Biomedical Engineering Reference
In-Depth Information
corrosion, and tissue reactions lead either to a mechanical failure or to aseptic loosening
of the implant. The main problem with the prosthetic joints lies in its wear and corrosion
during long-term use. The debris formed as a consequence of this wear results in tissue
inflammation, osteolysis, and finally loosening of the implants [157-162]. When two mate-
rial surfaces slide against each other and are in relative motion, the material with lower
hardness is worn out. The material will also come in contact with human body fluids.
Therefore, in prosthetic joints, the coating material should be hard and inert enough to
prevent wear out and corrosion. The coating material should also have good adhesion on
the substrate, especially in human body fluid. Besides amorphous carbon's biocompatibil-
ity, its excellent tribological properties [163-166] have made it a potential material to reduce
the wear rate and corrosion in the physiological environment.
There are experiments using hip simulators to determine friction and wear of amor-
phous carbon-coated hip joint balls sliding against ultra high molecular weight polyeth-
ylene (UHMWPE) or of metal-metal joints with both sides coated with amorphous carbon.
Tetrahedral amorphous carbon (ta-C) coated metal hip joint balls tested in 1 wt.% NaCl
water by pin-on-disk and in a hip joint simulator reduce the wear of the UHMWPE cup
by a factor of 10-100 [150,167]. In the case of a metal-metal joint with both sides coated, the
wear could be reduced by a factor of 10
5
. Additionally, the corrosion rate is significantly
lowered when the coated substrate is exposed to a saline solution equivalent to body fluids
in 37°C for 2 years [150]. Stainless steel femoral head is coated with a-C:H and the wear of
the UHMWPE cups in a hip joint simulator using distilled water after six million cycles is
determined [168]. The result is a decrease of wear by a factor of six. Tested in a knee wear
simulator using distilled water as a lubricant, a decrease of a factor of five in wear of the
UHMWPE by coating the cobalt chromium counter face with a-C:H can be obtained [169].
A comparative tribological test using ball-on-disk and pin-on-disk configuration, but in
dry conditions, to investigate the wear of different materials used in hip joint prostheses
against UHMWPE was conducted [170]. The wear rate of stainless steel, titanium, alumina,
zirconium oxide, a-C:H coated stainless steel, and a-C:H coated titanium were compared.
The results revealed the superiority of the a-C:H coatings. In a more recent tribological
study using pin-on-disk in simulated body fluid, it was found that coating both surfaces of
UHMWPE and Co-Cr-Mo implants by ta-C film enhanced the lifetime of the implant to
a considerable extent [171,172].
The wear of UHMWPE acetabular cups against CoCr, alumina, and a-C:H coated CoCr
with a biaxial hip wear simulator using diluted calf serum as the lubricant was studied
[173]. It was found that there is no significant difference in the wear of UHMWPE for all
three pairs of tested cups. The same result is obtained from another independent study
[174]. The corrosion resistance and hardness of an orthopedic material like Co-Cr-Mo
alloy can be significantly increased by ta-C coatings [175]. However, the wear resistance of
the ta-C coated Co-Cr-Mo against UHMWPE measured by a pin-on-disk method in air,
deionized water, and simulated body fluid did not show any significant improvement over
Co-Cr-Mo/UHMWPE sliding pairs. Another pin-on-disk study on the tribological per-
formance of UHMWPE against untreated, a-C coated by unbalanced magnetron sputter-
ing of graphite, nitrogen-implanted, thermal oxygen-treated, and oxygen diffusion-treated
Ti
6
Al
4
V alloy in distilled water environment was carried out [176]. There is improvement in
tribological behavior with all the surface engineering techniques, but the thermal oxygen-
treated Ti
6
Al
4
V alloy showed significantly less wear than the a-C coating.
Even though some studies reveal amorphous carbon film as a promising material to
reduce the wear of UHMWPE, the effect of the amorphous carbon coating on artificial
joints is still in debate. These conflicting findings may be the result of several issues. The
