Biomedical Engineering Reference
In-Depth Information
Suchanek et al. 1997), there are still some limitations. During the implant's service, the
through-thickness direction of HA coating is important for the apposition of bony tissue
and the contact zone should be of sufficient wear-resistance property that appears to be
necessary for the implant (Gualtieri et al. 1987). Also the incorporation of the bond coat
brings about complex operation and extra expense. Even if the functionally graded coating
(FGC) that comes from the extension of bond coat can fulfill the biological requirements,
there still exist problems of cost and production complexity. Unique incorporation of rein-
forcements could be more effective.
The tribological property of some bioinert ceramics used as prosthesis has been experi-
mentally investigated (Murakami et al. 1996; Fruh et al. 1997; Patel et al. 1997). Many materi-
als selected to strengthen sintered HA and optimum selections were determined according
to phase composition, biocompatibility and phase transformation (Suchanek et al. 1997). It
was reported that the incorporation of some glass into HA exhibited a major effect on HA
structure (Lopes et al. 1998). Some useful results were also obtained on the reliability of
alumina implantation in vivo (Gualtieri et al. 1987; Toni et al. 1987; Trentani et al. 1987).
For pure titanium alloy implant, it was usually found that corrosion was a considerable
factor in influencing the duration of the implant in bony tissue. Therefore, in many cases,
surface oxidization of the Ti alloy was utilized to promote corrosion resistance (Lausmaa
et al. 1986; Lausmaa et al. 1989). Many methods such as thermal oxidization have been
employed to produce the oxide layer on the metallic surface. It was found that biocompat-
ibility of the eventual implant seemed to associate with the highly stable protecting surface
oxide (Lausmaa et al. 1986). However, all these processes were limited by the thin layer
thickness and as a result, the implant may lose its surface protective layer in the long term.
Studies showed that titania ceramics were potentially useful as porous cell carrier mate-
rial whose properties, such as good permeability, serve to enhance cell vitality (Blum et al.
1996). The potential advantage offered by a porous ceramic implant is its inertness com-
bined with the mechanical stability of the highly convoluted interface developed when
bone grows into the pores of the ceramic. In porous materials, the exchange area is high
(depending on porosity), which requires a high chemical resistance in biological media. This
condition can be met by titania. Study on sintered HA/titania composites revealed effect of
sintering temperature on the phases of the composites (Figure 4.27) (Aoki 1994). The effect
of different titania materials on cell growth and distribution has also been reported (Blum
et al. 1996; Kanagaraja et al. 2001). Results indicate that the porosity of the ceramics should
be optimized to levels where release of wear particles does not occur during service. The
porous structure needs very coarse powder with a narrow size distribution, coarse grain
HAp
α-TCP
Perovskite
Rutile
Anatase
800
1000
1200
1400
Phase composition (wt.%)
FIGURE 4.27
Influence of TiO
2
on the phase composition of sintered HA. (From Aoki, H., Ishiyaku EuroAmerica, Inc., Tokyo,
St. Louis, 1994. With permission.)
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