Biomedical Engineering Reference
In-Depth Information
grain sizes, or better still, by using ceramic nanocomposites (Bal et al., 2009b;
Rahaman et al., 2007). In the present context, nanoscale ceramics are defined as
ceramics with grain sizes smaller than 50±100 nm. Ceramic nanocomposites are
defined as ceramic composites with more than one solid phase, in which at least
one of the phases has dimensions in the nanoscale range (<50±100 nm).
Because of the widespread use of Al
2
O
3
in orthopedic and engineering
applications, Al
2
O
3
-based nanocomposites have been the subject of considerable
research and development. Al
2
O
3
nanocomposites for potential bearing applica-
tions are typically dense two-phase materials, consisting of nanosize particles of
a hard ceramic phase (such as SiC or ZrO
2
), often 5±10 vol%, dispersed in a
fine-grained Al
2
O
3
matrix. While improvements in strength and fracture tough-
ness over the unreinforced ceramic (without nanoparticles) can be modest, the
improvement in the wear resistance can often be significant. For an equivalent
Al
2
O
3
grain size (2±3 m), the wear rate of Al
2
O
3
/SiC nanocomposites (5 vol%
SiC) was lower than that of Al
2
O
3
by a factor of 3±5 at high contact loads
(RodrÂguez et al., 1999). ZTA nanocomposites containing up to 10 vol% ZrO
2
nanoparticles were found to have a substantially higher fracture toughness
(10MPa m
1/2
) than Al
2
O
3
(4MPa m
1/2
), but the their wear resistance was not
significantly different from that for Al
2
O
3
(Taddei et al., 2006).
ZrO
2
/Al
2
O
3
nanocomposites, consisting of a ZrO
2
matrix stabilized with
10 mol% CeO
2
, and doped with 0.05 mol% TiO
2
, referred to as Ce-TZP, and
30 vol% of Al
2
O
3
as a second phase were reported to have a fracture toughness
that is 4±5 times that of Al
2
O
3
, and a flexural strength that is ~2 times the value
for Al
2
O
3
(Nawa et al., 1998; Tanaka et al., 2002).
7.6
Future trends
Promising new technologies based on non-oxide ceramics, composites of
existing ceramics, bearing surface modifications, and ceramic nanocomposites
can provide improved bioceramics for use in total hip and knee joint replace-
ment, and contribute to the longevity of these commonly performed surgical
procedures. As the number of prosthetic hip and knee joint surgery performed in
the US and overseas keeps increasing, the result will be a significant reduction in
healthcare costs, as well as a reduction in pain and suffering to patients, owing to
a reduction in the number of revision surgery.
Superior wear properties and reliability of ceramic bearing materials will also
contribute to newer designs of total hip and knee replacement implants that are
bone-conserving, allow greater function, and can be implanted with less invasive
surgical techniques. Improved wear resistance has already allowed the use of large
diameter femoral heads in total hip replacement, leading to increased arc of
movement and less risk of prosthesis dislocation. Newer designs will enable
ceramic bearings to be implanted into a wider range of patients, and to be used in a
wider range of repeat surgery resulting from failed metal bearings. Previous
