ASTM F2777-10
(Test Method)Standard Test Method for Evaluating Knee Bearing (Tibial Insert) Endurance and Deformation Under High Flexion
Standard Test Method for Evaluating Knee Bearing (Tibial Insert) Endurance and Deformation Under High Flexion
SIGNIFICANCE AND USE
This test method can be used to describe the effects of materials, manufacturing and design variables on the fatigue/cyclic creep performance of UHMWPE bearing components subject to substantial rotation in the transverse plan (relative to the tibial tray) for a relatively large number of cycles.
The loading and kinematics of bearing component designs in vivo will, in general, differ from the loading and kinematics defined in this test method. The results obtained here cannot be used to directly predict in vivo performance. However, this test method is designed to enable comparisons between the fatigue performance of different bearing component designs when tested under similar conditions.
The test described is applicable to any bicompartmental knee design including mobile bearing knees that have mechanisms in the tibial articulating component to constrain the posterior movement of the femoral component and a built in retention mechanism to keep the articulating component on the tibial plate.
SCOPE
1.1 This standard specifies a test method for determining the endurance properties and deformation, under specified laboratory conditions, of ultra high molecular weight polyethylene (UHMWPE) tibial bearing components used in bicompartmental or tricompartmental knee prosthesis designs.
1.2 This test method is intended to simulate near posterior edge loading similar to the type of loading that would occur during high flexion motions such as squatting or kneeling.
1.3 Although the methodology described attempts to identify physiological orientations and loading conditions, the interpretation of results is limited to an in vitro comparison between knee prosthesis designs and their ability to resist deformation and fracture under stated test conditions.
1.4 This test method applies to bearing components manufactured from UHMWPE.
1.5 This test method could be adapted to address unicompartmental total knee replacement (TKR) systems, provided that the designs of the unicompartmental systems have sufficient constraint to allow use of this test method. This test method does not include instructions for testing two unicompartmental knees as a bicompartmental system.
1.6 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.7 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 and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: F2777 − 10
StandardTest Method for
Evaluating Knee Bearing (Tibial Insert) Endurance and
Deformation Under High Flexion
This standard is issued under the fixed designation F2777; 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 Thisstandardspecifiesatestmethodfordeterminingthe 2.1 ASTM Standards:
endurance properties and deformation, under specified labora- F1223 Test Method for Determination of Total Knee Re-
tory conditions, of ultra high molecular weight polyethylene placement Constraint
(UHMWPE) tibial bearing components used in bicompartmen- F2003 Practice for Accelerated Aging of Ultra-High Mo-
tal or tricompartmental knee prosthesis designs. lecular Weight Polyethylene after Gamma Irradiation in
Air
1.2 This test method is intended to simulate near posterior
F2083 Specification for Knee Replacement Prosthesis
edge loading similar to the type of loading that would occur
2.2 Other Standards:
during high flexion motions such as squatting or kneeling.
ISO 4965 Axial Load Testing Machines—Dynamic Force
1.3 Although the methodology described attempts to iden-
Calibration—Strain Gauge Technique
tify physiological orientations and loading conditions, the
ISO 5833 Implants for Surgery—Acrylic Resin Cements
interpretation of results is limited to an in vitro comparison
3. Terminology
between knee prosthesis designs and their ability to resist
deformation and fracture under stated test conditions.
3.1 Definitions:
3.1.1 anatomic (mechanical) axis of the femur—the line
1.4 This test method applies to bearing components manu-
between the center of the femoral head and the center of the
factured from UHMWPE.
femoral knee.
1.5 This test method could be adapted to address unicom-
3.1.2 bearing centerline—the line running anteroposterior
partmental total knee replacement (TKR) systems, provided
that is the mirror line of the femoral articulating surface. For
that the designs of the unicompartmental systems have suffi-
asymmetric bearing tibial tray designs, the appropriate tibial
cient constraint to allow use of this test method. This test
tray centerline shall be determined and reported along with the
method does not include instructions for testing two unicom-
rationale for the location.
partmental knees as a bicompartmental system.
3.1.3 bearing retention mechanism—mechanical means for
1.6 The values stated in SI units are to be regarded as
preventing tibial tray/bearing disassociation.
standard. No other units of measurement are included in this
standard. 3.1.4 femoral component centerline—a line running antero-
posterior between the femoral condyles and parallel to the
1.7 This standard does not purport to address all of the
femoral condyles. The line should be equidistant between the
safety concerns, if any, associated with its use. It is the
condyles. For asymmetric or non parallel condyles designs, the
responsibility of the user of this standard to establish appro-
appropriatecenterlineshallbedetermined,andtherationalefor
priate safety and health practices and determine the applica-
that location reported.
bility of regulatory limitations prior to use.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
This test method is under the jurisdiction ofASTM Committee F04 on Medical contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
and Surgical Materials and Devices and is the direct responsibility of Subcommittee Standards volume information, refer to the standard’s Document Summary page on
F04.22 on Arthroplasty. the ASTM website.
Current edition approved Sept. 15, 2010. Published October 2010. DOI: Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
10.1520/F2777-10. 4th Floor, New York, NY 10036, http://www.ansi.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F2777 − 10
3.1.5 fixed bearing system—a knee prosthesis system com- 4.2 The loading and kinematics of bearing component
prised of a femoral component and a tibial component, where designs in vivo will, in general, differ from the loading and
thetibialarticulatingsurfaceisnotintendedtomoverelativeto kinematics defined in this test method. The results obtained
the tibial tray. here cannot be used to directly predict in vivo performance.
However, this test method is designed to enable comparisons
3.1.6 mobile bearing—the component between fixed femo-
between the fatigue performance of different bearing compo-
ral and tibial knee components with an articulating surface on
nent designs when tested under similar conditions.
both the inferior and superior sides.
4.3 The test described is applicable to any bicompartmental
3.1.7 mobile bearing knee system—akneeprosthesissystem
knee design including mobile bearing knees that have mecha-
comprised of a femoral component, a tibial component, and a
nisms in the tibial articulating component to constrain the
mobile bearing component that can rotate and/or translate
posterior movement of the femoral component and a built in
relative to the tibial component.
retention mechanism to keep the articulating component on the
3.1.8 posterior slope—the angle that the perpendicular axis
tibial plate.
of the tibial tray makes when it is tilted posteriorly away from
the tibial axis (see Fig. 1).
5. Apparatus and Materials
3.1.9 R value—the ratio of the minimum force to the
5.1 Testing machine, with the following characteristics:
maximum force (that is, R = minimum force/maximum force).
5.1.1 A sinusoidal, dynamic-forcing waveform.
3.1.10 tibial axis—nominal longitudinal axis of the tibia,
5.1.2 An error in applied force not greater than 62 % at the
whichcorrespondswiththecentralaxisofthemedullarycavity
maximum force magnitude (in accordance with ISO 4965).
of the proximal tibia.
5.1.3 The axial force peak representative of what could
3.1.11 tibial tray/bearing disassociation—unrecoverable
occur during daily activities of high flexion is a force of about
physical separation of the tibial bearing and tibial tray compo-
2275 N. During the tests, the values of the maximum and
nents as a result of bearing distraction or tilting.
minimumforcesshallbemaintainedtoanaccuracyof 62%of
the maximum force. The test shall be stopped if this accuracy
3.1.12 tibial tray centerline—a line running anteroposterior
that is the mirror line of the tibial articulating surface. For is not maintained.
5.1.4 The forcing accuracy must be maintained as bearing
asymmetric bearing tibial tray designs, the appropriate tibial
tray centerline shall be determined and reported along with the component deformation occurs.
5.1.5 Instrumentation to record the number of cycles.
rationale for the location.
5.2 Fixturing:
4. Significance and Use
5.2.1 Means of mounting and enclosing the test specimens
4.1 This test method can be used to describe the effects of using a corrosion-resistant material that is capable of holding
materials, manufacturing and design variables on the fatigue/ the femoral component and tibial tray.
cyclic creep performance of UHMWPE bearing components 5.2.2 The fixtures shall maintain the tibial and femoral
subject to substantial rotation in the transverse plan (relative to components in their required orientations for the duration of
the tibial tray) for a relatively large number of cycles. the test.
FIG. 1 Incline the Ttibial Tray Relative to the Tibial Axis at the Recommended Angle (Posterior Slope)
F2777 − 10
5.2.3 If necessary, bone cement (see ISO 5833) or a high- apartovertheentiresuperiorsurfaceoftheUHMWPEbearing.
strength epoxy may be used to lock the femoral and tibial The measurements should be made with the bearing at 20
components in their fixtures. 6 2°C.
5.2.4 The test apparatus or fixture should allow the force to
7.2 On one representative sample, perform the “A-P Draw
be applied through the center of the femoral component and
Test” (Section 9.2) and the “Rotary Laxity Test” (Section 9.4)
ensure equal force transmission through the medial and lateral
from Test Method F1223 at the same flexion angle used in 7.6
condyles.
of this test method.
5.3 Fluid Medium:
7.3 The UHMWPE bearing shall be conditioned in a deion-
5.3.1 The test assembly shall be immersed in deionized
ized water environment at 37 6 2°C prior to initiation of the
water at 37 6 2°C.
test for a long enough time to bring the bearing into equilib-
5.3.2 Water should be added as necessary to keep the test
rium with the fluid temperature.
components at the test temperature for the duration of the test.
7.4 Mount the tibial tray in the test machine. The main
6. Specimen Selection proximal planar surface shall be inclined at the posterior slope
recommended by the manufacturer (see Fig. 1). If more than
6.1 The metallic components shall follow the complete
one slope is recommended, the largest slope should be used.
manufacturing process (machining, surface treatment, laser
Mount the bearing component on the tibial tray using the
marking, passivation, cleaning, and so forth) until the steril-
method recommended by the manufacturer.
ization stage. Because sterilization has no known effect on the
NOTE 1—The tibial slope will generate a shear force and a resulting
mechanical properties for metallic components, it is not nec-
bending moment on the test frame actuator. This may cause a significant
essary for these to be sterilized. Unlike the metal components,
error of the load cell, depending on the sensitivity of the load cell to off
the UHMWPE components shall be sterilized in a manner axis loading. This should be addressed in the test setup.
consistent with the clinical use for such devices, as this may
7.5 Measure vertical distraction (when appropriate for the
affect the mechanical properties of the material.
design) and bearing tilt (Fig. 2).
6.2 The UHMWPE component(s) shall be artificially aged
7.5.1 To measure the vertical distraction, use appropriately-
according to Practice F2003, except when the mechanical sized feeler gauges, one set under each condyle to lift the
properties of the UHMWPE have been proven not to be
bearingawayfromthetibialplatekeepingtheposteriorsurface
detrimentally affected by aging. of the bearing parallel to the superior surface of the tibial plate,
until the gauges will not fit easily in the gap. The thickness of
6.3 Most of the knee systems allow the tibial tray to be
the feeler gauges is the vertical distraction value.
upsized, size for size or downsized relative to the bearing
7.5.2 To measure the posterior bearing tilt displacement,
component size. Consistent with the principle of this test
push the bearing posteriorly and raise the posterior edge of the
method, the smallest tibial tray compatible with a given
bearing by hand. Select a location on the posterior edge of the
bearing component size (according to the manufacturer) shall
bearing and measure the perpendicular distance from that
be used.
location to the tibial plate. Change in these displacements after
6.4 There may be some small variation in the amount of
testing may be useful as an indicator of damage.
cold flow of the bearing component depending on the tibial
7.6 Mount the femoral component in the test machine with
bearingthickness.However,thepossibleeffectofthecoldflow
an alignment such that the component is flexed in the sagittal
is the worst on the thinnest bearing components. Consequently,
plane at the maximum flexion angle (including the posterior
the thinnest bearing component in the knee system scope shall
slopeangle)themanufacturerrecommends(seeFig.3)accord-
be used in this test.
ing to the method in Section 6.1.3 of Specification F2083.
7. Procedure
7.7 The femoral component should be placed so that it
7.1 On one representative sample, make the initial measure- contacts the bearing component close to the posterior edge of
ments on the bearing surface to characterize the subsequent the bearing. The specific contact points between components
amount of bearing deformation after completion of the test. should be recorded and justified. At minimum, it should be
Use of a Coordinate Measuring Machine (CMM) is the demonstrated that the anterior-posterior placement of the
recommended method of making the measurements. The components would permit flexion and rotation of the femur to
measurements should be in the form of a grid of points, the prescribed angles without impingement between the femur
referenced to a fixed plane on the UHMWPE bearing, 1.5 mm and tibia.
FIG. 2 Vertical Distraction and Posterior Bearing Tilt Displacement
F2777 − 10
FIG. 3 Rotate the Femoral Component until the Maximum Flexion Angle is Reached
NOTE 2—If the mobile bearing knee design allows anterior-posterior
femoral condyles into contact with the bearing before applying
translation of the mobile bearing, translate the bearing component
force. It may also be necessary to achieve the 20° rotation by
posteriorly relative to the tibial tray (according to the maximum transla-
rotating the femoral component, as long as the appropriate
tion allowed by the knee system) to simulate a worst-case condition.
flexion and load line are correct.
7.8 Initially align the all components in neutral rotation to
7.9.3 The line of force application shall be set to pass
set the maximum flexion angle. In this position, the femoral
through the femoral component centerline intersecting at or
component, the bearing component and the tibial tray anterior-
posterior to the contact points.
posterior centerlines are coplanar (see Fig. 4).
7.10 Introduce the water to completely immerse the test
7.9 Rotational Alignment:
specimen contact surfaces.
7.9.1 For mobile bearing knee system designs simulate 20°
7.11 Start the test machine and apply a cyclical force with a
ofinternalrotationforthetibialtraywithrespecttothefemoral
peak of 2275 N to the bearing component with the femoral
and bearing components (see Fig. 5).
component at the specified force. For these tests, the ratio of
NOTE 3—The femoral component and the anteroposterior centerlines of
the minimum force magnitude to the maximum force magni-
the bearing component are still collinear.
tude should be 0.1.
7.9.2 Forfixeddesigns,thecomponentsshouldsimulate20°
7.12 Operate the testing machine at a fixed frequency
of internal rotation for the tibia tray with respect to the femoral
between 0.5 to 2.0 Hz.
component. If a smaller angle is used
...
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