Biomedical Engineering Reference
In-Depth Information
measured by bending tests, of more than 550MPa and a Vickers hardness of
more than 1800-2000HV. These hardness values are much higher than
those offered by metals such as CoCr or titanium alloys used in
orthopaedics, with hardness values bellow 500HV. Currently, the most
widely used ceramic in total hip arthroplasty is alumina. It is estimated that
by the year 2005 more than 5million Al
2
O
3
femoral heads had already been
implanted worldwide. But, together with its many advantages, alumina
presents some drawbacks. In addition to its low fracture toughness
(
4MPam
1/2
), it is very sensitive to the surface finish and the sphericity
of the bearing surfaces, to the tolerance between them and to the orientation
of the components. It is also sensitive to fracture due to fatigue if the
assemblage of the metal components is irregular or if there is not perfect
adaptation of the dimensions.
Due to these drawbacks, zirconium oxide or zirconia (ZrO
2
) was
introduced in the mid 1980s. Clarke et al. (2003) estimate that from 1985
to 1995 around 104,000 zirconia femoral heads were implanted. The 3mol%
yttria stabilized tetragonal zirconia, known as YTZP, offers the best values
of flexure resistance and fracture toughness among the bioceramics. It
presents resistance values in bending tests and fracture toughness almost
double those of alumina (Rahaman and Yao, 2007). It also shows an
excellent wear behaviour, better when combined with UHMWPE than that
of CoCr-UHMWPE couples, or even that of alumina (Cales, 2000;
Ferna´ ndez-Fairen et al., 2005). However, its use in implants has serious
limitations. The most important is its sensitivity to low-temperature
degradation or aging. The most dramatic case of aging was reported at
the beginning of 2002 for zirconia hip joints heads, when several hundreds of
implants failed within a short period (Chevalier, 2006). Aging is produced
through the slow surface transformation of the zirconia from its high-
temperature structure (tetragonal structure), obtained by the stabilization of
the ceramic with yttria, into the stable monoclinic phase in the presence of
water or water vapour. The phase transition entails a volume increase of the
grains, which causes microcracking and, usually,
~
failure and loss of
functionality of the implant.
Due to the modest failure resistance of alumina and the problem of YTZP
in terms of long-term reliability, there is a trend today to develop new
alternatives, especially for critical and/or new designs for which alumina and
YTZP do not satisfy all requirements.
Alumina-zirconia nanocomposites
There is a critical zirconia grain size below which no tetragonal
to
monoclinic phase transformation occurs. When the grain size is above 1
m,
the material behaves unstably and is susceptible to spontaneous tetragonal
μ
