Biomedical Engineering Reference
In-Depth Information
has a higher extent of dissolution than BCP1 [33]. In some cases, BCP ceramic
with similar HA/
-TCP ratios could present different dissolution rates [34]. This
phenomenon may be caused by processing variables (sintering time and tempera-
ture) that may affect total macroporosity and microporosity: the greater the
macroporosity and microporosity, the greater the extent of dissolution.
In
vivo
,
dissolution of BCP ceramics is manifested by a decrease in crystal size and an
increase in macro - and microporosity [9 - 12] .
β
4.3.3 Mechanical Properties
It is logical for the pore size and percentage of macroporosity of the BCP
ceramic to affect the mechanical properties [22]. The preparation method has also
been found to have a signifi cant infl uence on compressive strength. BCP ceramic
prepared from a single calcium-defi cient apatite phase is reported to have higher
compressive strength (2 to 12 MPa) than BCP ceramic prepared by mixing two
unsintered calcium phosphate preparations (2MPa): one which, after sintering at
1200 °C, results in only HA and the other which results in only
- TCP [34] . Initial
mechanical property is not the best criterion for evaluating the effi cacy of bone
ingrowth. For example, BCP with high mechanical properties because of low
microporosity (as a result of a high sintering temperature) may have reduced
bioresorption and bioactivity. On the contrary, it has been demonstrated that the
initial mechanical property of BCP increased two or three times (2 to 6 MPa) in a
few weeks after implantation thanks to the physical and chemical events of dis-
solution and biological precipitation into the micropores [12].
β
4.3.4 Bioactivity, Osteogenic Properties
The main attractive feature of bioactive bone graft materials such as BCP
ceramic is its ability to form a strong direct bond with host bone resulting in a
strong interface compared to bioinert or biotolerant materials which form a
fi brous interface [9-15]. The formation of the dynamic interface between bio-
active materials and host bone is believed to be the result of a sequence of events
involving interaction with cells and the formation of carbonate hydroxyapatite
(CHA), which is similar to bone mineral, by means of dissolution/precipitation
processes [12,15] .
Bioactivity is described as the property of a material to form carbonate
hydroxyapatite (CHA) on its surface
in
vitro
[15,35,36] or
in
vivo
[9,37 - 39] .
Osteoinductivity or osteogenic property is the property of the material to induce
bone formation
de
novo
or ectopically (in non-bone forming sites). Bioceramics
(calcium phosphates, bioactive glass) do not usually have osteoinductive proper-
ties [19]. However, several reports have shown the osteoinductive properties of
certain calcium phosphate bioceramics such as coralline HA (derived from coral)
or those observed in certain studies using BCP [40-41]. Reddi [42] explains these
apparent osteoinductive properties as the ability of particular ceramics to con-
centrate bone growth factors that are circulating in the biological fl uids, and these
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