ASTM F732-00(2006)
(Test Method)Standard Test Method for Wear Testing of Polymeric Materials Used in Total Joint Prostheses
Standard Test Method for Wear Testing of Polymeric Materials Used in Total Joint Prostheses
SIGNIFICANCE AND USE
This test method is intended to be performed in conjunction with pin-on-flat wear machines or similar machines that are designed to evaluate simplified specimen geometries.
This test method is designed to evaluate combinations of materials with respect to the amount of polymer wear, where quantifiable wear occurs primarily on the polymeric component. With some combinations of materials, significant wear of the counterface may occur, with subsequent embedding of counterface debris particles in the polymer. Such an occurrence will render the weight loss of the polymer specimen unreliable as an indicator of the polymer wear.
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 surface due to complex motion patterns, wear may be reported as volume loss of the polymeric specimen as a function of wear cycles (in which case a “wear cycle” shall be defined). Volume loss of the polymer specimen is determined by dividing the experimental weight loss by the density of the polymer. For ease of interpretation, wear should be reported as a function of both the number of wear cycles and the sliding distance, when possible.
The reference for the comparative evaluation of candidate materials shall be the wear rate of ultra-high-molecular-weight polyethylene (UHMWPE) conforming to Specification F 648 bearing against counterfaces of cobalt-chromium-molybdenum alloy (in accordance with Specifications F 75, F 799, or F 1537), having prosthetic-quality surface finish and lubricated with bovine blood serum (see 5.2).
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 test method 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.
General Information
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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation:F732–00 (Reapproved 2006)
Standard Test Method for
Wear Testing of Polymeric Materials Used in Total Joint
Prostheses
ThisstandardisissuedunderthefixeddesignationF732;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope F75 Specification for Cobalt-28 Chromium-6 Molybdenum
Alloy Castings and Casting Alloy for Surgical Implants
1.1 This test method describes a laboratory method for
(UNS R30075)
evaluating the wear properties of combinations of materials
F86 Practice for Surface Preparation and Marking of Me-
that are being considered for use as bearing surfaces of human
tallic Surgical Implants
total joint prostheses. The body of this test method contains
F648 Specification for Ultra-High-Molecular-Weight Poly-
general methods which apply to all types of prosthesis wear
ethylene Powder and Fabricated Form for Surgical Im-
applications while individual annexes describe specific wear
plants
test methods and clinical validation criteria tailored to each
F799 Specification for Cobalt-28Chromium-6Molybdenum
distinct wear application (for example, linear reciprocating
Alloy Forgings for Surgical Implants (UNS R31537,
motion, ball-cup (“hip-type”) wear, delamination wear, etc.). It
R31538, R31539)
is the intent of this test method to rank materials, within each
F1537 Specification for Wrought Cobalt-28Chromium-
wear application, for polymer wear rates under simulated
6MolybdenumAlloysforSurgicalImplants(UNSR31537,
physiological conditions. It must be recognized, however, that
UNS R31538, and UNS R31539)
contact geometries and wear motions are simplified using such
F2025 Practice for Gravimetric Measurement of Polymeric
methods. This test method, therefore, represents only an initial
Components for Wear Assessment
stage in the full wear characterization of a candidate material.
G40 Terminology Relating to Wear and Erosion
1.2 All candidate materials should be tested in an appropri-
ate joint simulator apparatus using prototype prostheses before
3. Terminology
being used in clinical trials in patients. The tests described in
3.1 Definitions of Terms Specific to This Standard:
thistestmethodareusedtoquicklyandreliablyscreenmaterial
3.1.1 wear—for the purpose of this test method, the pro-
combinations for wear performance in different orthopaedic
gressive loss of material from the polymer specimen as a result
wear applications prior to committing them to more expensive
of the oscillating motion against the counterface under load.
and time-consuming joint simulator testing. In addition, these
Wear may be generated by several mechanisms including
simplified tests can be used to relate material, surface finish, or
adhesion, two or three body abrasion, surface fatigue, or other
other parameters to wear behavior on a more practical basis
processes.
than is possible in joint simulator tests.
3.1.2 wear rate—the volume of material lost due to wear
2. Referenced Documents per unit of sliding distance (or per million wear cycles if
complex motion patterns result in a non-uniform sliding
2.1 ASTM Standards:
distance across the specimen; see 4.3).
D883 Terminology Relating to Plastics
4. Significance and Use
This test method is under the jurisdiction ofASTM Committee F04 on Medical 4.1 This test method is intended to be performed in con-
and Surgical Materials and Devices and is the direct responsibility of Subcommittee
junction with pin-on-flat wear machines or similar machines
F04.15 on Materials Test Methods.
that are designed to evaluate simplified specimen geometries.
Current edition approved March 1, 2006. Published April 2006. Originally
4.2 Thistestmethodisdesignedtoevaluatecombinationsof
approved in 1982. Last previous edition approved in 2000 as F732 – 00. DOI:
10.1520/F0732-00R06.
materials with respect to the amount of polymer wear, where
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
quantifiable wear occurs primarily on the polymeric compo-
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
nent. With some combinations of materials, significant wear of
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. the counterface may occur, with subsequent embedding of
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
F732–00 (2006)
counterfacedebrisparticlesinthepolymer.Suchanoccurrence that the serum contains a mass fraction of 0.2 to 0.3 % sodium
will render the weight loss of the polymer specimen unreliable azide (or other suitable antibacterial agent) to minimize bacte-
as an indicator of the polymer wear. rial degradation.
4.3 Wear is reported as volume loss of the polymeric
NOTE 2—Sodium azide is a poison and must be handled very carefully.
specimen as a function of sliding distance; however, if the
5.2.5 It is recommended that ethylene-diaminetetraacetic
sliding distance is not constant across the polymeric specimen
acid (EDTA) be added to the serum at a concentration of 20
surface due to complex motion patterns, wear may be reported
mM (7.45 g/L) to bind calcium in solution and minimize
asvolumelossofthepolymericspecimenasafunctionofwear
precipitation of calcium phosphate onto the bearing surfaces.
cycles (in which case a “wear cycle” shall be defined). Volume
The latter event has been shown to strongly affect the friction
loss of the polymer specimen is determined by dividing the
and wear properties, particularly of polyethylene/ceramic com-
experimental weight loss by the density of the polymer. For
binations (2).
ease of interpretation, wear should be reported as a function of
5.2.6 Additives such as sodium azide and EDTA shall be
both the number of wear cycles and the sliding distance, when
dissolvedindeionizedwaterandpassedthrougha0.2-µmfilter
possible.
before adding to bovine serum.
4.4 The reference for the comparative evaluation of candi-
5.2.7 The appropriate interval for replacing used serum
date materials shall be the wear rate of ultra-high-molecular-
depends on how long the serum maintains its composition (for
weight polyethylene (UHMWPE) conforming to Specification
example, lubricating properties) under the specific test
F648 bearing against counterfaces of cobalt-chromium-
conditions/materialsbeingusedandtheadditivespresentinthe
molybdenum alloy (in accordance with Specifications F75,
serum. There is no minimum replacement interval. The maxi-
F799,or F1537), having prosthetic-quality surface finish and
mum replacement interval is two weeks. The selected interval
lubricated with bovine blood serum (see 5.2).
must meet the validation requirements in the appropriate
5. Apparatus and Materials
annex.
5.2.8 Alubricantotherthanbovineserumshallbeusedonly
5.1 Orthopaedic Wear Application:
when it can be shown that the lubricant reproduces clinical
5.1.1 For linear reciprocating wear motion applications,
wear mechanisms as well or better than bovine serum. In such
refer to Annex A1.
case the lubricant shall be specified in the test report.
5.1.2 For fixed-bearing ball-cup (“hip-type”) wear motion
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 F86).
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.
6.2.4 Cleaning of Polymer Specimens—Prior to wear test-
ing, careful cleaning of the polymer specimens is important to
The boldface numbers in parentheses refer to a list of references at the end of
this test method. remove any contaminants that would not normally be present
F732–00 (2006)
on an actual prosthesis. During the wear test, the specimens wash, rinse, and drying fluids. This will provide the most
must be re-cleaned and dried before each wear measurement to accurate correction for fluid sorption by the wear specimens,
remove any extraneous material that might affect the accuracy and correction for any other factors which could affect wear
of the measurement. The required procedure for cleaning and measurements.
dryingofpolymericspecimens,asdefinedinPracticeF2025,is
7.6 After rinsing and drying, conduct wear measurements.
given in Annex A6.
7.7 Thoroughly rinse all test assembly surfaces which have
6.3 Soaking of Polymeric and Composite Specimens:
contacted bovine serum using deionized water.
6.3.1 Polymeric and composite specimens should be pre-
7.8 Inspect the bearing surfaces of the test specimens and
soaked in the wear test lubricant to minimize fluid-sorption
note the characteristics of the wear process. Visual, micro-
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 F2025 for the gravimetric measurement method).
test dates,
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