ASTM F897-84(1997)e1

e1
Designation: F 897 – 84 (Reapproved 1997)
Standard Test Method for
Measuring Fretting Corrosion of Osteosynthesis Plates and
Screws
This standard is issued under the fixed designation F 897; 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.
e NOTE—Sections 2.1, 4.3, and 7.1.2 were corrected editorially in October 2002.
1. Scope rank metal-to-metal couples for resistance to fretting corrosion,
or to study other material combinations.
1.1 This test method provides a screening test for determin-
1.6 This standard may involve hazardous materials, opera-
ing the amount of metal loss from plates and screws used for
tions, and equipment. This standard does not purport to
osteosynthesis (internal fixation of broken bones) due to
address all of the safety concerns associated with its use. It is
fretting corrosion in the contact area between the screw head
the responsibility of whoever uses this standard to consult and
and the plate hole countersink area. The implants are used in
establish appropriate safety and health practices and deter-
the form they would be used clinically. The machine described
mine the applicability of regulatory limitations prior to use.
generates a relative motion between plates and screws which
simulates one type of motion pattern that can occur when these
2. Referenced Documents
devices are used clinically.
2.1 ASTM Standards:
1.2 Since the environmental and stress conditions used in
D 1886 Test Methods for Nickel in Water
this test may not be identical to those experienced by bone
F 86 Practice for Surface Preparation and Marking of Me-
plates in the human body, this test may produce fretting
tallic Surgical Implants
corrosion rates that are lower or higher than those experienced
F 382 Specification and Test Method for Metallic Bone
in practice. The recommended axial load of 400 N was selected
Plates
as being in a range where the amount of fretting corrosion is
F 543 Specification for Cortical Bone Screws
not sensitive to small changes in axial load (1). The combi-
G 1 Practice for Preparing, Cleaning, and Evaluating Cor-
nation of the recommended load and angular displacement are
rosion Test Specimens
such that a measurable amount of fretting corrosion of surgical
alloys occurs in a comparatively short period of time (7 to 14
3. Summary of Test Method
days). (1-3)
3.1 A two-hole plate is attached to two plastic rods with
1.3 The device is designed so as to facilitate sterilization of
bone screws, with flexible spacers between the plate and the
the test specimens and test chambers to permit testing with
rods, placed in a glass beaker, and the beaker sealed with a
proteinaceous solutions that would become contaminated with
flexible rubber cover. This assembly is steam sterilized, and
microbial growth in nonsterile conditions.
then a sterile solution is injected through the rubber cover into
1.4 The specimens used can be standard osteosynthesis
the beaker. This assembly is then mounted in the fretting
implants or can be materials fabricated into the appropriate
apparatus which, when set in motion, produces a rocking
shapes.
motion and, therefore, a small cyclic displacement between the
1.5 This test method may be used for testing the fretting
mating surfaces of the plate and screws. The amount of fretting
corrosion of metal plates and screws of similar or different
corrosion is determined at the end of the test by measurement
alloy compositions, or it may be used for testing the fretting
of the weight loss of the plates and screws and by chemical
corrosion of metal-nonmetal combinations. This test method
analysis of the solutions.
may also be used for wear or degradation studies of nonme-
tallic materials. This test may be used as a screening test to
4. Significance and Use
rank the corrosivities of saline or proteinaceous solutions, or to
4.1 It is well known from examination of implants after use
that plates and screws used for osteosynthesis are subjected to
This test method is under the jurisdiction of ASTM Committee F04 on Medical metal loss due to corrosion at the plate-screw interfaces. One of
and Surgical Materials and Devicesand is the direct responsibility of Subcommittee
F04.15 on Material Test Methods.
Current edition approved Sept. 28, 1984. Published February 1985. Annual Book of ASTM Standards, Vol 11.01.
2 4
The boldface numbers in parentheses refer to the list of references at the end Annual Book of ASTM Standards, Vol 13.01.
of this standard. 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.
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F 897 – 84 (1997)
the mechanisms of this corrosive attack is fretting corrosion (250 6 4°F), and equipped with a thermometer, pressure gage,
due to relative motion (micromotion) between the screw heads vent cock, and a rack to hold the test assemblies above the
and plate-hole countersinks. water level.
4.2 It is also known that release of corrosion products into
5.2 Microbalance, with a 0.01-mg scale.
the tissues surrounding an implant may have adverse effects on 5.3 Fretting Apparatus, as described in 5.3.1-5.3.4 and
local tissue or have systemic effects. Thus, it is important to
illustrated in Fig. 1.
minimize the amount of tissue exposure to corrosion products. 5.3.1 The fretting apparatus is driven by a slow speed gear
4.3 Screws and plates are available in different configura-
motor connected to a horizontal rotating shaft. Round disks
tions in accordance with Specifications F 543 and F 382. This with machined flats (cams) are mounted on the shaft as shown
test method may be used to evaluate the effects of different
in Fig. 1. For multiple specimen testing, there may be more
combinations of screw and plate designs. As new materials and than one cam on the drive shaft.
device designs are developed for use in the treatment of
5.3.2 The flats on the cams are machined so as to produce 2°
fractured bones, it is important to determine the effects these of relative motion between the posts of the test assemblies.
developments have on the amount of metal loss due to fretting
NOTE 1—A suggested combination of short post length and plunger
corrosion.
displacement is a 5-cm post with a 1.9-mm displacement.
4.4 This test method provides a standardized screening test
5.3.3 The shaft rotation rate and the number of machined
for ranking metal plates and screws in terms of resistance to
flats shall be such that the flats produce one oscillation of the
fretting corrosion and for determining the influence of different
plunger per second.
solutions on fretting corrosion rates.
5.3.4 Test assembly holding and driving frames are mounted
4.5 This test method may also be used to generate corrosion
symmetrically on each side of the rotating cams. The oscillat-
products either for chemical analysis of the products or for
ing plunger is springloaded and held in the guide sleeve. The
testing for biological reactions to corrosion products using
hole in the top plate is slotted to permit adjustment of the
animal or cell culture methods.
position of the test assemblies.
4.6 It is well known that fretting corrosion rates depend on
5.4 Test Assemblies, consisting of two plastic rods, and two
normal load or pressure, frequency, sliding amplitude, materi-
flexible spacers, the two-hole plate, two bone screws, one
als, surface treatments, and environmental factors. (4) There-
beaker, and the rubber cover.
fore, when determining the effect of changing one of these
5.4.1 The longer rod is threaded at one end to mate with a
parameters (for example, material or environment), all others
mounting screw, while the other end is threaded to mate with
must be kept constant to facilitate interpretation of the results.
the bone screw.
5. Apparatus
5.4.2 The shorter rod has a reduced diameter at one end to
5.1 Steam Autoclave, capable of maintaining 121 6 2°C mate with the oscillating horizontal plunger, while the other
NOTE 1—Figure shows assembly drawings of one pair of test positions on each side of a cam, and of the relationships between the screws, plate,
spacers, and polyacetal rods.
FIG. 1 Two-Hole Plate Fretting (THPF) Testing System
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F 897 – 84 (1997)
able bone screws. Heads should be spherical, although other
shapes may be used.
7.2.2 Screws may be fabricated from rod stock in accor-
dance with Specification F 543.
7.3 Test specimens may be used in the condition as received
from the implant manufacturer; custom fabricated specimens
should be prepared in accordance with Practice F 86.
8. Procedure
8.1 Test Assembly Preparation (see Fig. 1b):
8.1.1 Clean the plates and screws ultrasonically with deter-
gent or other degreasing agent to ensure that they are free from
grease and dirt. Rinse them with distilled water, and immedi-
ately dry them in warm air.
FIG. 2 Assembled Test Chamber with Rubber Seal
8.1.2 Weigh the plate and each screw separately. Then
weigh the three together on a microbalance to an accuracy of
end is threaded to mate with a bone screw.
0.01 mg.
5.4.3 The flexible spacers made of, for example, polydim-
8.1.3 Attach the plates to the posts with the rubber spacers
ethylsiloxane or buna-n, are used to maintain axial loads on the
and bone screws. Tighten the screws so as to create a 400 6 50
screws and to permit the necessary axial displacements asso-
N axial load on the screws. After a correlation has been
ciated with the rocking motion of the screws, while at the same
developed for relating torque and axial load for the particular
time preventing fatigue failure of the screws. The screws are
metal(s), screw head, and plate hole configuration used, deter-
tightened such that there is a 400 6 50 N load on the screws;
mine the load indirectly by measurement of the screw torque.
a different load may be used, but in such cases the load must be
8.1.4 Place the assembled test specimens in a borosilicate
reported (see 10.1). In actual operation, it may be easier to
beaker, add the test solution, and seal the top with the rubber
measure the screw torque rather than the axial load; a method
cover with the tops of
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