Biomedical Engineering Reference
In-Depth Information
LeGeros and Daculsi, 1990; LeGeros and LeGeros, 2008; LeGeros et al., 2008; Osborne and
Newesely, 1980).
“Bioactivity,” the property that allows the material to directly bond with bone, was first
observed and described by Hench et al. (1977) with silica-based bioactive glasses. In vitro,
bioactivity is demonstrated by the formation of carbonate apatite on the surfaces of materi-
als after immersion in serum (LeGeros et al. 1991, 1992) or in simulated body fluid (Kokubo
et al., 1990) as shown in Figure 7.6. In vitro
cell culture studies showed greater cell prolifer-
ation and differentiation (gene expression of markers for bone formation) of bone-forming
cells on surfaces coated with apatite (Boyan et al., 1998; Brunette, 1988; Cooper et al., 1999;
deOliviera and Nanci, 2004; Masaki et al., 2005; DaSilva et al., 2003). In vivo, bioactivity is
shown by the formation of the nanocrystals of carbonate apatite associated with the bioac-
tive material (Heughebaert et al., 1988; LeGeros and Daculsi, 1990; LeGeros et al
.,
1991) as
shown in Figure 7.6a.
The bone mineral, idealized as an HA, is a carbonate-substituted apatite or carbonate
apatite (LeGeros, 1981). The rationale for using calcium phosphates (especially HA,
β
-TCP,
or biphasic calcium phosphate, BCP—an intimate mixture of HA and
β
-TCP) as bone sub-
stitute material or as coating on implant is the similarity in SEM image showing a gap
(occupied by fibrous tissue) between Ti alloy implant surface and host bone indicating
absence of true osseointegration (Daculsi et al., 1995) property to bone in terms of com-
position (mainly calcium and phosphate ions) and in osteoconductive property (which
serves as a template for forming new bone). The release of calcium and phosphate ions and
subsequent formation of nanoapatite crystals, similar to the nanocarbonate apatite crys-
tals of bone (Figure 7.6a), may be a critical step in bone-bonding of the bioactive ceramic
or bioactive coating (LeGeros, 2008). The carbonate apatite layer that forms on the cal-
cium phosphate coating after implantation facilitates the adhesion of proteins on which
the osteoprogenitor cells can attach, proliferate, differentiate, and produce extracellular
matrix that eventually leads to biomineralization or bone formation (LeGeros, 2008).
Thus, the rationale for depositing calcium phosphate coatings on implants is to provide
a bioactive surface that will ensure direct bonding with bone. To improve the adhesion
of the coating with the metal substrate, the implant surfaces are first roughened either
by acid-etching, grit-blasting, or deposition of a layer of cpTi or TiO
2
powder by plasma-
spraying or arc deposition.
Plasma-Sprayed HA Coatings
Although by themselves, calcium phosphate ceramics or bioactive glasses have very desir-
able properties, they are not strong enough to be used in load-bearing areas (deGroot, 1987;
HA
Ti Alloy
3005 15 kV X1, 500 10 µm WD25
(b)
(a)
FIGURE 7.7
(a) Representation of the plasma-spray technique for depositing coating on implant. HA beads (inset) are par-
tially melted and partially transformed to different calcium phosphate phases, principally, ACP (From LeGeros
et al.,
Ceram. Trans
., 48, 173-189, 1995. With permission.) (b) SEM image of the plasma-sprayed HA surface.
