Friction of Gamma Irradiated Conventional and Remelted Highly Crosslinked UHMWPE against Orthopaedic
Grade Co-Cr alloy and Zirconia/Alumina Composites Bearing Materials for Total Joint Replacements
1
Kim, D W; +1Lee, K Y; 2Lee, S J
+1Sejong University, Seoul, Korea, 2Inje University, Gimhae, Korea
[email protected]
INTRODUCTION:
Ultra high molecular weight polyethylene (UHMWPE) has been
successfully employed as a bearing liner material in total joint
replacement over the last four decades. There was tremendous
improvement of wear resistance in UHMWPE from gamma irradiated
conventional one to highly crosslinked one. Also many kinds of opposite
bearing materials have been used such as Co-Cr alloy and ceramic
composites. There have been many efforts to minimize wear from
bearing material pairs, for it has been proved that submicron sized wear
debris caused osteolysis in periprosthetic tissue and subsequent aseptic
loosening of total joint replacement. However, studies characterizing the
frictional behaviors of these bearing material pairs are lacking. In this
study, the short-term frictional behaviors of two typical types of gamma
irradiated conventional and remelted highly crosslinked UHMWPEs
against Co-Cr alloy and low-temperature degradation free zirconia/
alumina ceramic composite were compared under three levels of contact
pressures.
METHODS:
Two groups of UHMWPE pin specimens were prepared from two
kinds of UHMWPEs in a right angle cylindrical shape. One group of pin
specimens was machined from compression molded sheet with GUR
1020 resin and gamma irradiated at the dose of 4.0Mrad. The other
group of pin specimens was machined from the commercial hip liners
made with remelted highly crosslinked UHMWPE. And two groups of
disk specimens were prepared from two kinds of metal and ceramic
materials in a flat disk shape. One group of disk specimens was
machined from orthopaedic grade Co-Cr alloy rod. The other group of
disk specimens was made with low temperature degradation (LTD)-free
zirconia/alumina ceramic composite. All Co-Cr alloy and LTD-free
zirconia/alumina ceramic composite disk specimens were polished
below the surface roughness (Ra) of 0.003㎛.
Friction tests were conducted by using a pin-on-disk type tribotester in
a fully immersed lubrication condition with a mixture of bovine serum
and sodium azide solution. Three levels of contact pressures of 5, 10,
and 20MPa were applied to each pair of contact material. All friction
tests were performed in a repeat pass rotational sliding motion with a
speed of 60 rpm. The coefficients of friction were determined from the
measured frictional force divided by applied normal force. All frictional
data were measured and stored during the entire test duration of about 8
minutes, and average coefficients of friction were calculated between
250-350 seconds after the measured frictional forces reached to the
steady state, and statistically analyzed.
RESULTS:
Average coefficients of friction of two kinds of UHMWPE pin
specimens against two kinds of disk specimens were summarized for
three levels of contact pressures in Table 1 and plotted in Figure 1. It is
surprising that all test results of coefficients of friction were in the range
of 0.01-0.04 which was as low as for the natural cartilage and cartilage
contact. The highest coefficient of friction was 0.041 and the lowest one
was 0.013 from all kinds of contact pairs. And also average coefficients
of friction decreased as contact pressure increased for all kinds of
contact pairs.
Comparing the results in the viewpoint of UHMWPE, remelted highly
crosslinked UHMWPE pins showed lower coefficients of friction than
gamma irradiated conventional UHMWPE against both Co-Cr alloy and
LTD-free zirconia/alumina ceramic composite disks for all levels of
contact pressures. In the viewpoint of disk material, average coefficients
of friction of gamma irradiated conventional UHMWPE against Co-Cr
alloy were much higher than those against LTD-free zirconia/alumina
ceramic composite disks for all levels of contact pressures. However,
there was no significant difference in average coefficients of friction of
remelted highly crosslinked UHMWPE against Co-Cr alloy and LTDfree zirconia/alumina ceramic composite disks for all levels of contact
pressures except the case of contact pressure of 10MPa.
Table 1. Average coefficients of friction and standard deviations of each
contact pair for three levels of contact pressures.
Figure 1. Average coefficients of friction and standard deviations of
each contact pair for three levels of contact pressures.
DISCUSSION:
In other friction studies on UHMWPE against Co-Cr alloy and
ceramics, Saikko [1] repoted the average coefficients of friction of 0.080.16 in distilled water under 4.8MPa in an oscillating motion. Yao at. al.
[2] repoted the average coefficients of friction of 0.02-0.025 in distilled
water, diluted or undiluted bovine calf serum, bovine synovial fluid, and
human synovial fluid under 4.9N in a rotational motion. Gispert at. al.
[3] reported the average coefficients of friction of 0.028-0.086 in Hank’s
solution (HBSS), HBSS+hyaluronic acid (HA), and HBSS+bovine
serum albumin (BSA)+HA under 0.39, 0.88, and 1.53MPa in a
rotational motion. All of these studies have been done under relatively
lower level of contact pressures, and their results of coefficients of
friction in a bovine serum fully-immersed lubricated condition were
equivalent to or higher than those of this study.
Arranging the results of coefficients of friction for each contact pair
from the lowest to the highest order, it can be listed as highly
crosslinked PE + Z/A composite, highly crosslinked PE + Co-Cr alloy,
conventional PE + Z/A composite, conventional PE + Co-Cr alloy. It is
interesting that this order of frictional behavior is same as the order of
contact pairs that having the wear resistance from the highest to the
lowest order.
ACKNOWLEDGMENT:
This work was supported by the Korea Science and Engineering
Foundation (KOSEF) grant funded by the Korea government (MEST)
(2009-0080928).
REFERENCES:
[1] Saikko V., Wear, 166:169-178 (1993).
[2] Yao JQ, et. al., Wear, 255:780-784 (2003).
[3] Gispert MP, et. al., Wear, 260:149-58 )2006).
Poster No. 2255 • 56th Annual Meeting of the Orthopaedic Research Society