Biomedical Engineering Reference
In-Depth Information
Organization (ISO) requirements for alumina implants.
Extensive testing has shown that alumina implants that
meet or exceed ISO standards have excellent resistance to
dynamic and impact fatigue and also resist subcritical
crack growth (Drre and Dawihl, 1980). An increase in
average grain size to
>
17
m
m can decrease mechanical
properties by about 20%. High concentrations of sintering
aids must be avoided because they remain in the grain
boundaries and degrade fatigue resistance.
Methods exist for lifetime predictions and statistical
design of proof tests for load-bearing ceramics. Applica-
tions of these techniques show that load limits for spe-
cific prostheses can be set for an Al
2
O
3
device based upon
the flexural strength of the material and its use envi-
ronment (Ritter
et al.
, 1979). Load-bearing lifetimes of
30 years at 12,000-N loads have been predicted (Christel
et al.,
1988). Results from aging and fatigue studies show
that it is essential that Al
2
O
3
implants be produced at the
highest possible standards of quality assurance, especially
if they are to be used as orthopedic prostheses in younger
patients.
Alumina has been used in orthopedic surgery for
nearly 20 years (Miller
et al.
, 1996). Its use has been
motivated largely by two factors: its excellent type 1
biocompatibility and very thin capsule formation
(
Fig. 3.2.10-2
), which permits cementless fixation of
prostheses; and its exceptionally low coefficients of
friction and wear rates.
The superb tribiologic properties (friction and wear)
of alumina occur only when the grains are very small
(
<
4
m
m) and have a very narrow size distribution. These
conditions lead to very low surface roughness values
(Ra
<
40.02
m
m,
Table 3.2.10-4
). If large grains are
present, they can pull out and lead to very rapid wear of
bearing surfaces owing to local dry friction.
Alumina on load-bearing, wearing surfaces, such as in
hip prostheses, must have a very high degree of sphe-
ricity, which is produced by grinding and polishing the
two mating surfaces together. For example, the alumina
ball and socket in a hip prosthesis are polished together
and used as a pair. The long-term coefficient of friction of
an alumina-alumina joint decreases with time and ap-
proaches the values of a normal joint. This leads to wear
on alumina-articulating surfaces being nearly 10 times
lower than metal-PE surfaces (
Fig. 3.2.10-4
).
Low wear rates have led to widespread use in Europe
of alumina noncemented cups press-fitted into the ace-
tabulum of the hip. The cups are stabilized by the growth
of bone into grooves or around pegs. The mating femoral
ball surface is also made of alumina, which is bonded to
a metallic stem. Long-term results in general are good,
especially for younger patients. However, Christel
et al.
(1988) caution that stress shielding, owing to the high
elastic modulus of alumina, may be responsible for can-
cellous bone atrophy and loosening of the acetabular cup
in old patients with senile osteoporosis or rheumatoid
arthritis. Consequently, it is essential that the age of the
patient, nature of the disease of the joint, and bio-
mechanics of the repair be considered carefully before
any prosthesis is used, including alumina ceramics.
Zirconia (ZrO
2
) is also used as the articulating ball in
total hip prostheses. The potential advantages of zirconia
in load-bearing prostheses are its lower modulus of
elasticity and higher strength (Hench and Wilson, 1993).
There are insufficient data to determine whether these
properties will result in higher clinical success rates over
long times (
>
15 years).
Table 3.2.10-4 Physical characteristics of Al
2
O
3
bioceramics
High
alumina
ceramics
ISO
Standard
6474
Alumina content
(% by weight)
>99.8
99.50
Density (g/cm
3
)
>3.93
3.90
Average grain size (mm)
3-6
<7
Ra (mm)
a
0.02
Hardness (Vickers
hardness number, VHN)
2300
>2000
Compressive strength
(MPa)
4500
0.15
Friction
Wear
Bending strength (MPa)
(after testing in Ringer's
solution)
550
400
Metal-polyethyiene
0.10
10
AI
2
O
3
- AI
2
O
3
Young's modulus (GPa)
380
0.05
5
Natural joint
Fracture toughness
(K1C) (MPa
12
)
5-6
AI
2
O
3
- AI
2
O
3
0
10,00
0
0
10
100
1,000
Testing time (hr)
Slow crack growth
10-52
Fig. 3.2.10-4 Time dependence of coefficient of friction and
wear of alumina-alumina versus metal-PE hip joint (in vitro
testing).
a
Surface roughness value.
