Biomedical Engineering Reference
In-Depth Information
Ca 10 (PO 4 ) 6 (OH) 2 → 3Ca 3 (PO 4 ) 2 + CaO + H 2 O↑
(4.14)
(Liu et al. 1994; Gross et al. 1998a).
Due to decomposition, HA coatings can be accompanied by phases such as oxyapatite,
tetracalcium phosphate, tricalcium phosphate, calcium oxide, and so forth. The dehydrox-
ylated HA (OHA), TTCP, and TCP obtained from high-temperature processing undergo
dissolution and degradation more rapidly than HA in an aqueous environment, which
decreases chemical stability and enhances degradation of the implants in vivo (Lin et al.
2000). Like other amorphous phases, the ACP in the as-sprayed coating is thermodynami-
cally metastable. It enhances adhesion to the substrate but dissolves quickly in body fluids,
and thus adversely affects bone formation (Guipont et al. 2002). Since HA coatings with
lower crystallinity would result in an increased dissolution, a high crystallinity level is
desirable in order for the materials to have good bioactive properties (Feng et al. 2000) and/
or good biointegration by minimizing the soluble phases. Furthermore, the existence of too
much amorphous phases at the interface between the HA deposit and metal alloy implant
could be detrimental to the long-term survivability of the implant based on that after some
time of implantation, the interface may directly contact with bony tissue (Khor and Cheang
1994a). An appropriate thermal treatment could induce crystallization to occur. That is
why the as-sprayed coatings are usually subjected to a postheat-treating cycle. Therefore,
it is important to control the relative content of the different phases in as-sprayed coatings.
Generally, the appearance of CaO in as-sprayed coatings suggests the loss of P 2 O 5 from the
starting powders during spraying. Loss of water gives a hydroxyapatite-oxyapatite solid
solution in which chains of OH are replaced by chains of OH , O 2− , and vacancies, and the
range of composition in equilibrium is given by Ca 10 (PO 4 ) 6 (OH) 2−2 x O x x , where x > 0.75,
⊗ = vacancy (McPherson et al. 1995; Antolottin et al. 1998).
Besides decomposition, another problem pertinent to the thermal-sprayed HA coating is
the formation of an amorphous phase (Reis et al. 1996; Gross et al. 1998b; Yang et al. 1995),
along with the generation of other nonbioactive calcium phosphate phases. Generally, the
reason for the formation of amorphous phase in HA coating was believed to be related
to the entire melting of the powders and subsequent rapid solidification (Cao et al. 1996).
Radin et al. (1992) found that the formation of the amorphous phase was apparently associ-
ated with partial dehydroxylation of HA during plasma spray process.
The presence of ACP phase is theoretically undesirable because natural HA in bone is
crystalline (Cheang et al. 1996a). Furthermore, a highly crystalline coating is more stable
over time in vivo, while a less crystalline coating is subject to slow degradation in the long
term but facilitates its substitution by newly formed bone (Tranquilli et al. 1994) because
the dissolvability of ACP is far better than that of crystalline one (Ducheyne et al. 1993).
Therefore, controlling the amount and location of ACP is very important for proper func-
tion of the coated appliance. However, Gross et al. (1988a) stated that ACP was good for
absorbing the mechanical mismatch, improve fatigue behavior, and promote fast bone
remodeling and attachment. Thus it seemed that HA coatings with limited amorphous
phase are preferred in clinical applications (Bioceramics Workshop 1999).
The prevention of phase transformation to ACP could be achieved through the mini-
mal removal of hydroxide and phosphate from HA during processing (Gross et al. 1998a).
Especially during plasma spraying, the extreme heating and cooling conditions can pro-
duce metastable phases as a result of rapid cooling from high temperatures. Gross and
Berndt proposed a visualized model (Gross et al. 1998b), shown in Figure 4.32, to illustrate
the equilibrium phase diagram of HA materials during plasma spraying. It is found that
phase transformations are produced by (1) preferential removal of hydroxyl and phosphate
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