ASTM F2694-16(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: F2694 − 16 (Reapproved 2020)
Standard Practice for
Functional and Wear Evaluation of Motion-Preserving
Lumbar Total Facet Prostheses
This standard is issued under the fixed designation F2694; 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
1.1 This practice provides guidance for the functional, 2.1 ASTM Standards:
kinematic, and wear testing of motion-preserving total facet F561 Practice for Retrieval and Analysis of Medical
prostheses for the lumbar spine.These implants are intended to Devices, and Associated Tissues and Fluids
allow motion and lend support to the functional spinal unit(s) F732 Test Method for Wear Testing of Polymeric Materials
through replacement of the natural facets. Used in Total Joint Prostheses
F1714 GuideforGravimetricWearAssessmentofProsthetic
1.2 Thispracticeisnotintendedtoaddresstheboneimplant
Hip Designs in Simulator Devices
interface or the static characteristics of the prosthesis compo-
F1877 Practice for Characterization of Particles
nents. Fatigue characteristics are included, but only as a
F2346 Test Methods for Static and Dynamic Characteriza-
by-product of cyclic wear testing under facet load and thus are
tion of Spinal Artificial Discs
not addressed in the typical process of generating a Stress-Life
F2423 Guide for Functional, Kinematic, and Wear Assess-
(S-N) characterization.
ment of Total Disc Prostheses
1.3 Biocompatibility of the materials used in a total facet
prosthesis are not addressed in this practice. 3. Terminology
1.4 The values stated in SI units are to be regarded as 3.1 All functional and kinematic testing terminology is
standard. No other units of measurement are included in this consistent with the referenced standards, unless otherwise
standard. stated.
1.4.1 The values stated in SI units are to be regarded as the
3.2 Definitions of Terms:
standard with the exception of angular measurements, which
3.2.1 mechanical failure, n—failure associated with a defect
may be reported in either degrees or radians.
in the material (for example, fatigue crack) or the bonding
1.5 This standard does not purport to address all of the
between materials that may or may not produce functional
safety concerns, if any, associated with its use. It is the
failure. [F2423]
responsibility of the user of this standard to establish appro-
3.2.2 runout (cycles), n—maximum number of cycles that a
priate safety, health, and environmental practices and deter-
test needs to be carried to if functional failure has not yet
mine the applicability of regulatory limitations prior to use.
occurred. [F2423]
1.6 This international standard was developed in accor-
3.3 Definitions of Terms Specific to This Standard:
dance with internationally recognized principles on standard-
3.3.1 coordinate systems/axes, n—global XYZ orthogonal
ization established in the Decision on Principles for the
axes are defined following a right-handed Cartesian coordinate
Development of International Standards, Guides and Recom-
system in which the XY plane is parallel to and co-planar with
mendations issued by the World Trade Organization Technical
the superior endplate of the inferior vertebral body. The global
Barriers to Trade (TBT) Committee.
axes are fixed relative to the inferior vertebral body, which in
this practice is also considered to be stationary with respect to
ThispracticeisunderthejurisdictionofASTMCommitteeF04onMedicaland
Surgical Materials and Devices and is the direct responsibility of Subcommittee
F04.25 on Spinal Devices. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Oct. 1, 2020. Published November 2020. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 2007. Last previous edition approved in 2016 as F2694 – 16. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/F2694-16R20. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F2694 − 16 (2020)
thetestmachine’sframe.Lowercaseletters, xyz,denotealocal 3.3.13 net volumetric wear NV of wear specimen (mm ),
i
moving orthogonal coordinate system attached to the superior n—NV = NW/ρ at end of cycle interval i; where ρ = mass
i i
vertebralbodywithdirectionsinitiallycoincidentwiththoseof density (for example, units of g/mm ) of the wear material.
the global XYZ axes, respectively. The 3D motion of the
3.3.14 wear, n—progressive loss of material from the de-
superiorrelativetotheinferiorvertebraisspecifiedandistobe
vice(s) or device components as a result of relative motion at
measured in terms of sequential Eulerian angular rotations
the surface with another body as measured by the change in
about the xyz axes, respectively (z axial rotation, x lateral bend,
mass of the total facet prosthesis or components of the total
and y flexion-extension). See Figs. 1 and 2.
facet prosthesis. In the case of a non-articulating, compliant
total facet prosthesis, wear is defined simply as the loss of
3.3.1.1 origin, n—center of the global coordinate system
that is located at the posterior medial position on the superior material from the prosthesis. Note that inferior and superior
bone interface components are excluded from this definition
endplate of the inferior vertebral body.
(see 5.2.2).
3.3.1.2 X-axis, n—the positive X-axis is directed anteriorly
3.3.15 facet load, n—anterior-posterior (AP) directed force
relative to the specimen’s initial unloaded position. See Figs. 1
(applied in the direction of the global X-axis) representing the
and 2.
resultant in the mid-sagittal XZ plane applied by the superior
3.3.1.3 Y-axis, n—the positive Y-axis is directed laterally
vertebra that simulates the in-vivoAPshear load F transmitted
x
(toward the left) relative to the specimen’s initial unloaded
from superior to inferior vertebra and resisted by the total facet
position. See Figs. 1 and 2.
prosthesis.
3.3.1.4 Z-axis, n—the positive Z-axis is directed superiorly
relative to the specimen’s initial unloaded position. See Figs. 1
4. Summary of Practice
and 2.
4.1 This practice can be used to describe the function,
3.3.2 fluid absorption, n—fluid absorbed by the device
kinematics, and wear behavior of total facet prostheses sub-
material during testing or while implanted in vivo.
jected to cyclic loading/motion for relatively large numbers of
cycles. (For example, various designs of total facet prostheses,
3.3.3 functional failure, n—permanent deformation or wear
as well as the effects of materials, manufacturing techniques
that renders the total facet prosthesis assembly ineffective or
and other design variables on one particular design can be
unable to perform its intended function.
studied using this practice.)
3.3.4 interval net volumetric wear rate, VR, during cycle
i
interval i (mm /million cycles), n—VR = WR/ρ; where ρ = 4.2 This practice is intended to be applicable to total facet
i i
prostheses that support and transmit motion by means of an
mass density (for example, units of g/mm ) of the wear
material. articulating joint or by use of compliant materials. Ceramics,
metals, and/or polymers may be used in total facet prosthesis
3.3.5 interval net wear rate, WR, during cycle interval i
i
design, and it is the goal of this practice to enable a kinematic
(g/million cycles), n—WR =((NW – NW )/(number of cycles
i i i-1
wear comparison of these devices, regardless of material and
in interval i))·106; for i=1, NW =0.
i-1
type of device.
3.3.6 total facet prosthesis, n—nonbiologic structure in-
tended to restore the support and motion of the natural
5. Significance and Use
vertebral facet joint.
5.1 Total Facet Prosthesis Components—The total facet
3.3.7 kinematics profile, n—relative motion between adja-
replacement may comprise a variety of shapes and configura-
cent vertebral bodies that the total facet prosthesis is subjected
tions. Its forms may include, but are not limited to: ball-and-
to while being tested.
socket articulating joints, joints having a free-floating or
semi-constrained third body, metallic load-bearing surfaces,
3.3.8 load profile, n—loading that the device experiences
while being tested under a defined kinematics profile or the and spring and dampening mechanisms. Additionally, it may
have a unilateral or bilateral design.
loading that the total facet prosthesis is subject to if tested in
load control.
5.2 Spinal Testing Apparatus:
5.2.1 Test Chambers—In case of a multispecimen machine,
3.3.9 radius of rotation, n—the distance between the center
of rotation and the functional position (for example, load- each chamber shall be isolated to prevent cross-contamination
bearing contact point) of the total facet prosthesis, for a given of the test specimens. The chamber shall be made entirely of
motion (that is, flexion/extension, lateral bending, or axial corrosion-resistantmaterials,suchasacrylicplasticorstainless
rotation). steel, and shall be removable from the machine for thorough
cleaning between tests.
3.3.10 weight S of soak control specimen (g), n—S initial
i 0
5.2.2 Component Clamping/Fixturing—Since the purpose
and S at end of cycle interval i.
i
of the test is to characterize the wear and kinematic function of
3.3.11 weight W of wear specimen (g), n—W initial and W
i 0 i
the total facet prosthesis, the method for mounting components
at end of cycle interval i.
in the test chamber shall not compromise the accuracy of
3.3.12 net wear NW of wear specimen (g), n—NW =(W – assessment of the weight loss or stiffness variation during the
i i 0
W)+(S – S ); loss in weight of the wear specimen corrected test. For example, prostheses having complicated superior and
i i 0
for fluid absorption at end of cycle interval i. inferior surfaces for contacting bone (for example, sintered
F2694 − 16 (2020)
beads, hydroxylapatite (HA) coating, plasma spray) may be thattheserumcontainamassfractionofasuitableantibacterial
specially manufactured to modify that surface in a manner that agent to minimize bacterial degradation. Alternate lubricants
does not affect the wear simulation. (other than bovine serum solution) should be evaluated to
5.2.3 The device should be securely (rigidly) attached at its determine appropriate storage conditions.
bone-implant interface to the mating test fixtures. 6.1.3 It is recommended that ethylene-diaminetetraacetic
5.2.4 The motion of the superior test fixture (more posterior acid (EDTA) be added to the serum at a concentration of 20
fixture in Figs. 1 and 2) relative to the inferior testing fixture mM (7.45 g/L) to bind calcium in solution and minimize
shall be constrained in three-dimensional space except for the precipitation of calcium phosphate onto the bearing surfaces.
components in the direction of specified test motions/loads. The latter event has been shown to affect the friction and wear
5.2.5 Load and Motion: properties strongly, particularly of polyethylene/ceramic com-
5.2.5.1 Facet loads (f ) are initially applied in the direction binations. The addition of EDTA to other testing mediums
x
of the positive X-axis. should be evaluated.
5.2.5.2 Flexion load and motion are positive moment and 6.1.4 The bulk temperature of the testing medium shall be
rotation about the Y-axis. maintained at 37 6 3 °C unless otherwise justified.
5.2.5.3 Extensionloadandmotionarenegativemomentand 6.1.5 The user may wish to reference Test Method F732 for
rotation about the Y-axis.
additional guidance on serum preparation.
5.2.5.4 Lateral bend load and motion are positive and
6.2 The user is cautioned that internal heating of the
negative moments and rotations about the X-axis.
prosthesis may cause localized temperatures to fall outside the
5.2.5.5 Axial rotation load and motion are positive and
37 6 3 °C of the testing medium. Internal local temperatures
negative moments and rotations about the Z-axis.
may depend on a number of factors including, but not limited
5.2.6 Frequency—Test frequency shall be determined and
to: joint friction, material hysteresis, conductivity of the
justified by the user of this practice, and shall not exceed 2 Hz
device-fixture materials, design, and test frequency. Localized
without adequate justification ensuring that the applied motion
elevated temperatures may have an effect on the mechanical as
(load) profiles remain within specified tolerances and that the
well as wear properties of the prosthesis. If the device
total facet prosthesis’s wear and functional characteristics are
experiences localized elevated temperatures, the user shall
not significantly affected. See X1.6.
describe the effect the selected frequency and resultant local-
5.2.7 Cycle Counter—One complete motion is the entire
ized temperature have on the test results or justify that the
range from starting position through the range of motion (or
effects are insignificant. Refer to X1.5 for further information.
loadwheninloadcontrol)andreturningtothestartingposition
(load). Cycles are to be counted using an automated counting
7. Sampling and Test Specimens
device.
7.1 It is suggested that a minimum sample size of six be
6. Reagents and Materials used for each kinematic/load profile. However, note that, as for
any experimental comparison, the total number of needed
6.1 Testing Medium:
specimenswilldependonthemagnitudeofthedifferencetobe
6.1.1 A solution containing bovine serum diluted to a
established,therepeatabilityoftheresults(standarddeviation),
protein concentration of 20 g/L in deionized water shall be
and the level of statistical significance desired.
used as the testing medium.
6.1.2 To retard bacterial degradation, freeze and store the 7.2 The test assemblies (that is, total facet prosthesis com-
serum until needed for testing. In addition, it is recommended ponentsinthetestedconfiguration)shallbelabeledsotheycan
NOTE 1—This setup would require two rotational actuators and one translational actuator.
FIG. 1 Diagrams of Possible Test Apparatus for Allowing Simultaneous Lateral Bending and Axial Rotation Motions
with Anterior-Posterior Directed Facet Loading
F2694 − 16 (2020)
NOTE 1—This setup would require two rotational actuators and one translational actuator.
FIG. 2 Diagrams of Possible Test Apparatus for Allowing Simultaneous Flexion-Extension and Lateral Bending Motions
with Anterior-Posterior Directed Facet Loading
be traced and shall be kept in a clean environment to avoid 9.2 Record weights, W and S , as the initial weights of the
0 0
contamination. The test assembly can be disassembled to wear and soak controls, respectively. Place the loaded soak
facilitate examination of surface conditions. control specimens in holders in a soak chamber of the testing
medium, such that the total surface area exposed to the testing
8. Preparation of Apparatus
medium is the same as that of the wear components when
mounted in the spinal testing apparatus. Maintain the soak
8.1 Thefunctionalportionofthedevicetobetestedmustbe
chamber temperature at 37 6 3 °C (see 6.2), or specify and
produced using manufacturing methods equivalent to those of
justify if different.
the implantable form of the total facet prosthesis, including
sterilization.
9.3 As a weight control for the testing, a minimum of two
identicalloadedsoakcontrolspecimensintestingmedium(see
8.2 It is permissible to exclude nonfunctional features that
6.1) shall be used. In other words, the loaded soak control
may interfere with obtaining wear/functional measurements.
specimen must be loaded statically with the same facet load
For example, bone-implant interfaces such as HA, plasma-
vectorasshowninFigs.1and2sinceitiswellknownthatload
spray titanium, and beads may be omitted since they may
can significantly affect fluid absorption.
abrade the fixtures and thus produce an unwanted mixture of
NOTE 1—The user of this practice may justify not performing control
functional and nonfunctional component wear particles (see
testsincertaincircumstances(forexample,all-metalcomponents).Before
5.2.2).
and at all specified time intervals (determined by the user) of the presoak
period (defined in Guide F1714), the wear components and soak controls
8.3 It is permissible to make entirely different bone-implant
should be removed from the soak bath, cleaned, dried, and weighed three
interface components (that is, superior and inferior surfaces)
times, in rotation, keeping the same specimen sequence each time. The
providedthatthemodificationisproperlyjustifiedanddoesnot
average of the three weights may be used for the wear calculations. An
interfere with an accurate measurement of the wear and
analytical balance with a sensitivity of 610 µg shall be used. This degree
of sensitivity for weighing is necessary to detect the slight loss in weight
functionalcharacteristicsofthedevice.Forexample,aballand
of polymers, such as Ultra High Molecular Weight Polyethylene
socket joint prosthesis having the polished articulation compo-
(UHMWPE), which may wear 30 µg or less per million cycles (1).
nent (that is, functional surfaces or features of the device) and
9.4 For all components, measure the geometry of relevant
an opposite side that mounts directly to the testing apparatus,
functional surfaces or features before starting the test. For
may be manufactured, thereby simplifying the fixturing de-
example, articulating joints should have measurements of the
mands.
bearing area.Visual, microscopic, profilometric, replication, or
8.4 The requirements of Guide F1714, Section 5, Specimen
other inspection techniques can be used. Prostheses having
Preparation, shall be followed.
bonded polymer cores should have measurements of the
external geometry such as starting circumference (to calculate
9. Procedure
changes caused by equatorial bulging) and prosthesis height.
9.1 Alwaysweighspecimensintheclean,drycondition(see
9.5 Testing medium, temperature, and removal periods for
Annex A4 of Guide F1714). Keep the components in a
weighing components shall be identical for all control and test
dust-free container and handle with clean tools or gloves or
specimens.
both to prevent contamination that might affect the weight
measurement. Weigh each wear and control component three
times in rotation to detect random errors in the weighing
The boldface numbers in parentheses refer to the list of references at the end of
process. this standard.
F2694 − 16 (2020)
TABLE 1 Test Profiles and Associated Parameters for Total
9.6 Unless otherwise justified by intended use and service
Facet Prostheses
life expectancy of the total facet prosthesis, all tests should be
Displace-
conductedtoarunoutof10 000 000cycles(seeAppendixX1).
ment Load
Radius
Facet Control: Control:
9.7 The testing medium shall be collected for subsequent
of
Test Profile Load, Range of Applied
analysis of wear particulates at least once every one million
Rotation,
N (2-6) Motion Moments,
mm
A
cycles and shall be replaced with fresh testing medium.
(ROM), Nm
degrees
9.8 Place the prostheses in the spinal testing apparatus, add
B E
Flexion/Extension 400 15º total ±10 44.3
testing medium, and subject the total facet prostheses to each C,D
motion
B D E
of the tests as listed in 9.10. The prostheses shall be visually
Rotation 400 ±3 (6, 7) ±10 52.1
B D E
Lateral Bending 400 ±6 (6, 7) ±10 27.5
analyzed at a minimum once per 1 000 000 cycles, with
A
Approximated based on a review of ROM (p. 111) and average flexibility and
mechanical failures noted. A mechanical failure (for example,
stiffness coefficients (p. 47) (8) as done for Test Methods F2346.
considerable wear of the bearing surface) may not necessitate
B
This load represents combined loading for two facets (see X1.7). In the scenario
termination of the test since this practice attempts to charac-
where a unilateral prosthesis is used to replace the two natural facets, the
combined load should be applied in its entirety. In a bilateral design it may be
terize the time-dependent wear properties of the device. The
distributed between the two facets.
test shall be terminated if functional failure (for example, gross
C
Depending on device design, the balance of ROM should be appropriate and
fracture or a bearing seizes) occurs.
justified to the expected ROM in a clinical situation (9).
D
Preferred test modes for each motion (see 9.10.1).
9.9 Anew, unused specimen is used to start each test series. E
Approximate distance to natural facet location based on location of a fixed
center of rotation (COR) at the anterior third of the disc and centralized in the
9.10 Tests:
medial-lateral direction (see X1.8).
9.10.1 A facet load shall be an applied compressive force
through the total facet prosthesis (f ). The specific methodol-
x
ogy for fixturing and applying the facet load will dictate the
the device in flexion/extension loading for 10 000 000 cycles,
resultantloadandbendingmomentthedevicewillbesubjected
followed by lateral bend testing for 10 000 000 cycles on the
to throughout the motion profile.
samedeviceandfinallyrotationaltestingfor10 000 000cycles
9.10.2 The facet load for all testing shall be applied with the
on the same device).
use of a mechanism that can apply a constant magnitude of
(2) Theusermaywishtoperformatestinwhichthedevice
force (65 %) throughout the ranges of motion that the test rig
is tested following one of the prescribed single motions
will undergo during testing. Pneumatic or hydraulic cylinders,
followed by a coupled test (on the same device) for the
by virtue of their ability to apply a nearly constant force but
remainingtwomotions.Bywayofexample,theusermaywish
allow movement of th
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