Biomedical Engineering Reference
In-Depth Information
D'Antonio et al., 2003; Garino, 2000), the risk of catastrophic failure of Al
2
O
3
bearings in vivo, while rare, is still unacceptably high. Even a small risk of
unexpected brittle failure in vivo discourages the widespread adoption of
ceramic bearings, given the prospect of expensive litigation and complex repeat
surgery. The development of alternative ceramics and ceramic composites,
deposition of ceramic coatings on metals, and surface modification of metals to
produce ceramic surfaces, are the main approaches that have been investigated
to achieve the low wear of ceramic articulating surfaces with reduced risk of
catastrophic failure.
7.2
Ceramics for bearing applications
Bearings for total joint replacement must remain mechanically and chemically
stable in vivo for greater than 10 years, while enduring 2 million or more gait-
related cycles of loading each year. The loads vary from three times the body
weight (3 kN) for normal walking, to eight times the body weight (8 kN) for
jogging or stumbling (Bergmann et al., 1993). High elastic modulus is required
to resist deformation and to maintain shape, whereas high strength, high
resistance to fracture, and high resistance to mechanical fatigue are required to
minimize the risk of failure. For bioinertness and biocompatibility, the material
should have high corrosion resistance in the body fluids. High hardness and the
ability to be polished to a smooth surface finish are required for long-term wear
resistance and low friction. Good wetting (low contact angle) between the
bearing surface and the synovial fluids of the joint
is necessary for good
lubrication.
The strong bonding of the atoms by ionic and covalent bonds, and the
crystalline structure are responsible for the desirable mechanical properties (high
compressive strength, elastic modulus, and hardness), and chemical inertness in
vivo of ceramic bearing materials such as Al
2
O
3
, ZrO
2
, and Si
3
N
4
. The bio-
compatibility of these materials is related to the high chemical stability of these
materials, which confers resistance to corrosion and reliable in vivo behavior
over time. The surfaces of these ceramics have polar hydroxyl (±OH) groups that
promote interaction with the aqueous body fluids to provide a lubricating layer.
7.2.1
Physical and mechanical properties
Table 7.2 compares the several physical and mechanical properties of Al
2
O
3
,
ZrO
2
, and Si
3
N
4
ceramics with those for CoCr alloys and for bone. Modern
Al
2
O
3
bearings, fabricated by sintering and hot isostatic pressing, have a dense,
fine-grained microstructure, providing a flexural strength >400MPa, and an
elastic modulus of >450 GPa. Al
2
O
3
has a Vickers hardness number of more
than 14±16 GPa, compared with a value of less than 3±4 GPa for common
orthopedic metals such as CoCr and Ti alloys. On the other hand the low fracture
