Biomedical Engineering Reference
In-Depth Information
growth factors induce bone formation. Ripamonti [43] and Kuboki [44] indepen-
dently postulated that the geometry of the material is a critical parameter in bone
induction. Others have speculated that low oxygen tension in the central region
of implants might provoke a dedifferentiation of pericytes from blood microves-
sels into osteoblasts [45]. It has been also postulated that the nanostructured
rough surface or the surface charge of implants might cause the asymmetrical
division of stem cells into osteoblasts [46].
Surface microstructure appears to be a common property of the materials
that induce ectopic bone formation. Recent studies have indicated the critical
role played by micropores on ceramic-induced osteogenesis. For example, it has
been reported that bone formation occurred in dog muscle inside porous calcium
phosphate ceramics with surface microporosity but bone was not observed inside
the dense surface of macroporous ceramics [47]. It has also been reported that
metal implants coated with a microporous layer of octacalcium phosphate could
induce ectopic bone in goat muscle, while a smooth layer of carbonated apatite on
these porous metal implants was not able to induce bone formation [48]. In all the
previous experiments, ectopic bone formation occurred inside the macroporous
ceramic blocks.
It has been demonstrated that sintering temperature has a drastic effect on
the microporosity of calcium phosphate ceramics: the higher the sintering tem-
perature, the denser the ceramic surface. To evaluate microporosity, how it is pro-
duced and the role it plays, the authors precipitated calcium-defi cient apatite
(CDA) prepared to provide BCP with an HA/
- TCP ratio of 60/40 + 2, then sin-
tered between 1000 ° C and 1200 ° C; discs (25 mm diameter) were prepared for an
in
vitro
test and machined into implants (3 mm diameter) for
in
vivo
implantation.
The discs were distributed into fi ve groups: D1, D2, D3, D4 and D5, and sintered
using various conditions (heating rate and temperature) (Figure 4.2) (Table 4.2).
All groups were subjected to the same rate of temperature rise (5 ° C/min), cool-
ing rate (1 °C/min) and total sintering period (5 h). The discs in groups D2, D3 and
D4 were heated to 900 °C then allowed to remain at this temperature for 3 h (D2)
or 12 h (D3 and D4). The fi nal temperature was 1050 °C for groups D1, D2 and
D3, and 1200 °C for groups D4 and D5. XRD and FTIR analyses of the sintered
discs showed only the BCP phase with an HA/
β
-TCP ratio of 60/40. Surface area,
microporosity percent, cell coverage are summarized in Table 4.3.
β
TABLE
4.3.
SSA m
2
/g
±
0.01
% micropores
% cell coverage
dissolution ppm Ca, 60 min
D1
3.5
80
35 ± 1.2
7.8
D2
3.2
60
35 ± 1.0
7.9
D3
3.1
50
28 ± 1.5
6.9
D4
0.3
10
20 ± 2.0
3.8
D5
0.8
10
16 ± 2.0
3.1
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