Biomedical Engineering Reference
In-Depth Information
TABLE 1.1
Calcium-Phosphate Phases with Corresponding Ca/P Ratios
Name
Formula
Ca/PRatio
Hydroxyapatite (HA)
Ca
10
(PO
4
)
6
(OH)
2
1.67
Fluorapatite
Ca
10
(PO
4
)
6
F
2
1.67
Chlorapatite
Ca
10
(PO
4
)
6
Cl
2
1.67
A-type carbonated apatite (unhydroxylated)
Ca
10
(PO
4
)
6
CO
3
1.67
B-type carbonated hydroxyapatite (dahllite)
Ca
10−
x
[(PO
4
)
6−2
x
(CO
3
)
2
x
](OH)
2
≥1.67
Mixed A- and B-type carbonated apatites
Ca
10−
x
[(PO
4
)
6−2
x
(CO
3
)
2
x
]CO
3
≥1.67
HPO
4
containing apatite
Ca
10−
x
[(PO
4
)
6−
x
(HPO
4
)
x
](OH)
2−
x
≤1.67
Monohydrate calcium phosphate (MCPH)
Ca(H
2
PO
4
)
2
H
2
O
0.50
Monocalcium phosphate (MCP
Ca(H
2
PO
4
)
2
0.50
Dicalcium phosphate dihydrate (DCPD)
Ca(HPO
4
)2H
2
O
1.00
Tricalcium phosphate (TCP)
α- and β-Ca
3
(PO
4
)
2
1.50
Octacalcium phosphate (OCP)
Ca
8
H(PO
4
)
6
5H
2
O
1.33
Source:
Segvich et al., in
Biomaterials and Biomedical Engineering
, Ahmed et al. (eds.), TTP, Switzerland, pp. 327-373,
2008. With permission.
can arise from lattice substitutions of calcium, phosphate, and hydroxide groups with
magnesium and carbonate ions. These substitutions can also give rise to altered solubil-
ity of the mineral phase. Since bone mineral contains calcium and alkali reserves, this
enhanced solubility can buffer systemic changes in Ca
2+
, H
3
PO
4
, and CO
2
(Neuman and
Neuman 1957). For instance, during acidosis, the mineral can give up a carbonate ion for
a hydronium ion to supplement blood buffers. The crystalline phase contains carbonate
lattice substitutions that account for 2 to 7 wt.% of biological apatite (Segvich, Luong, and
Kohn 2008). The consensus is that carbonate substitutes directly into the lattice through a
type B (Table 1.1) substitution that is most commonly found in biological apatite (LeGeros
2002). The organic matrix is predominantly comprised of collagen, of which type I colla-
gen is the major component, with trace amounts of other collagen isoforms present during
certain developmental stages. Noncollagenous proteins comprise the remaining 10% to
15% of total bone protein content and include proteoglycans, glycosylated proteins, and
γ-carboxylated proteins. These noncollagenous proteins are involved in directing organic
matrix assembly, maintaining structural integrity of the tissue, sequestering and interact-
ing with growth factors, and regulating bone metabolism and mineralization.
Bone ECM also functions as a reservoir of growth factors that are secreted by the cells
(Biondi et al. 2008). Growth factors are a major class of hormones that mediate growth,
division, and proliferation, and can be involved in endocrine, autocrine, and paracrine sig-
naling (Silverthorn 2003). Cell stimulation by growth factors is influenced by concentration
gradients and stage of development at which the active molecules are present. For instance,
the role of TGF-β1 in osteogenesis and bone remodeling varies with concentration. TGF-β1
is present in high concentrations during early fracture repair process, but levels off in later
stages (Allori, Sailon, and Warren 2008). In the early stages of repair, TGF-β1 promotes
division of fibroblasts, osteoblast recruitment, and differentiation. TGF-β1 also inhib-
its osteoclast proliferation and differentiation. In later stages of wound healing, TGF-β1
promotes osteoclastogenesis. Similarly, BMP2 promotes chemotaxis and cell proliferation
at low concentrations and cell differentiation and bone formation at high concentrations
(Allori, Sailon, and Warren 2008).
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