Biomedical Engineering Reference
In-Depth Information
part is composed majority by collagen (about 94 %) and small parts of noncol-
lagenous proteins (about 4 %) and bone cells (about 2 %). The collagen present in
the bone matrix is of type I, although small quantities of type V collagen can be
found. Collagen fibres are arranged along lines of mechanical stress according to
Wolff's Law [
2
]. It is the orientation and the quantity of these fibres that confers to
the bone mechanical properties such as elasticity and high ultimate traction and
shear stress. The collagen fibres are synthesized by the osteoblasts (bone cells). For
instance, the cortical bone and the periosteum are bonded through collagen fibres,
the Sharpey fibres.
Although it represents only 5 % of the organic bone matrix, the noncollagenous
proteins play an important role in the bone metabolism and in the mineralization of
the bone matrix. Only 2 % of the organic matrix are bone cells, but without such
living elements the bone could not repair, rebuild, remodel, reshape itself or even
grow.
The bone inorganic part consists mainly of a carbonate-rich hydroxyapatite,
also called bone apatite, which is smaller and less perfect in crystal arrangement
than pure hydroxyapatite, Ca
10
ð
PO
4
Þ
6
ð
OH
Þ
2
. However it is due to this crystalline
imperfections that bone apatite is biological more appropriate than pure
hydroxyapatite, once it is more readily available for metabolic activity and for
body fluid exchanges. Other minerals are also present in the bone mineral matrix,
but in much lower percentage: sodium, magnesium, potassium, fluoride and
chloride. The inorganic components confer to the bone structure the hardness, the
stiffness and the high ultimate compression stress.
6.1.2.2 Bone Cells
The bone cells are divided, generally, in five cell-types: osteoprogenitors, osteo-
blasts, osteocytes, bone-lining cells and osteoclasts. The role of these cells in the
bone reabsorption process is presented in Fig.
6.3
.
The osteoprogenitors are immature non-specialized cells which differentiate to
origin the osteoblasts [
3
]. These cells are in the genesis of all bone structures
during the growth process, since the conception, and are generated in the bone
marrow, or in other connective tissues. The osteoprogenitors can be found in the
interior layer of the periosteum, in the endosteum and in the Haversian's and
Volkmann's canals.
The osteoblasts are mononuclear cells responsible for the bone formation and
for the mineralization of the osteoid matrix. These cells are in the origin of the
osteocytes. The osteoblasts, produced by the osteoprogenitors in the periosteum,
the endosteum and the bone marrow, can be found in the bone inner surface
(endosteum) or outer surface (periosteum) in a laminar distribution. When active
the osteoblasts are cuboidal or polygonal shaped [
1
]. The number of osteoblast
cells tends to decrease as individuals become elder, therefore decreasing the
natural renovation of the bone tissue [
4
].
