Biomedical Engineering Reference
In-Depth Information
Figure 10.1.
Components for total hip replacement and current material combinations for
total hip replacement [CeramTec, 2002].
produced by the wearing action, the ceramic-ceramic coupling should be pre-
ferred for young, heavy and active patients.
10.3 CERAMICS FOR TOTAL HIP JOINT REPLACEMENT
10.3.1 Alumina Implants
The fi rst ceramic ball-heads were made of Al
2
O
3
, which today is still the most
commonly-used ceramic. Alumina has a very good corrosion resistance, which is
most important in biomedical applications [Black, 1992]. The hardness is very
high, resulting in a very low wear rate. In fact, when testing the current generation
of pure alumina ball heads and acetabular cups in a hip joint simulator, it becomes
very hard to characterize the wear either from the loss of any material from the
mating surfaces or by collecting the wear debris. The elastic modulus is 396 GPa,
providing rigid and non-deformable implants [Willmann, 1996]. To obtain high
strength Al
2
O
3
(
450 MPa), high purity alumina is used with a very low concentra-
tion of sintering additives (
>
0.5 wt %) because the residual glass phase on the
grain boundaries can degrade and mechanical strength will be lost. A reduction in
grain size improves drastically the strength and therefore very small grain size
and a narrow grain size distribution are essential. The average grain size of current
medical grade alumina is 1.4
<
μ
m. To limit grain growth, Al
2
O
3
is doped with MgO
(
0.5 wt %). Despite these favorable properties, alumina is brittle [Kingery, 1976],
which is a disadvantage compared to zirconia.
<
10.3.2 Zirconia Implants
The fi rst ball-head made out of tetragonal zirconia polycrystalline (TZP) material
was introduced by Christel et al. [Christel et al., 1988]. Until recently, about 25%
of the total number of hip implant operations per year in Europe were performed
with TZP [Cordingley et al., 2003]. Whereas the fl exural strength of medical grade
Al
2
O
3
is in the range of 500-580 MPa, that of ZrO
2
is two to three times higher
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