cadera / hip - Active Congress.......

MARTES / TUESDAY
16
standards. Furthermore, the necessity is
evident for developing a new standard for
screening wear tests based on the latest
fi ndings, such as multidirectional motion or
lubricant composition
Materials And Methods
A pin-on-disk (POD) wear test machine is a
common wear test method, which has been
widely used to evaluate the wear of polymers
in biotribology. In a POD the polymer, usually
in the form of a pin, slide over the surface of a
rotating disk. Two basic confi gurations can be
distinguished; the pin may be loaded along
its major axis, in a direction either normal to
or parallel with the axis of rotation of the disk.
Hence, the contact area is produced on the
edge (horizontal POD confi guration) or the
face (vertical POD confi guration) of the disk.
According to this defi nition, the wear test
method proposed here was a horizontal POD
(here in after called simply as POD I).
In the POD test device, a wheel or ring in
vertical position wears against an underlying
pin of polyethylene. Figure 1a shows
schematically the loading/motion confi guration.
The contact geometry at the test start
is non-conformal, that is a line-contact. In
this sense, these wear test devices would be
more representative for a knee wear device
than for a hip wear device, where the contact
geometry is conformal (also designated as
congruent).
There is no standard regarding this type
of wear device and similar studies are not
present in the literature. Thus, comparison
and analysis of the test conditions proposed
cannot be made. Due to its confi guration,
the closest available standard would be
the ASTM G137-97, with identical contact
geometry and intended too for ranking the
resistance of plastic materials in sliding wear.
However, this standard does not cover biotribological
conditions.
The multidirectional POD (POD II) wear test
method is based on the former unidirectional
POD (POD I) wear test method. However, in
the POD II test method the pin rotates too,
see Figure 1b, and as consequence of the
rotation of the pin this device has a biaxial
pattern motion (i.e. multidirectional). In the
literature, no wear device similar to this can
be found making from it a unique screening
wear test device. The tasks with this wear
test method are in principle the same as for
the former POD I. With the POD II tests,
additionally the effect of the motion type in
the wear resistance of the UHMWPE can
be studied.
The test with the POD test method was performed
as follows. The disk rotates continuously
against an underlying pin of UHMWPE.
A load of ~150 N (15 Kg) was applied to the
pins. It gives a maximum Herztian contact
pressure (po) of ~5 MPa. The relative surface
velocity was 100 rpm for the disk and
for the multidirectional test, the pin velocity
was 99 rpm. The frequency of the disk was
1 Hz. The wear of the UHMWPE pins was
mainly determined by profi lometric measurements.
Weight loss measurements were also
performed after the completion of the test.
The test length was 34 Km.
To the test lubricant distilled water was
added during the test for compensating
water evaporation. As test lubricant, a solution
consisting of bovine serum and distilled
water was used, which had a total protein
concentration of 30 mg/ml. The serum was
purchased at Sigma-Aldrich SrI (Calf serum,
bovine donor; product No.C9676). The soak
adsorption of the UHMWPE pins was determined
using an additional control pin, which
was loaded identically as the UHMWPE
pins in the RPOF machine, but no motion
was applied. The cleaning and drying of the
UHMWPE pins was performed according
to the ASTM 1715 standard. Weighing was
carried out with a Mettler Toledo AT261DeltaRange
® microbalance with an accuracy
of ±10 μg.