Contact Pressure and Sliding Velocity Dependency of the

Contact Pressure and Sliding Velocity Dependency of the Coefficient of Friction in Metal-Polyethylene Articulation
1
+1Schwenke, T; 1Sauerberg, I; 1Klabunde, R; 1Seebeck, J; 2Morlock, M
Zimmer GmbH, Switzerland, 2Technical University Hamburg-Harburg, Germany
[email protected]
Introduction
Wear of the polyethylene component is one limiting factor for the
longevity of artificial knee or hip joint [1]. The coefficient of friction
(COF) between the two articulating implant components is an important
parameter in the generation of wear—it is an integral part in the
formulation of friction energy that is induced into the surface and that
potentially leads to material removal [2]. Therefore, the relationship
between COF and system parameters such as contact stress and sliding
velocity is essential for implant material characterization and for
accurate numerical simulations of wear [3].
The present experimental study investigated the correlation of COF
with contact pressure and with sliding velocity, using a pin-on-disk test
setup. It was hypothesized that a) the COF decreases with increasing
contact pressure and that b) the COF decreases with increasing sliding
velocity.
Materials and Methods
Experiments were conducted on a six-station wear screening device
(OrthoPOD–AMTI, Inc., Watertown, MA, USA). Cylindrical polyethylene pins articulated against flat metal disks (Figure 1). The pins were
machined from non-crosslinked ultra-high molecular weight
polyethylene (PE, GUR 1020) and the disks were machined from
wrought low-carbon cobalt-chromium alloy (CoCr); both are
commercially available implant materials. The articulating disk surfaces
were highly polished.
During testing the PE pins were pressed
against the disks under constant vertical load
while the disks performed a linear reciprocal
motion at constant velocity (except for
acceleration and deceleration at the ends of
the motion path).
Six different contact pressures were
applied on two pin sizes (Table 1). Three
sliding velocity levels were investigated for
each contact pressure. Five pins were tested
in each pressure-velocity-group and every
test was repeated once. In order to
investigate the influence of pin diameter on
the resulting COF, two pressure levels of the Figure 1: Pin-on-disk recismaller pins were set close to those applied procating articulation under
constant vertical load.
on the larger pins. The samples were
mounted in chambers and lubricated with a mix of bovine calf serum
and deionized water, set at a final protein content of 30 g/l. EDTA and
sodium azide were added as this fluid recipe is commonly used for wear
testing of artificial implants [4]. The chambers were maintained at
37 ± 2°C (body temperature) during testing. Friction measurement
consisted of three reciprocations along the specified path, while the
shear force was measured at an acquisition rate of 200 Hz. Individual
and mean friction coefficients for each pressure- velocity-combination
were computed (Matlab–The MathWorks, Inc., Natick, MA, USA).
Roughness of pin and disk articulating surfaces were assessed in the
center of each sample pre- and post-test using a white-light
interferometer (NewView 6000–Zygo Corp., Middlefield, CT, USA).
Differences between sample groups were statistically analyzed to test for
their significance (Student’s t-test, Minitab–Minitab, Inc., State College,
PA, USA). The significance level was chosen at α = 0.050.
Table 1: Test matrix of applied vertical loads and sliding velocities. Each
pressure-velocity-group consisted of five pins that were tested twice.
Pin Diameter
[mm]
6.4
9.0
Vertical Load
[N]
126
191
318
34
69
140
282
422
Contact Pressure
[MPa]
3.92
5.94
9.89
0.53
1.08
2.2
4.43
6.63
Sliding Velocity
[mm/s]
1, 10, 30
1, 10, 30
Results
The pre-test average roughness value Ra of the CoCr disk was
0.004 ± 0.002 μm and did not change significantly post-test (p = 0.263).
The Ra-values of the PE pins decreased from pre- to post-test, as listed in
Table 2. This change was statistically significant for each pin size, as
indicated by the probability level p.
The COF decreased with increasing contact pressure (Figure 2). The
6.4 mm pins followed the trend of the 9.0 mm pins, except that their
COFs were slightly higher. For any given contact pressure, a trend was
observed of decreasing COF with increasing sliding velocity (Figure 3).
This trend, however, was only expressed in three out of five 9.0 mm pin
groups, while it was found for all 6.4 mm pins.
Table 2: Pre- and post-test average surface roughness Ra for the two pin groups.
Pin Diameter
[mm]
6.4
9.0
Surface Roughness Ra [μm]
Pre-Test
Post-Test
2.207 ± 0.081
1.997 ± 0.105
3.424 ± 0.308
2.548 ± 0.532
Probability Level
p
0.009
0.008
Figure 2: COF over contact pressure for the two pin diameters. Shown is the
average COF for each group.
Figure 3: Average COF over sliding velocity for the two pin diameters. Shown is
the average COF for each group.
Discussion
In line with the hypotheses, the COF decreased with increasing
contact pressure and a general trend was found of decreasing COF with
increasing sliding velocity in the articulation of non-crosslinked
polyethylene against highly polished CoCr. The decrease in COF is an
indication for a decrease in accumulative wear, which has been reported
in experimental wear investigations [5]. An increase in contact pressure
is associated with increasing wear in numerical wear simulations [3].
These models need to be refined based on the results of the present
study. The change of pin surface roughness after few reciprocations was
further interesting to note. The impact of pin surface roughness on COF
and ultimately on wear requires further investigation. In summary, the
results from this study are useful for the development of improved
numerical representations of metal-on-polyethylene articulating wear.
References
[1] Jacobs JJ et al., J Am Acad Orthop Surg, 2(4):212–20, 1994; [2]
Wang A et al., Wear, 248(1-2):38–47, 2001; [3] Fregly BJ et al., J
Biomech, 38(2):305–14, 2005; [4] Schwenke T et al., Proc Inst Mech
Eng [H], 219:457–64, 2005. [5] Saikko V et al., Proc. Inst Mech Eng
[H], 220(7):723-31, 2006.
Poster No. 2330 • 56th Annual Meeting of the Orthopaedic Research Society

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