ASTM F732-00(2011)
(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 F648 bearing against counterfaces of cobalt-chromium-molybdenum alloy (in accordance with Specifications F75, F799, or F1537), 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.
1.3 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in non-conformance with the standard.
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 2011)
Standard Test Method for
Wear Testing of Polymeric Materials Used in Total Joint
Prostheses
ThisstandardisissuedunderthefixeddesignationF732;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoptionor,inthecaseofrevision,theyearoflastrevision.Anumberinparenthesesindicatestheyearoflastreapproval.Asuperscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2. Referenced Documents
1.1 This test method describes a laboratory method for
2.1 ASTM Standards:
evaluating the wear properties of combinations of materials
D883Terminology Relating to Plastics
that are being considered for use as bearing surfaces of human
F75Specification for Cobalt-28 Chromium-6 Molybdenum
total joint prostheses. The body of this test method contains
Alloy Castings and Casting Alloy for Surgical Implants
general methods which apply to all types of prosthesis wear
(UNS R30075)
applications while individual annexes describe specific wear
F86Practice for Surface Preparation and Marking of Metal-
test methods and clinical validation criteria tailored to each
lic Surgical Implants
distinct wear application (for example, linear reciprocating
F648Specification for Ultra-High-Molecular-Weight Poly-
motion,ball-cup(“hip-type”)wear,delaminationwear,etc.).It
ethylene Powder and Fabricated Form for Surgical Im-
is the intent of this test method to rank materials, within each
plants
wear application, for polymer wear rates under simulated
F799Specification for Cobalt-28Chromium-6Molybdenum
physiological conditions. It must be recognized, however, that
Alloy Forgings for Surgical Implants (UNS R31537,
contactgeometriesandwearmotionsaresimplifiedusingsuch
R31538, R31539)
methods.This test method, therefore, represents only an initial
F1537 Specification for Wrought Cobalt-28Chromium-
stage in the full wear characterization of a candidate material.
6Molybdenum Alloys for Surgical Implants (UNS
R31537, UNS R31538, and UNS R31539)
1.2 All candidate materials should be tested in an appropri-
F2025Practice for Gravimetric Measurement of Polymeric
ate joint simulator apparatus using prototype prostheses before
Components for Wear Assessment
being used in clinical trials in patients. The tests described in
G40Terminology Relating to Wear and Erosion
thistestmethodareusedtoquicklyandreliablyscreenmaterial
combinations for wear performance in different orthopaedic
3. Terminology
wear applications prior to committing them to more expensive
and time-consuming joint simulator testing. In addition, these
3.1 Definitions of Terms Specific to This Standard:
simplifiedtestscanbeusedtorelatematerial,surfacefinish,or
3.1.1 wear—forthepurposeofthistestmethod,theprogres-
other parameters to wear behavior on a more practical basis
sive loss of material from the polymer specimen as a result of
than is possible in joint simulator tests.
theoscillatingmotionagainstthecounterfaceunderload.Wear
1.3 The values stated in either SI units or inch-pound units
may be generated by several mechanisms including adhesion,
are to be regarded separately as standard. The values stated in
twoorthreebodyabrasion,surfacefatigue,orotherprocesses.
each system may not be exact equivalents; therefore, each
3.1.2 wear rate—thevolumeofmateriallostduetowearper
system shall be used independently of the other. Combining
unit of sliding distance (or per million wear cycles if complex
values from the two systems may result in non-conformance
motion patterns result in a non-uniform sliding distance across
with the standard.
the specimen; see 4.3).
ThistestmethodisunderthejurisdictionofASTMCommitteeF04onMedical
andSurgicalMaterialsandDevicesandisthedirectresponsibilityofSubcommittee
F04.15 on Material Test Methods. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
CurrenteditionapprovedJune1,2011.PublishedJuly2011.Originallyapproved contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
in 1982. Last previous edition approved in 2006 as F732–00 (2006). DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/F0732-00R11. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F732 − 00 (2011)
4. Significance and Use be filtered to remove hard, abrasive, particulate contaminants
that might otherwise affect the wear properties of the speci-
4.1 This test method is intended to be performed in con-
mens being tested.
junction with pin-on-flat wear machines or similar machines
5.2.3 Maintain the volume, concentration, and temperature
that are designed to evaluate simplified specimen geometries.
of the lubricant nearly constant throughout the test. This may
4.2 Thistestmethodisdesignedtoevaluatecombinationsof
be accomplished by sealing the chambers so that water does
materials with respect to the amount of polymer wear, where
notevaporate,byperiodicallyorcontinuouslyreplacingevapo-
quantifiable wear occurs primarily on the polymeric compo-
rated water with deionized water, or by recirculating the
nent.With some combinations of materials, significant wear of
lubricant in a sealed environment.
the counterface may occur, with subsequent embedding of
5.2.4 To retard bacterial degradation, freeze and store the
counterfacedebrisparticlesinthepolymer.Suchanoccurrence
serum until needed for testing. In addition, it is recommended
will render the weight loss of the polymer specimen unreliable
that the serum contains a mass fraction of 0.2 to 0.3 % sodium
as an indicator of the polymer wear.
azide (or other suitable antibacterial agent) to minimize bacte-
4.3 Wear is reported as volume loss of the polymeric rial degradation.
specimen as a function of sliding distance; however, if the
NOTE 2—Sodium azide is a poison and must be handled very carefully.
sliding distance is not constant across the polymeric specimen
5.2.5 It is recommended that ethylene-diaminetetraacetic
surface due to complex motion patterns, wear may be reported
acid (EDTA) be added to the serum at a concentration of 20
asvolumelossofthepolymericspecimenasafunctionofwear
mM [7.45 g/L] to bind calcium in solution and minimize
cycles(inwhichcasea“wearcycle”shallbedefined).Volume
precipitation of calcium phosphate onto the bearing surfaces.
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
andwearproperties,particularlyofpolyethylene/ceramiccom-
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
F648 bearing against counterfaces of cobalt-chromium-
example, lubricating properties) under the specific test
molybdenum alloy (in accordance with Specifications F75,
conditions/materialsbeingusedandtheadditivespresentinthe
F799,or F1537), 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
6.1 The governing rule for specimen preparation is that the
applications, refer to Annex A3.
fabricationprocessparallelsthatusedorintendedforuseinthe
NOTE 1—Other types of applications may be addressed in later
productionofactualprostheses,inordertoproduceaspecimen
revisions.
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
waterofupto75%(volumefraction)maybeappropriate.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-
face finish of specimens may be characterized by profilometry,
photomicrography, replication by various plastics, or other
The boldface numbers in parentheses refer to a list of references at the end of
this test method. techniques.
F732 − 00 (2011)
6.2.3 Sterilization—Sterilize the specimens in a manner chambers. This temperature shall be 37 6 3°C unless justifi-
typical of that in clinical use for such devices unless it can be cation can be provided that use of a different temperature will
proven that this has no effect on wear properties of the not affect the results.
materials. Report sterilization processing parameters with the
7.3 Placetheweartestspecimensintheirtestchambers,add
aging time prior to each test, if known. Sterilization of all test
the lubricant, and activate load(s) and motion(s).
and control specimens within a specific test group should be
7.4 As testing is commenced, monitor the specimens for
done simultaneously (in a single container), when possible, to
signsoferraticbehaviorthatmightrequireearlyterminationof
minimize variation among the specimens.
the test.
6.2.4 Cleaning of Polymer Specimens—Prior to wear
testing,carefulcleaningofthepolymerspecimensisimportant
7.5 Remove the wear and soak specimens at desired
to remove any contaminants that would not normally be
intervals, wash, rinse, concurrently in accordance with the
present on an actual prosthesis. During the wear test, the
procedure in Annex A6 (also defined in Practice F2025). It is
specimens must be re-cleaned and dried before each wear
important that both the wear and soak components be treated
measurement to remove any extraneous material that might
identically to ensure that they have the same exposure to the
affect the accuracy of the measurement. The required proce-
wash, rinse, and drying fluids. This will provide the most
dure for cleaning and drying of polymeric specimens, as
accurate correction for fluid sorption by the wear specimens,
defined in Practice F2025, is given in Annex A6.
and correction for any other factors which could affect wear
measurements.
6.3 Soaking of Polymeric and Composite Specimens:
6.3.1 Polymeric and composite specimens should be pre-
7.6 After rinsing and drying, conduct wear measurements.
soaked in the wear test lubricant to minimize fluid-sorption
7.7 Thoroughly rinse all test assembly surfaces which have
during the wear test. Without presoaking, specimens made
contacted bovine serum using deionized water.
from very low-wear polymers such as UHMWPE could show
7.8 Inspect the bearing surfaces of the test specimens and
a net increase in weight or volume during the initial wear
note the characteristics of the wear process. Visual,
intervals due to fluid sorption (1, 3). The error due to fluid
microscopic, profilometric, replication, or other inspection
sorption can be reduced through presoaking and use of control
techniques can be used. Care must be taken, however, that the
soak specimens. The length of presoaking depends on the
surfaces do not become contaminated or damaged by any
variability and magnitude of fluid sorption encountered (3).A
substance or technique that might affect the subsequent wear
minimum of one control soak specimen per material condition
properties. If contamination occurs, thoroughly reclean the
is required.
specimens prior to restarting the wear test.
6.4 Counterfaces of Metal Alloys, Ceramic, or Other Mate-
7.9 Replace the wear specimens, maintaining original
rials:
couples and orientation, and soak control(s) in fresh lubricant
6.4.1 Characterization—Pretest characterization of the
and continue wear cycling.
counterface material shall include recording of fabrication
variables, such as composition, forming method (forging,
7.10 The appropriate wear test duration depends on the
casting,molding,etc.)andanypostformingprocessing,suchas
objective of the specific test, the duration of run-in effects, the
annealing. Obtain data on material properties relevant to wear
linearity of wear rates, and the potential for wear mechanism
(for example, grain structure, hardness, and percentage of
transitions. The minimum duration shall be two million wear
contaminants).
cycles. The minimum number of wear measurements, subse-
6.4.2 Surface Finish—In tests that are intended to evaluate
quent to the initial measurement shall be four.
an alternate counterface material bearing against the standard
UHMWPE,ensurethatthecounterfacefinishisappropriatefor
8. Report
components intended for clinical use. In test of alternate
8.1 Materials:
materialswhereareferencemetalorceramicisused,polishthe
8.1.1 Provide material traceability information from a raw
counterface to the prosthesis quality.
material and fabrication or manufacturing standpoint for each
6.4.3 Ensure that cleaning of specimens produces a surface
material counterface. Examples of such information i
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