Biomedical Engineering Reference
In-Depth Information
toughness of Al
2
O
3
(4MPa m
1/2
), compared with a value of 50±100MPa m
1/2
for
CoCr alloys, is a limitation.
ZrO
2
stabilized in the tetragonal phase with Y
2
O
3
, referred to as yttria-
stabilized tetragonal zirconia polycrystals, Y-TZP, has flexural strength and
fracture toughness values that are almost twice those for Al
2
O
3
. However, use of
Y-TZP in THA and TKA has severe limitations, due to degradation in a moist
atmosphere which leads to microcracking and loss of strength. Following an
unusually high rate of clinical failures which resulted from alteration of the
manufacturing variables, Y-TZP was withdrawn from clinical use (Masonis et
al., 2004). Si
3
N
4
bearings are currently undergoing clinical trials for applications
in THA. Control of the manufacturing variables can lead to the production of
Si
3
N
4
with a dense microstructure of elongated grains, referred to as `in situ
toughened' Si
3
N
4
, with a flexural strength as high as 1 GPa and a fracture
toughness of 10±12MPa m
1/2
.
7.2.2 Wear of ceramic bearings
Al
2
O
3
femoral heads articulating against UHMWPE reduce wear by at least a
factor of 2 when compared to the standard CoCr femoral head, and Al
2
O
3
±Al
2
O
3
bearing couples provide the least wear rate, with a value that is often more than
50±100 times lower than that of the standard CoCr±UHMWPE couple (Table
7.3) (Willmann et al., 1996). Si
3
N
4
±Si
3
N
4
and Si
3
N
4
±CoCr bearing couples
have shown wear rates that are comparable to those of Al
2
O
3
±Al
2
O
3
bearings
(Bal et al., 2008, 2009a; Clarke et al., 2000).
Since the 1970s, the standard bearing couple has consisted of a CoCr femoral
head articulating against a UHMWPE liner. In some countries, the femoral head
consists of stainless steel. The CoCr±UHMWPE couple has the advantage of
low cost. However, the wear rate is relatively high and depends on the activity of
the patient and on the diameter of the femoral head. The lifetime is typically not
more than 10 years, so this implant is often recommended for elderly or inactive
patients. Because of their low wear rates and low prevalence of osteolysis,
Table 7.3 Volumetric wear at 1010
6
cycles for bearing couples in
identical hip simulators and testing protocols
Volumetric wear (mm
3
)
Bearing couple
CoCr±UHMWPE
62
Al
2
O
3
±UHMWPE
<30
CoCr±CoCr
6.5
Al
2
O
3
±Al
2
O
3
0.35±0.6
Si
3
N
4
±Si
3
N
4
0.65
Si
3
N
4
±CoCr
0.47
FromBal et al. (2008, 2009a); Clarke et al. (2000).
