Biomedical Engineering Reference
In-Depth Information
Jarcho, 1981). The rationale for depositing calcium phosphate coatings on metal implants is
to combine the strength of the metal and the bioactivity of the calcium phosphate coating.
The plasma-spray technique (Figure 7.7) is the most commonly used method for depositing
calcium phosphate coating on commercial orthopedic and dental implants that have been
either grit-blasted or acid-etched before the deposition of coating (deGroot, 1987; Jarcho, 1992;
Lacefield, 1988; Serekian, 1993). Dense HA spherical beads or granules are fed into an electric
arc-plasma gas atmosphere. The high temperature (10,000 to 30,000°C) causes the surface of
the HA beads or particulates to partially melt and transform to other forms of calcium phos-
phates, principally a noncrystalline phase, amorphous calcium phosphate (ACP) and small
amounts of other crystalline calcium phosphate phases (
β
-TCP,
α
-TCP; tetracalcium phos-
phate, TTCP), and sometimes also calcium oxide, CaO, as shown by the reaction below:
Ca PO OH
(
) (
)
→
Ca PO OH
(
) (
)
+
Ca
x
PO
(
)
y
+
Ca PO
(
)
+
Ca P O
10
4 6
2
10
4 6
2
4
3
4 2
4 2
9
HA source
HA
β
-TCP,
α
-TCP
TTCP
ACP
X-ray diffraction analyses of plasma-sprayed HA coatings on orthopedic and dental
implants show that the composition of the coating is mainly HA and ACP with vary-
ing HA/ACP ratios (LeGeros et al., 1994, 1995, 1998). Variation in the HA/ACP ratios in
the coating depends on the extent of melting of the HA, which in turn depends on sev-
eral operating parameters including distance of the substrate (implant) from the plasma,
position of entry (within or outside the nozzle) of the HA particles in the plasma stream,
geometry of the substrate, and/or the type of gasses used as the carrier gas (Lacefield, 1988;
LeGeros et al., 1998). The plasma-sprayed HA coating is inhomogeneous in composition,
(d)
(a)
(e)
(b)
b
(c)
(f )
25 27 29 31 33 35° 2
θ
Diffraction angle
22
26
30
34
38
FIGURE 7.8
X-ray diffraction profiles: (a) HA used as the source material for plasma-spraying; (b) outermost layer of the
plasma-sprayed coating; (c) coating layer closest to the Ti alloy substrate. In (b) and (c), ACP (area under dotted
line) denotes the amorphous phase, ACP. Note that the HA/ACP ratio is higher in the outermost layer. Panels
(d), (e), and (f) are x-ray diffraction profiles of plasma-sprayed HA coatings on different commercial dental
implants showing variability of the HA and ACP components in the different coatings. (From LeGeros et al.,
Ceram. Trans
., 48, 173-189, 1995. With permission.)
