ASTM F732-00
(Test Method)Standard Test Method for Wear Testing of Polymeric Materials for Use in Total Joint Prostheses
Standard Test Method for Wear Testing of Polymeric Materials for Use in Total Joint Prostheses
SCOPE
1.1 This test method describes a laboratory method for evaluating the wear properties of combinations of materials that are being considered for use as bearing surfaces of human total joint prostheses. The body of this standard contains general methods which apply to all types of prosthesis wear applications while individual annexes describe specific wear test methods and clinical validation criteria tailored to each distinct wear application (for example, linear reciprocating motion, ball-cup ("hip-type") wear, delamination wear, etc.). It is the intent of this test method to rank materials, within each wear application, for polymer wear rates under simulated physiological conditions. It must be recognized, however, that contact geometries and wear motions are simplified using such methods. This test method, therefore, represents only an initial stage in the full wear characterization of a candidate material.
1.2 All candidate materials should be tested in an appropriate joint simulator apparatus using prototype prostheses before being used in clinical trials in patients. The tests described in this test method are used to quickly and reliably screen material combinations for wear performance in different orthopaedic wear applications prior to committing them to more expensive and time-consuming joint simulator testing. In addition, these simplified tests can be used to relate material, surface finish, or other parameters to wear behavior on a more practical basis than is possible in joint simulator tests.
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Designation:F732–00
Standard Test Method for
Wear Testing of Polymeric Materials Used in Total Joint
Prostheses
This standard is issued under the fixed designation F 732; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope ethylene Powder and Fabricated Form for Surgical Im-
plants
1.1 This test method describes a laboratory method for
F 799 Specification for Thermomechanically Processed
evaluating the wear properties of combinations of materials
Cobalt-Chrome-MolybdenumAlloy for Surgical Implants
that are being considered for use as bearing surfaces of human
F 1537 Specification for Wrought Cobalt-28-Chromium-6
total joint prostheses. The body of this standard contains
Molybdenum Alloy for Surgical Implants
general methods which apply to all types of prosthesis wear
F 2025 Practice for Gravimetric Measurement of Polymeric
applications while individual annexes describe specific wear
Components for Wear Assessment
test methods and clinical validation criteria tailored to each
G 40 Terminology Relating to Erosion and Wear
distinct wear application (for example, linear reciprocating
motion, ball-cup (“hip-type”) wear, delamination wear, etc.). It
3. Terminology
is the intent of this test method to rank materials, within each
3.1 Definitions of Terms Specific to This Standard:
wear application, for polymer wear rates under simulated
3.1.1 wear—for the purpose of this test method, the pro-
physiological conditions. It must be recognized, however, that
gressive loss of material from the polymer specimen as a result
contact geometries and wear motions are simplified using such
of the oscillating motion against the counterface under load.
methods. This test method, therefore, represents only an initial
Wear may be generated by several mechanisms including
stage in the full wear characterization of a candidate material.
adhesion, two or three body abrasion, surface fatigue, or other
1.2 All candidate materials should be tested in an appropri-
processes.
ate joint simulator apparatus using prototype prostheses before
3.1.2 wear rate—the volume of material lost due to wear
being used in clinical trials in patients. The tests described in
per unit of sliding distance (or per million wear cycles if
thistestmethodareusedtoquicklyandreliablyscreenmaterial
complex motion patterns result in a non-uniform sliding
combinations for wear performance in different orthopaedic
distance across the specimen; see 4.3).
wear applications prior to committing them to more expensive
and time-consuming joint simulator testing. In addition, these
4. Significance and Use
simplified tests can be used to relate material, surface finish, or
4.1 This test method is intended to be performed in con-
other parameters to wear behavior on a more practical basis
junction with pin-on-flat wear machines or similar machines
than is possible in joint simulator tests.
that are designed to evaluate simplified specimen geometries.
2. Referenced Documents 4.2 Thistestmethodisdesignedtoevaluatecombinationsof
materials with respect to the amount of polymer wear, where
2.1 ASTM Standards:
2 quantifiable wear occurs primarily on the polymeric compo-
D 883 Definitions of Terms Relating to Plastics
nent. With some combinations of materials, significant wear of
F 75 SpecificationforCastCobalt-Chromium-Molybdenum
3 the counterface may occur, with subsequent embedding of
Alloy for Surgical Implant Applications
counterfacedebrisparticlesinthepolymer.Suchanoccurrence
F 86 Practice for Surface Preparation and Marking of
will render the weight loss of the polymer specimen unreliable
Metallic Surgical Implants
as an indicator of the polymer wear.
F 648 Specification for Ultra-High-Molecular-Weight Poly-
4.3 Wear is reported as volume loss of the polymeric
specimen as a function of sliding distance; however, if the
sliding distance is not constant across the polymeric specimen
This test method is under the jurisdiction ofASTM Committee F04 on Medical surface due to complex motion patterns, wear may be reported
and Surgical Materials and Devices and is the direct responsibility of Subcommittee
asvolumelossofthepolymericspecimenasafunctionofwear
F04.15 on Materials Test Methods.
cycles (in which case a “wear cycle” shall be defined). Volume
Current edition approved May 10, 2000. Published August 2000. Originally
published as F 732–82. Last previous edition F 732–98.
Annual Book of ASTM Standards, Vol 08.01.
3 4
Annual Book of ASTM Standards, Vol 13.01. Annual Book of ASTM Standards, Vol 03.02.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
F732–00
loss of the polymer specimen is determined by dividing the The latter event has been shown to strongly affect the friction
experimental weight loss by the density of the polymer. For and wear properties, particularly of polyethylene/ceramic com-
ease of interpretation, wear should be reported as a function of binations (2).
both the number of wear cycles and the sliding distance, when 5.2.6 Additives such as sodium azide and EDTA shall be
possible. dissolvedindeionizedwaterandpassedthrougha0.2-µmfilter
4.4 The reference for the comparative evaluation of candi- before adding to bovine serum.
date materials shall be the wear rate of ultra-high-molecular- 5.2.7 The appropriate interval for replacing used serum
weight polyethylene (UHMWPE) conforming to Specification depends on how long the serum maintains its composition (for
F 648 bearing against counterfaces of cobalt-chromium- example, lubricating properties) under the specific test
molybdenum alloy (in accordance with Specifications F 75, conditions/materialsbeingusedandtheadditivespresentinthe
F 799, or F 1537), having prosthetic-quality surface finish and serum. There is no minimum replacement interval. The maxi-
lubricated with bovine blood serum (see 5.2). mum replacement interval is two weeks. The selected interval
must meet the validation requirements in the appropriate
5. Apparatus and Materials
annex.
5.1 Orthopaedic Wear Application: 5.2.8 Alubricantotherthanbovineserumshallbeusedonly
5.1.1 For linear reciprocating wear motion applications, when it can be shown that the lubricant reproduces clinical
refer to Annex A1. wear mechanisms as well or better than bovine serum. In such
5.1.2 For fixed-bearing ball-cup (“hip-type”) wear motion case the lubricant shall be specified in the test report.
applications, refer to Annex A2.
6. Preparation of Specimens
5.1.3 For nominally linear motion delamination wear appli-
6.1 The governing rule for specimen preparation is that the
cations, refer to Annex A3.
fabricationprocessparallelsthatusedorintendedforuseinthe
NOTE 1—Other types of applications may be addressed in later revi-
productionofactualprostheses,inordertoproduceaspecimen
sions.
with comparable bulk material properties and surface charac-
5.2 Lubricant (see also Annex A4):
teristics (see Practice F 86).
5.2.1 The specimen shall be lubricated with bovine blood
6.2 Polymers and Composites:
serum unless an alternative medium can be justified as de-
6.2.1 Obtain a fabrication history for each polymeric or
scribed in section 5.2.8. Since different sera differ in compo-
composite specimen, including information such as grade,
sition (protein concentration, etc.), dilution with deionized
batch number, and processing variables, including method of
water of up to 75 % (volume fraction) may be appropriate.The
forming (extruding, molding, etc.), temperature, pressure, and
appropriate dilution shall be based on satisfaction of the
forming time used, articulation surface preparation methods
clinical validation criteria in the appropriate annex.
(see Annex A5) and any post-forming treatments, including
5.2.2 A filter-sterilized serum rather than pooled serum
sterilization.
should be used since the former is less likely to contain
6.2.2 Pre-test characterization may include measurement of
hemolyzed blood material, which has been shown to adversely
bulk material properties, such as molecular-weight range and
affect the lubricating properties of the serum (1) . Serum must
distribution, percent crystallinity, density, or others. The sur-
be filtered to remove hard, abrasive, particulate contaminants
face finish of specimens may be characterized by profilometry,
that might otherwise affect the wear properties of the speci-
photomicrography, replication by various plastics, or other
mens being tested.
techniques.
5.2.3 Maintain the volume, concentration, and temperature
6.2.3 Sterilization—Sterilize the specimens in a manner
of the lubricant nearly constant throughout the test. This may
typical of that in clinical use for such devices unless it can be
be accomplished by sealing the chambers so that water does
proven that this has no effect on wear properties of the
notevaporate,byperiodicallyorcontinuouslyreplacingevapo-
materials. Report sterilization processing parameters with the
rated water with deionized water, or by recirculating the
aging time prior to each test, if known. Sterilization of all test
lubricant in a sealed environment.
and control specimens within a specific test group should be
5.2.4 To retard bacterial degradation, freeze and store the
done simultaneously (in a single container), when possible, to
serum until needed for testing. In addition, it is recommended
minimize variation among the specimens.
that the serum contains a mass fraction of 0.2 to 0.3 % sodium
6.2.4 Cleaning of Polymer Specimens— Prior to wear
azide (or other suitable antibacterial agent) to minimize bacte-
testing, careful cleaning of the polymer specimens is important
rial degradation.
to remove any contaminants that would not normally be
present on an actual prosthesis. During the wear test, the
NOTE 2—Sodium azide is a poison and must be handled very carefully.
specimens must be re-cleaned and dried before each wear
5.2.5 It is recommended that ethylene-diaminetetraacetic
measurement to remove any extraneous material that might
acid (EDTA) be added to the serum at a concentration of 20
affect the accuracy of the measurement. The required proce-
mM (7.45 g/L) to bind calcium in solution and minimize
dure for cleaning and drying of polymeric specimens, as
precipitation of calcium phosphate onto the bearing surfaces.
defined in Practice F 2025, is given in Annex A6.
6.3 Soaking of Polymeric and Composite Specimens:
6.3.1 Polymeric and composite specimens should be pre-
The boldface numbers in parentheses refer to a list of references at the end of
this test method. soaked in the wear test lubricant to minimize fluid-sorption
F732–00
during the wear test. Without presoaking, specimens made scopic, profilometric, replication, or other inspection tech-
from very low-wear polymers such as UHMWPE could show niques can be used. Care must be taken, however, that the
a net increase in weight or volume during the initial wear surfaces do not become contaminated or damaged by any
intervals due to fluid sorption (1, 3). The error due to fluid substance or technique that might affect the subsequent wear
sorption can be reduced through presoaking and use of control properties. If contamination occurs, thoroughly reclean the
soak specimens. The length of presoaking depends on the specimens prior to restarting the wear test.
variability and magnitude of fluid sorption encountered (3).A
7.9 Replace the wear specimens, maintaining original
minimum of one control soak specimen per material condition couples and orientation, and soak control(s) in fresh lubricant
is required.
and continue wear cycling.
6.4 Counterfaces of Metal Alloys, Ceramic, or Other Mate-
7.10 The appropriate wear test duration depends on the
rials:
objective of the specific test, the duration of run-in effects, the
6.4.1 Characterization—Pretest characterization of the
linearity of wear rates, and the potential for wear mechanism
counterface material shall include recording of fabrication
transitions. The minimum duration shall be two million wear
variables, such as composition, forming method (forging,
cycles. The minimum number of wear measurements, subse-
casting,molding,etc.)andanypostformingprocessing,suchas
quent to the initial measurement shall be four.
annealing. Obtain data on material properties relevant to wear
(for example, grain structure, hardness, and percentage of
8. Report
contaminants).
8.1 Materials:
6.4.2 Surface Finish—In tests that are intended to evaluate
8.1.1 Provide material traceability information from a raw
an alternate counterface material bearing against the standard
material and fabrication or manufacturing standpoint for each
UHMWPE,ensurethatthecounterfacefinishisappropriatefor
material counterface. Examples of such information include
components intended for clinical use. In test of alternate
material grade, batch number, and processing variables.
materialswhereareferencemetalorceramicisused,polishthe
8.1.2 Pretest characterization for a plastic counterface may
counterface to the prosthesis quality.
include measurement of bulk material properties, such as
6.4.3 Ensure that cleaning of specimens produces a surface
molecular-weight average, range, and distribution, percent
free of any particles, oils, greases, or other contaminants that
crystallinity, density, degree of oxidation, or others. The
might influence the wear process.
surface finish of both counterfaces may be characterized by
7. Procedure profilometry, photomicrography, replication, or other appli-
cable techniques. Surface finish of the harder counterface shall
7.1 Make any initial measurements required to determine
be reported.
the subsequent amount of wear of the polymeric specimen (see
8.1.3 Reportthemethodofsterilization,thesterilizationand
Practice F 2025 for the gravimetric measurement method).
test dates, if known, and the means of storage post-sterilization
7.2 Place the control soak specimen(s) in a soak chamber of
and pretest.
test lubricant, such that the total surface area exposed to the
8.2 Test Apparatus—Report the number of stations on the
lubricant is equal to that of the wear specimens when mounted
machine and the number of stations used for this test. Report if
in the test chambers. Maintain the soak chamber lubricant
replicate tests were conducted during more than one test series.
temperature at the same nominal temperature as the test
Describe the mechanisms used to generate motions and forces,
chambers. This temperature shall be 37 6 3°C unless justifi-
the
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