Biomedical Engineering Reference
In-Depth Information
Organic
30%
Mineral
50%
Water and
pores 20%
Figure 3.1
Three constituent phases of bone: organic, min-
eral, and water/pore space collectively contribute to the over-
all mechanical behavior of the material bone. The volume
percent of each component is reported [6].
3.2.1
Organic
The organic phase is dominated by the presence of collagen type I (
90% by mass).
Although noncollagenous proteins and glycoproteins are present, they are likely
to have little effect on the overall mechanical behavior of the material. Osteoblasts
deposit the collagenmatrix in a three-dimensional formation through self-assembly
from individual tropocollagen molecules (triple helices). A 67 nm characteristic
banding indicates the presence of gaps between the lengths of collagen fibrils
and is intrinsically due to the quarter-staggering of 300 nm long triple helices [9].
The fibrils organize and cross-link to form larger fibers [9], which contribute to
the viscoelastic response and toughness of bone [10]. The noncollagenous organic
phase in bone ismostly composed of glycoproteins, osteopontin, and proteoglycans.
This nonfibrillar organic matrix may ''glue'' mineralized collagen fibers together
and may also serve as a source of sacrificial bonds, thus increasing the fracture
energy [11].
≈
3.2.2
Mineral
The mineral phase of bone is itself heterogeneous. Multiple forms of poorly
crystalline carbonate apatite mineralize and effectively stiffen the organic matrix.
The primary phase of hydroxyapatite Ca
10
(PO
4
)
6
(OH)
2
[12, 13] exists within amilieu
of calcium phosphate materials that physically connect through a continuous and
rigid structure after the removal of the organic phase. Carbonate (CO
3
2
−
)is
commonly substituted into the apatite lattice for OH
−
or PO
4
3
−
and a common
