Biomedical Engineering Reference
In-Depth Information
Even though it was found that a bioinert ceramic such as pure alumina prosthesis (Heimann
et al. 1998) could not reach an intimate contact with bone owing to its bioinert surface, it
is capable of impeding possible diffusion of metal alloy implant to the tissue. Due to the
attractive properties of the bioinert ceramics, the clinical-purposed application of these
materials has attracted many investigations (Murakami et al. 1996; Patel et al. 1995; Toni
et al. 1987; Trentani et al. 1987; Drouin et al. 1997; Labat et al. 1995). They have been widely
employed as the skeleton component for maintaining external and internal load in a com-
posite implant.
Many types of bioinert ceramics have been explored as implant materials. Alumina was
first used as an implant material in 1964 (Heimke et al. 1987), and some of its modifica-
tions were studied experimentally in the late 1960s. Since then, more and more ceramics
were attempted for clinical applications. Partially stabilized zirconia has high mechani-
cal strength, low radioactivity, low crystal phase transformation, and high toughness.
However, phase transformation among its multiphases—cubic, monoclinic, and tetrag-
onal—can cause the degradation of mechanical strength, thus decreasing its reliability
as an implant. Many factors could influence the transformation, even though the tensile
stress does not affect it (Fujisawa et al. 1996).
Generally, bioceramics are used as bond coat (Kurzweg et al. 1998; Lamy et al. 1996) or
additives to form a composite structure with HA. Bond coat was capable of effectively
improving the adhesion properties of HA coatings (Cheang et al. 1996b; Heimann et al.
1998; Tsui et al. 1998). An increase in an order of 20%, up to 15.49 MPa with a bond coat of
Ca 2 SiO 4 has been reported (Lamy et al. 1996). The adhesion was even further increased by
the introduction of Ti bond coat with a thickness of 100 μm (Tsui et al. 1998). In addition
to its contribution to the adhesion improvement, which is mostly considered in clinical
applications of the ceramic coatings, bond coat could also (Hench 1987; Kurzweg et al. 1998;
Lamy et al. 1996):
1. Prevent direct contact between Ti and HA since this is believed to catalyze ther-
mal transformation of HA toward tri- or tetracalcium phosphate or even nonbio-
tolerant CaO
2. Reduce the release of metal ions from the substrate to surrounding living tissue
that has been shown to induce massive hepatic degeneration in mice and impaired
development of human osteoblasts
3. Reduce the thermal gradient at the substrate/coating interface caused by the rapid
quenching of molten particle splats that leads to deposition of ACP with a concur-
rent decrease in resorption resistance and hence to reduced in vivo performance
4. Prevent a steep gradient in the coefficients of thermal expansion between sub-
strate and coating that promotes the formation of strong tensile forces in the coat-
ing giving rise to crack generation, chipping, and/or delamination
5. Cushion damage by cracking and delamination of the coating initiated by cyclic
micromotions of the implant during movement of patient in the initial phase of
the healing process
Therefore, it is highly desirable to engineer the substrate/HA coating interface in such a
way that by application of a suitable thin biocompatible bond coat layer the advantages
addressed above can be realized (Oonishi et al. 1987).
Accompanying with the positive effect brought about by the bond coat, such as the
improvement of peeling strength by 20% to 100% (Heimann et al. 1998; Lamy et al. 1996;
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