ASTM F2723-21

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it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
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Designation: F2723 − 13a F2723 − 21
Standard Test Method for
Evaluating Mobile Bearing Knee Tibial Baseplate/Bearing
Resistance to Dynamic Disassociation
This standard is issued under the fixed designation F2723; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method describes a laboratory method for evaluating the potential for mobile bearing knee tibial baseplate/bearing
disassociation under repeated forces.
1.2 The test described is applicable to any bicompartmental mobile bearing knee with a bearing retention mechanism. With
modification, the test can be applied to a unicompartmental mobile bearing knee with a bearing retention mechanism.
1.3 Although the methodology described does not replicate all physiological force conditions, it is a means of in vitroin-vitro
comparison of mobile bearing knee designs and the strength of the bearing retention mechanism between the tibial baseplate and
bearing components under the stated test conditions.
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Terminology
2.1 Definitions:
2.1.1 bearing axis—the line connecting the lowest points on both the lateral and medial condyles of the superior surface of the
mobile bearing.
2.1.2 bearing retention mechanism—mechanical means preventing tibial baseplate/bearing disassociation.
2.1.3 inferior articulating interfaces—any interface in which relative motion occurs between the underside of the mobile bearing
component and the tibial tray.
This test method is under the jurisdiction of ASTM Committee F04 on Medical and Surgical Materials and Devices and is the direct responsibility of Subcommittee
F04.22 on Arthroplasty.
Current edition approved July 15, 2013March 1, 2021. Published August 2013March 2021. Originally approved in 2008. Last previous version approved in 2013 as
F2723 – 13.F2723 – 13a. DOI: 10.1520/F2723-13A.10.1520/F2723-21.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F2723 − 21
2.1.4 limiting position—the position of the femoral component relative to the bearing at which the shear force is at a maximum
with anterior-posterior (AP) movement of the femoral component on the bearing.
2.1.5 mobile bearing—the component between fixed femoral and tibial knee components with an articulating surface on both the
inferior and superior sides.
2.1.6 mobile bearing knee system—a knee prosthesis system, comprised of comprising a tibial component, a mobile bearing
component that can rotate or rotate and translate relative to the tibial component, and a femoral component.
2.1.7 superior articulating interfaces—any interface in which relative motion occurs between the topside of the mobile bearing
component and the femoral bearing component.
2.1.8 tibial baseplate/bearing disassociation—unrecoverable physical separation of the bearing and tibial baseplate components
as a result of bearing distraction or tilting.
2.1.9 2-axistwo-axis orthogonal load frame—a test machine capable of applying forces and displacements that act at 90° to each
other.
3. Significance and Use
3.1 This test method includes the use of static and fatigue shear and bending force conditions to evaluate the bearing retention
mechanism of a mobile bearing knee design and its ability to prevent disassociation.
3.2 In general, disassociation does not occur during activities where the contact locations are within the boundaries of the bearing
surfaces. Disassociation is most likely to occur with forces at the edges of the bearing component or with large AP shear forces
on a posterior stabilized knee tibial component post. Extreme bearing rotation, bone/bearing impingement, severe varus or valgus
moments, high flexion, or any combination of the above these can increase the likelihood of disassociation.
3.3 The test method described is applicable to any bicompartmental mobile bearing knee with a bearing retention mechanism.
With modification, the test can be applied to a unicompartmental mobile bearing knee with a bearing retention mechanism.
4. Apparatus and Materials
4.1 A 2-axistwo-axis orthogonal load frame with feedback control on both axes beis required for dislocation testing. The machine
must be able to record force and displacement in both axes.
4.1.1 Component Size—Test specimens should be chosen to maximize the force on the bearing retention mechanism.
Considerations should include bearing thickness (a thicker bearing would tend to increase the forces on the locking mechanism,
but could also increase the material support for the locking mechanism), bearing profile/sizeprofile/size, and tibial baseplate
profile/size (a large bearing on a small tibial baseplate would tend to increase the overhang with rotation).
4.1.2 Component Quantity—The minimum number of test samples shall be five.
4.2 Component Configurations—The mobile bearing knee components should be assembled, as they would be for in-vivo use.
4.2.1 The femoral component flexion angle should be chosen to maximize the forces on the bearing retention mechanism. An
engineering analysis may be necessary to determine the appropriate femoral flexion angle that creates the largest shear and/or
bending forces on the retention mechanisms. For example, for a gait congruent design, a 0° flexion angle might distribute forces
on both the anterior and the posterior sides of the locking mechanism, minimizing any bending forces. A flexion angle of greater
than 90° may maximize the posterior position of the femoral component and consequently increase bending forces on the retention
mechanism.
4.2.2 The tibial baseplate should be positioned with the recommended posterior slope. For knee systems where more than one
posterior slope is recommended, the largest slope should be used.
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4.2.3 Component Fixtures—The femoral component is fixed at the desired flexion angle. The tibial baseplate should be fixtured
with the appropriate posterior slope. The tibial fixtures must allow the tibial baseplate to be fixed in relative rotation to the bearing
and the femoral components. The test specimen coordinate system is shown in Fig. 1. Fixtures should not inhibit free motion of
the bearing, even with substantial deformation if it should occur.
4.2.4 Applied Force—The vertical axial force should be maintained within 63 % for the duration of the test. The test apparatus
or fixtures should allow the force to be applied through the center of the femoral component (V , Fig. 1) to be distributed to the
c
contact points with the tibial component. The peak cyclic horizontal force applied to the tibial baseplate should be maintained
within 63 % for the duration of the test.
4.2.5 Displacement Measurement—Displacement sensing devices should be capable of measuring the relative motion between the
femoral and tibial baseplate in the anterior-posterior direction.
4.2.6 Oscillating Frequency—The cyclic horizontal force should be applied at a frequency of 0.5 to 3.0 cycles per second (0.5 to
3.0 Hz).
4.2.7 Cycle Counter—The test apparatus should be equipped with a cycle counter to record the total number of horizontal test
cycles.
5. Test Specimens
5.1 The total knee replacement (TKR) should be the manufacturer’s designated “standard” or “medium” size unless the bearing
retention mechanism varies with the size of the knee. If the retention mechanism does vary, an engineering analysis should be
conducted to justify a “worst case.”
5.2 The implant shall be in its original packaging as supplied to the user by the manufacturer.
5.3 If the implant is not available in its package state, the condition of the device shall meet all geometrygeometrical and material
specifications, but may contain slight surface irregularities (that is, “cosmetic rejects”) not considered influential in those regions
of the device deemed critical to the specific test.
6. Conditioning
6.1 Expose the test specimens to a clean atmosphere at a temperature of 37 6 2°C2 °C for 24 h prior to testing.
FIG. 1 Coordinate System and Force Locations
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6.2 The test shall be run in a bath at 37 6 2°C2 °C that covers the tibial bearing surface. The bath can be either bovine serum,
mineral oil, olive oil, or deionized water.shall be water (for example, reverse osmosis (RO) water, deionized (DI) water, or distilled
water). Before testing, the implant must be moved cyclically three times in the desired direction before data are acquired. These
three repetitions can be performed by hand. This procedure is intended to distribute lubricant between the bearing surface and the
tibial component. If the bearing is installed on the tibial component in the presence of lubricant it can be omitted.
7. Procedure
7.1 Assemble the bearing and tibial baseplate.
7.2 Measure vertical distraction (when appropriate for the design) and posterior bearing tilt displacement (Fig. 2). Change in these
displacements after testing may be useful as an indicator of damage.
7.2.1 To measure the vertical distraction, use plastic blade-type feeler gauges of an appropriate range to measure the amount of
distraction. The gauge blades should be long enough to completely fit under the full length of the bearing. One set shall be placed
under each condyle to lift the bearing away from the tibial plate keeping the posterior surface of the bearing parallel to the superior
surface of the tibial plate, until the gauges will not fit in the gap without force. The thickness of the feeler gauges is the vertical
distraction value.
7.2.2 To measure the posterior bearing tilt displacement, push the bearing posteriorly and raise the posterior edge of the bearing
by hand. Select a location on the inferior-posterior edge of the bearing and measure the perpendicular distance from that location
to the tibial plate. The value of that measurement is the posterior bearing tilt displacement.
7.3 Place test specimens in test apparatus using the appropriate fixtures.
7.3.1 The femoral component should be fixed at the desired flexion angle relative to the tibial baseplate.
7.3.2 The tibial baseplate and bearing should be approximately centered under the femoral component. The tibial baseplate should
be axially aligned with the femoral component, that is, 0° of relative rotation. The bearing can be allowed to rotate slightly less
than 5° when the force is applied to accommodate the possible variation in the flexion radii of the cond
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