Ceramics and composites - Orthopaedic Biomaterials in Research and Practice - page 201

polycrystalline materials, with small, controlled-size crystallites, are the
strongest and toughest, but defect-free single crystals may be stronger, if
they lack notch sensitivity.
The following terms are used to describe material response (to
implantation):

Nonresorbable (or “bioinert”): Essentially unaffected by chemical
effects of implantation.
Reactive (or “bioactive”): React with the local host environment to
produce altered surface properties, which may elicit a desired host
response.
Resorbable (or “bioresorbable”): Dissolve with time in vivo. For rea-
sons discussed in more detail later (see Chapter 12), the compound
“bio-” form terms are unsatisfactory and should be avoided in
common usage; none of these material responses are apparently
cell mediated.

Inert ceramics Ceramics currently used in orthopaedic applications include alu-

mina (Al 2 O 3 ), zirconia (ZrO 2 ), and hydroxyapatite Ca 10 (PO 4 ) 6 (OH) 2 .
Hydroxyapatite is a reactive ceramic and will be discussed in the next
section. Alumina and zirconia, on the other hand, are both bioinert and
are continuing to find use as articulating components in hip arthroplasty
for select patient profiles. Both of these ceramics have enormous poten-
tial to reduce wear rates relative to metallic and polymeric articulating
surfaces, though they also carry risks of sudden fracture that are the
focus of continual improvement in the materials and design. Intolerable
squeaking is also a risk with ceramic implants, which has been attrib-
uted to edge loading, potentially from malpositioning of the implant and
other causes, such as rim design.
Alumina . High-density alumina (Al 2 O 3 ) is a fine-grained polycrys-
talline ceramic that has found use for load-bearing prosthesis for large
joint arthroplasty over the last 30 years. The material, standardized in
1984 (ISO 6474), is produced by sintering alumina powder at tempera-
tures between 1600°C and 1800°C. A small amount (<0.5%) of magnesia
(MgO) is added to limit grain growth during sintering. The advantages
to alumina include its good biocompatibility, high hardness, and low
coefficient of friction, leading to exceptionally low wear rates in arthro-
plasty devices. The low coefficient of friction for alumina and other
ceramics is attributed to its hydrophilic surface, high wettability, and
fluid-film lubrication capability. Linear wear rates of current-generation
alumina-on-alumina total hip arthroplasty (THA) components are 4000
times lower than traditional metal-on-polyethylene components.
The use of alumina ceramics in orthopaedics is extensive, particu-
larly in total hip replacements. This has been traced back to 1970 when
the first all-ceramic total hip replacement prostheses were introduced
in France. In the early 1980s, the US Food and Drug Administration
approved the first ceramic hip (Mittelmeier); however, by the mid-1980s,
it was no longer accepted clinically in the United States. This was attrib-
uted primarily to the high mechanical/fixation failure rates. Then, in the