Biomedical Engineering Reference
In-Depth Information
properties are enhanced, but biocompatibility issues are still contro-
versial (Streicher et al., 2007; Garmendia et al., 2008, 2009, 2010, 2011).
Ceramic in polymer composites: especially relevant for tissue engineer-
ing applications.
.
16.3 Orthopaedic and dental implants
Ceramics such as zirconia (ZrO
2
) and alumina (Al
2
O
3
) appear to be ideally
suited for the fabrication of orthopaedic implants because of their hardness,
low wear rates and excellent biocompatibility. For dental applications,
where aesthetic requirements (colour, translucency) are also essential, the
use of zirconia is preferred.
16.3.1 Bearing materials for orthopaedic implants
Alumina and zirconia ceramic materials have been used as joint substitutes
for over 30 years as an alternative to CoCr-UHMWPE bearing pairs. These
ceramics have crystal structures where atoms are joined by the combination
of strong ionic and covalent bonds. Due to the existence of these bonds they
show exceptional mechanical properties (high compressive strength, elastic
modulus and hardness) and they are chemically inert in vivo. Their
biocompatibility is also related to their high chemical stability, which
confers resistance to corrosion and reliability in the in vivo behaviour during
the lifetime of the implant (Rahaman and Yao, 2007). Also, the surfaces of
these oxides present polar hydroxyl groups (-OH), which promote
interaction with aqueous body fluids, providing a lubricating layer. In
addition, ceramics can be polished to tight tolerances and, due to the
hardness of the material, they are not affected by the wear particles than can
be generated due to the wear of bone cement, for example (Skinner, 2006).
The drawback of alumina and zirconia ceramics is their intrinsic
brittleness, which can lead to catastrophic failure in vivo and limits their
use in orthopaedic applications. However, the incidence of brittle failures
decreases with improvements in the quality of materials, manufacturing
techniques and implant design. Nevertheless, their low fracture toughness,
combined with their susceptibility to failure due to slow crack growth under
stresses below their fracture resistance, remains a problem regarding the
reliability of ceramic bearings (Rahaman and Yao, 2007).
The use of ceramics in joint prostheses began in the early 1970s. It was
observed that the low production of polyethylene wear debris in contact
with the ceramic solved the problem of loss of the prosthesis as a result of
osteolysis or bone loss. Boutin in France and Mittelmeier in Germany began
using aluminum oxide or alumina (Al
2
O
3
) as a constituent material of the
bearing surfaces of total hip prostheses. Alumina shows a resistance, as
