents
described are unsuitable.
2.1 ASTM Standards:
1.5 The science of nucleus device design is still very young
D2990Test Methods forTensile, Compressive, and Flexural
and includes technology that is changing more quickly than
Creep and Creep-Rupture of Plastics
this guide can be modified. Therefore, the user must carefully D6204Test Method for Rubber—Measurement of Unvulca-
consider the applicability of this guide to the user’s particular
nized Rheological Properties Using Rotorless Shear Rhe-
device; the guide may not be appropriate for every device. For ometers
example,atthetimeofpublication,thisguidedoesnotaddress
E6Terminology Relating to Methods of MechanicalTesting
thenucleusreplacementandnucleusaugmentationdevicesthat E111Test Method for Young’s Modulus, Tangent Modulus,
are designed to be partially or completely resorbable in the
and Chord Modulus
body. However, some of the test recommended in this guide E132TestMethodforPoisson’sRatioatRoomTemperature
may be applicable to evaluate such devices. It has not been
E328Test Methods for Stress Relaxation for Materials and
demonstrated that mechanical failure of nucleus devices is Structures
related to adverse clinical results. Therefore this standard
E1823TerminologyRelatingtoFatigueandFractureTesting
should be used with care in evaluating proposed nucleus F561 Practice for Retrieval and Analysis of Medical
devices.
Devices, and Associated Tissues and Fluids
F1582Terminology Relating to Spinal Implants
ThistestmethodisunderthejurisdictionofASTMCommitteeF04onMedical
andSurgicalMaterialsandDevicesandisthedirectresponsibilityofSubcommittee
F04.25 on Spinal Devices. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Feb. 1, 2020. Published April 2020. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 2010. Last previous edition approved in 2010 as F2789 – 10 (2015). Standards volume information, refer to the standard’s Document Summary page on
DOI: 10.1520/F2789-10R20. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F2789 − 10 (2020)
F1714GuideforGravimetricWearAssessmentofProsthetic 3.2.4 extrusion, n—a condition during testing when a por-
Hip Designs in Simulator Devices tion of a device displaces through a surrounding membrane or
F1877Practice for Characterization of Particles enclosure but does not separate from the rest of the device.
F1980Guide for Accelerated Aging of Sterile Barrier Sys- Extrusion may be considered a specific type of migration and
tems for Medical Devices
forthepurposesofthisstandardisonlyusefulwhenthetesting
F2267TestMethodforMeasuringLoadInducedSubsidence is being conducted within a surrogate annulus or enclosure.
of Intervertebral Body Fusion Device Under Static Axial
3.2.5 fatigue life, n—The number of cycles, N, that the
Compression
nucleus device can sustain at a particular load or moment
F2346Test Methods for Static and Dynamic Characteriza-
before functional or mechanical failure occurs.
tion of Spinal Artificial Discs
3.2.6 functional failure, n—A failure that renders the
F2423Guide for Functional, Kinematic, and Wear Assess-
nucleusdeviceineffectiveorunabletoresistloadorfunctionas
ment of Total Disc Prostheses
predetermined within desired parameters (for example, perma-
2.2 Other Standards:
nent deformation, dissociation, dehydration, expulsion, extru-
ISO 10993Biological Evaluation of Medical Devices: Parts
sion or fracture), or both.
1–20
ISO 18192–1Implants for Surgery—Wear ofTotal Interver- 3.2.6.1 Discussion—Functional failure may or may not be
tebral Spinal Disc Prostheses correlated with clinical failure.
3.2.7 hysteresis, n—The resultant loop on a force displace-
3. Terminology
ment plot that is created from a mechanical test performed on
3.1 For definition of terms, refer to Terminologies E6,
a viscoelastic material.The area inside the loop can be used to
E1823, and F1582.
determine the energy absorption.
3.2 Definitions:
3.2.8 mechanical failure, n—A failure associated with the
3.2.1 coordinate system/axes, n—Three orthogonal axes are
onset of a defect in the material (for example, a fatigue
defined by Terminology F1582. The center of the coordinate
fracture, a static fracture, or surface wear).
system is located at the geometric center of the native disc.
3.2.8.1 Discussion—Amechanicalfailurecanoccurwithout
Because of design intent, or procedural limitations, the device
there being a functional failure.
might not be implanted at the center of the native disc;
therefore, the geometric center of the disc might not be the 3.2.9 migration, n—A condition during testing when a
device displaces from its original position during testing.
geometric center of the device. For uniformity in comparison
between devices, it is important that the origin be placed with Migration may or may not be considered a specific type of
functional failure. The user is expected to define their criteria
respect to the disc, not the device. This is done so that all
loading is consistently applied and measurement made with for acceptable levels of migration and provide rationale for
thosecriteria.Seealsodefinitionsforexpulsion,extrusion,and
respect to the anatomy of the spine, and not with respect to the
device. The XY plane bisects the sagittal plane between subsidence.
superior and inferior surfaces that are intended to simulate the
3.2.10 nucleus device, n—A generic term that refers to all
adjacent vertebral endplates. The positive X axis is to be
types of devices intended to replace or augment the nucleus
directed anteriorly. The positive Z axis is to be directed
pulposus in the intervertebral disc.Adjectives can be added to
superiorly. Shear components of loading are defined to be the
the term “nucleus device” to more thoroughly describe the
components parallel to the XY plane. The compressive axial
device’s intended function. Terms 3.2.10.1 through 3.2.10.9
force is defined to be the component in either the positive or
will be used to address specific types of nucleus devices
negative Z direction depending on the test frame set-up.
throughouttherestofthisguide.Thesetermsmaynotapplyto
Torsional load is defined as the component of moment about
all nucleus devices and some combinations of terms may be
the Z axis.
applicabletocertaindevices.However,thistermshouldnotbe
3.2.2 energy absorption, n—The work or energy (in joules)
used interchangeably with annular repair device.
that a material can store, temporarily or permanently, after a
3.2.10.1 complete nucleus replacement device, n—A
given stress is applied and then released.
nucleus device that is designed to replace most or all (≥50%
3.2.3 expulsion, n—a condition during testing when the
by volume) of the nucleus pulposus of the intervertebral disc.
deviceoracomponentofthedevicebecomesfullydisplacedor
3.2.10.2 partial nucleus replacement device, n—A nucleus
dislodged from its implanted position (that is, in the direction
device that is designed to replace some (<50% by volume) of
of shear) through a surrogate annulus, or enclosure used to
the nucleus pulposus of the intervertebral disc.
simulate an annular boundary. Expulsion may be considered a
specific type of migration and for the purposes of this standard
3.2.10.3 nucleus augmentation device, n—Anucleus device
is only useful when the testing is being conducted within a
that is designed to supplement or augment, but not replace, the
surrogate annulus or enclosure.
existing nucleus pulposus in the intervertebral disc.
3.2.10.4 encapsulated nucleus device, n—A nucleus device
that includes an outer jacket, bag, or a similar casing, which in
Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
4th Floor, New York, NY 10036, http://www.ansi.org. turn interfaces directly with the in vivo environment.
F2789 − 10 (2020)
3.2.10.5 open nucleus device, n—A nucleus device that is 4. Summary of Test Method
not encased. The material interfaces directly with the in vivo
4.1 The tests for characterizing the performance of nucleus
environment.
devices can include one or more of the following: static and
3.2.10.6 in situ formed nucleus device, n—Anucleus device
dynamic axial compression, axial torsion, and shear tests,
that is introduced into the disc space without a predetermined
functional range of motion, subsidence, mechanical behavior
geometry. This may include injectable, in situ curing or
change due to aging, swelling pressure, and viscoelastic
polymerizing nucleus devices.
testing. Table 1 summarizes these tests with reference to
sections where they are described in more detail.Additionally,
3.2.10.7 preformed nucleus device, n—A nucleus device
Table 1 also lists additional reference documents that may be
that is introduced into the disc space already in a
applicable to each particular test.
predetermined, but not necessarily final, geometry with all
chemical processes completed prior to insertion.
4.2 Sometestsmaynotbeapplicableforalltypesofnucleus
devices.
3.2.10.8 non-hydrated nucleus device, n—Anucleus device
thatdoesnotrequirewatertobepresenttoachieveitsintended
4.3 Where appropriate, a surrogate annulus may be used to
purposes.
further characterize the nucleus device.
3.2.10.9 hydrated nucleus device, n—Anucleus device that
4.4 Alltestsshallbeperformedonthenucleusdeviceinthe
requires water to be present to achieve its intended purposes.
same shape, size, and condition as it would be used clinically
3.2.11 Range of Motion (ROM), n—The difference between
unless adequately justified (that is, if gamma radiation is to be
the minimum and maximum displacement or angular displace-
used to sterilize the device, or the device is meant to function
ment of the nucleus device that occurs during a test. This
in a hydrated state, then all tests should be performed on
parameter may be useful when a surrogate annulus is used for
gamma-irradiated or hydrated parts or a justification shall be
testing.
made).
3.2.12 secant stiffness, n—For a given applied load or
4.5 Nucleus devices shall be tested statically to failure and
applied displacement: [(maximum load) – (minimum load)]/
also tested cyclically to estimate the maximum run out load or
[(maximum displacement) – (minimum displacement)].
momentat10×10 cycles.Dependingonthetestandintended
use, the devices can be tested in force control or in position
3.2.13 stiffness, n—The slope of the linear portion of the
control,butineithercase,thecontrolmodeshouldbejustified.
load-displacement curve or of the moment-angular displace-
mentcurveatasegmentwithinnormalphysiologicparameters.
5. Significance and Use
If there is no linear portion, then stiffness may be estimated
using other standard methods such as those found in Test
5.1 Nucleus devices are generally designed to augment the
Method E111 (chord or tangential stiffness, or both) within
mechanical function of native degenerated nucleus material or
normal physiologic parameters.
to replace tissue that has been removed during a surgical
3.2.14 subsidence, n—Settling or migration of the device procedure. This guide outlines methods for evaluating many
into the inferior or superior interfaces adjacent to the device. different types of devices. Comparisons between devices must
Subsidencemaybeconsideredaspecifictypeofmigrationand, be made cautiously and with careful analysis, taking into
for the purposes of this standard, is only useful when the account the effects that design and functional differences can
matingendplates,fixturesorsurrogateannulushaveamodulus have on the testing configurations and overall performance,
that allows subsidence to occur. andthepossibilitythatmechanicalfailuremaynotberelatedto
TABLE 1 Summary of Test Methods
Test Grouping Test Type Boundary and Sample Section of this Standard Applicable Standard or
Conditions Reference
Static Axial Compression As Manufactured 7.2 Test Methods F2346
Axial Torsion With Surrogate Annulus 7.1 and 7.2
Shear Simulated Aged 7.2 and 7.7
Bending With Surrogate Annulus 7.1, 7.2, and 7.7
and Simulated Aged
Dynamic Axial Compression As Manufactured 7.3 Test Methods F2346, Guide
(Fatigue and Wear) Axial Torsion With Surrogate Annulus 7.1 and 7.3 F2423
Shear Simulated Aged 7.3 and 7.7 and
Bending With Surrogate Annulus 7.1, 7.3, and 7.7 ISO 18192–1
and Simulated Aged
Functional Testing Functional Range of Motion As Manufactured 7.3.6 Wilke, 1998 (1)
Lifting Force (After simulated aging and with (7.1 and 7.7 where applicable) Catellani, 1989 (2)
Viscoelastic Testing surrogate annulus where 7.4 Test Methods D2990
Subsidence applicable) (7.1 and 7.7 where applicable) Test Method F2267
7.5
(7.1 and 7.7 where applicable)
7.6
(7.1 and 7.7 where applicable)
F2789 − 10 (2020)
clinical failure and inversely, that mechanical success may not 7.1.2 If a surrogate annulus is used, it should be character-
be related to clinical success. ized without the nucleus replacement device present for
comparison to available published in vitro data for the human
5.2 These tests are conducted in vitro to allow for analysis
annulus (for example, stiffness and radial bulge) (3, 4).
of the mechanical performance of the nucleus device under
7.1.3 Fordynamicandfatiguetesting,thefatiguelifeofthe
specific testing modalities. The loads applied may differ from
annulus shall be quantified. If it is determined that the
thecomplexloadingseeninvivo,andthereforetheresultsfrom
surrogate annulus will not survive 10 × 10 cycles in a fatigue
these tests may not directly predict in vivo performance.
or wear test, a suggested replacement interval shall be deter-
5.3 These tests are used to quantify the static and dynamic
mined. For example, if an annulus is found to survive3×10
properties and performance of different implant designs. The
cycles, a replacement interval of 2.5 × 10 cycles may be
mechanical tests are conducted in vitro using simplified loads
chosen.
and moments. Fatigue testing in a simulated body fluid or
7.1.4 Where appropriate, the viscoelastic response of the
saline may have fretting, aging, corroding, or lubricating
surrogate annulus (for example, stress relaxation and creep)
effects on the device and thereby affect the relative perfor-
shall be quantified.
mance of tested devices. Hence, the test environment and the
7.1.5 Where necessary (particularly in hydrated nucleus
effect of that environment, whether a simulated body fluid,
pulposus replacements), the surrogate annulus shall allow
normal saline bath (9 g NaCl per 1000 mLH O), or dry, is an
appropriate fluid availability to the nucleus pulposus replace-
important characteristic of the test and must be reported
ment.
accurately.
7.1.6 The surrogate annulus should be comprised of a
material that is easily distinguishable from the device under
5.4 Dynamic testing methods should be designed to answer
the following questions, including but not limited to: Does the test. Where the materials are similar, standard particle charac-
terization techniques may not be adequate to effectively
device still function as intended after cycling? Does it retain
adequateperformancecharacteristics(forexample,mechanical characterize particle size, shape or morphology to distinguish
between the two materials.
and kinematic properties such as ROM)? Did the device wear
or degrade? If there is evidence of wear or degradation of the
7.2 Static Testing:
device, it should be identified and quantified with reasonable
7.2.1 Axial compression, axial torsion, compression/shear,
methods generally available. The user shall distinguish be-
flexion/extension, lateral bending tests should be performed in
tween particulates generated by the device and particulates
either force/moment control or displacement/angle control.
generated by the test model and fixtures if technically feasible.
7.2.2 Refer to Test Methods F2346, Guide F2423, and ISO
18192–1 for suggested load/moment and displacement/angle
6. Sampling and Test Specimens
inputs.
7.2.3 Thetestset-upoftheaxialcompression,axialtorsion,
6.1 It is suggested that a minimum sample size of five be
used for each form of testing described in Section 7. However, and compression/shear test should follow the set-up and
fixtures described in Test Methods F2346. Any necessary
notethat,asforanyexperimentalcomparison,thetotalnumber
deviationsshouldbenoted(forexample,ifasurrogateannulus
of needed specimens will depend on the magnitude of the
is used as shown in X1.2, a polyacetal test block may be
difference to be established, the repeatability of the results
unnecessary).
(standard deviation), and the level of statistical significance
7.2.4 For viscoelastic or strain rate sensitive materials, the
desired.
effect of loading rate should be considered and characterized.
6.2 The test assemblies (that is, nucleus pulposus test
This might include testing at different strain rates or impact
samples in their tested configuration) shall be labeled so they
loading as compared to a typical static test, which might be
canbetraced,andshallbekeptinacleanenvironmenttoavoid
performed at a rate of 1-25 mm/min. Elevated strain rates or
contamination. The test assembly can be disassembled to
impact rates should be justified.
facilitate examination of surface conditions.
7.3 Dynamic Testing:
TEST METHODS
7.3.1 Dynamic tests should be completed using methods
definedinTestMethodsF2346,GuideF2423,orISO18192–1,
7. Procedure
or combinations thereof, noting any necessary deviations.
7.3.2 Where possible, the nucleus pulposus replacement
7.1 Use of a Surrogate Annulus:
device shall be tested in combined flexion/extension, lateral
7.1.1 Since most nucleus devices are designed to work with
bending, axial rotation and axial loading. The test setup for
anintactorpartlyintactannulusfibrosus,theuseofasurrogate
combinations of flexion/extension, lateral bending, axial rota-
annulus to perform the tests below may be considered. This
tionandaxialloadingshouldfollowtheguidelinesdescribedin
annulus can be modeled in the test set-up if applicable (see
Guide F2423 or ISO 18192–1. All tests without a dynamic
X1.2 for references that detail examples of lumbar test mod-
compression component should be completed with a static
els).Theuseofasimulatedannulusmaybenecessarytoallow
for testing of an open, in situ formed device and to test a
nucleusdeviceinload/momentcontrol.However,itmaynotbe
necessary if the tests are performed in displacement/angle
The boldface numbers in parentheses refer to a list of references at the end of
control. this standard.
F2789 − 10 (2020)
axial compressive preload. The preload or displacement (or 7.4.1 An assessment of axial lifting force exerted by a
angulation), or both, for each test should be justified. hydrated nucleus replacement device during the absorption
7.3.3 Test Methods F2346 states that the end of the test is process may be performed by placing the specimen between
defined as a functional failure or the attainment of 10 × 10 two discs in a rigid cage. Axial lifting force is performed on
cycles. If a mechanical failure (for example, fatigue crack, hydratednucleusreplacementdevicesbyplacingthespecimen
surfacewear)thatisnotafunctionalfailureoccurs,itshouldbe between two discs in a rigid cage.Aforce transducer placed in
reported in detail. However, the test should be continued until line with the cage can be used to measure the force exerted by
afunctionalfailureortheattainmentof10×10 cyclesoccurs. thedevicewhenitisplacedincontactwiththechosensolution.
7.3.4 Testing should be performed in a physiologic solution A method developed by Catellani, et al. describes a test
if possible. The environment should be maintained at body apparatus and procedure for quantifying lifting force (2).A
temperature (37 6 3ºC), as many materials exhibit different diagram of a suggested apparatus is provided as Fig. 1; further
properties at different temperatures. detailsregardingtheapparatusandaprocedureareprovidedas
7.3.5 If an analysis of wear or degradation products of the 7.4.1.1- 7.4.1.4 and 7.4.1.5, respectively.
nucleusdeviceisperformedontheenvironmentalsolution,the 7.4.1.1 The objective of the test systems shown in Fig. 1 is
user should be able to distinguish between particulates gener-
todeterminetheamountofwaterabsorbedandtheliftingforce
ated by the device and particulates generated by the surrogate
generated during the absorption process.
annulus or fixtures, or both.
7.4.1.2 The frame is used for mounting the load cell and
7.3.6 Kinematic and functional evaluation should be per-
sample chamber, which consists of a stainless steel cage and a
formed by examining and comparing the range of motion,
glass disc on which the sample sits. It also provides a
stiffness, secant stiffness, or the hysteresis of the device, or
controlled means by which to lower the sample chamber
combinations thereof, at the start and finish of the test. These
assembly into solution.
evaluations can also be conducted at intermediate points as an
7.4.1.3 Thesteelcageassemblyprovidesarigidinterfaceto
option. Depending on the device, one or more of the metrics
the load cell such that, as the sample absorbs fluid, the force
above may be determined to be useful for adequate character-
generated by the increasing volume of the sample is measured
ization of the device (1).
on the load cell.
7.3.7 An analysis of wear or degradation should be done
7.4.1.4 The system utilizes a balance with a mounted water
according to methods described in Practice F561, Guide
bathintowhichthesampleislower
...