Biomedical Engineering Reference
In-Depth Information
TABLE 2.5
Physical Property Requirements of Alumina
and Partially Stabilized Zirconia
Properties
Alumina
Zirconia
Elastic modulus (GPa)
380
190
Flexural strength (GPa)
1.0
>0.4
Hardness, Mohs
9
6.5
Density (g/cm
3
)
3.8-3.9
5.95
Grain size (μm)
4.0
0.6
Source:
Adapted from Park J.B. 1993. Personal communication.
Note:
Both the ceramics contain 3 mol% Y
2
O
3
.
candidate for use in joint replacements (Park and Lakes, 1992). Aluminum oxide implants in bones of
rhesus monkeys have shown no signs of rejection or toxicity for 350 days (Hentrich et al., 1971; Graves
et al., 1972). One of the most popular uses for aluminum oxide is in total hip protheses. Aluminum oxide
hip protheses with an ultra-high-molecular-weight polyethylene (UHMWPE) socket have been claimed
to be a better device than a metal prostheses with a UHMWPE socket (Oonishi, 1992). However, the
key for success of any implant, besides the correct surgical implantation, is the highest possible quality
control during fabrication of the material and the production of the implant (Hench, 1991).
2.2.3 Zirconia (ZrO
2
)
Pure zirconia can be obtained from chemical conversion of zircon (ZrSiO
4
), which is an abundant
mineral deposit (Park and Lakes, 1992). Zirconia has a high melting temperature (
T
m
= 2953 K) and
chemical stability with
a
= 5.145 Å,
b
= 0.521 Å,
c
= 5.311 Å, and β = 99°14 (Park and Lakes, 1992). It
undergoes a large volume change during phase changes at high temperature in pure form; therefore, a
dopant oxide such as Y
2
O
3
is used to stabilize the high-temperature (cubic) phase. We have used 6 mol%
Y
2
O
3
as dopant to make zirconia for implantation in bone (Hentrich et al., 1971). Zirconia produced in
this manner is referred to as
partially stabilized
zirconia (Drennan and Steele, 1991). However, the physi-
cal properties of zirconia are somewhat inferior to that of alumina (Table 2.5).
High-density zirconia oxide showed excellent compatibility with autogenous rhesus monkey bone and
was completely nonreactive to the body environment for the duration of the 350-day study (Hentrich
et al., 1971). Zirconia has shown excellent biocompatibility and good wear and friction when combined
with UHMWPE (Kumar et al., 1989; Murakami and Ohtsuki, 1989).
2.2.4 Carbons
Carbons can be made in many allotropic forms: crystalline diamond, graphite, noncrystalline glassy
carbon, and quasicrystalline pyrolitic carbon. Among these, only pyrolitic carbon is widely utilized
for implant fabrication; it is normally used as a surface coating. It is also possible to coat surfaces with
diamond. Although the techniques of coating with diamond have the potential to revolutionize medical
device manufacturing, it is not yet commercially available (Park and Lakes, 1992).
The crystalline structure of carbon, as used in implants, is similar to the graphite structure shown in
Figure 2.1. The planar hexagonal arrays are formed by strong covalent bonds in which one of the valence
electrons or atoms is free to move, resulting in high but anisotropic electric conductivity. Since the
bonding between the layers is stronger than the van der Waals force, it has been suggested that the layers
are
cross-linked
. However, the remarkable lubricating property of graphite cannot be attained unless the
cross-links are eliminated (Park and Lakes, 1992).
The poorly crystalline carbons are thought to contain unassociated or unoriented carbon atoms. The
hexagonal layers are not perfectly arranged, as shown in Figure 2.2. Properties of individual crystallites
