Biomedical Engineering Reference
In-Depth Information
coefficient (0.044-0.115) and wear rate (1-5 mm
3
/year
in vivo
) that
make alumina a good candidate for joint replacement materials despite
its brittleness. Nevertheless, its high elastic modulus may be
responsible for bone atrophy and implant loosening in old patients.
Finally, another limitation of alumina-based biomaterials is its high
manufacturing cost [LI 98].
The main advantage of zirconia (ZrO
2
) is its lower elastic modulus
(Table 3.3). Moreover, zirconia can exist in different phases
(monoclinic, tetragonal and cubic) which can be tailored to enhance
material toughness and strength. Pure zirconia is monoclinic at room
temperature, but the addition of other oxides, such as CaO, MgO and
Y
2
O
3
, allows the generation of multiphasic materials. Furthermore, by
stabilizing zirconia in cubic or tetragonal phases, the additives prevent
from the material volume change and cracking that occurs during
cooling down and phase transformation. Zirconia-based medical
devices are divided into two categories: partially stabilized zirconia
(PSZ) and tetragonal zirconia polycrystals (TZPs). PSZ is constituted
of a cubic major phase and precipitates of monoclinic and tetragonal
phases at grain boundaries or within the cubic matrix grains. Y-TZP
materials contain 2-3% Y
2
O
3
and are completely constituted of
tetragonal grains in the order 0.4-0.8 µm. Both PSZ and Y-TZP have
been used as medical implants, but Y-TPZ materials are the most
selected into the market ball heads. Indeed, the flexural strength
(950 MPa) and the fracture toughness (10.5 MPa.m
1/2
) of Y-TPZ are
almost the double of alumina-based ceramics, leading to a decrease in
sensitiveness to stress concentration at contact points [PIC 99].
Moreover, in comparison to alumina, Y-TPZ shows lower friction
coefficient (0.028-0.082), finer grain size and better-controlled
microstructure without any residual porosity. As a consequence,
Y-TPZs are good candidates to replace alumina in orthopedic
applications or dental crowns. However, the main drawback of
zirconia is that it may be weakened due to phase transformations that
may arise under loading leading to surface degradation, or that occur
through Zr-OH bonding when subjected to aqueous environment
[PAR 07] Chapter 9.
