Cortical Bone Mechanics and Composition: Effects of Age and Gender - Skeletal Aging and Osteoporosis: Biomechanics and Mechanobiology - page 54

Osteonal
lamellae

Periosteal
lamellae

Osteon

Interstitial
tissue

Cortical
bone

Endosteal
lamellae

Haversian
canal

Fig. 1 Hierarchical structure of cortical bone that is composed of lamellae, osteon, and

interstitial tissues

The mineral crystals in bone are plate-like in shape and quite small, having the
length, width, and thickness of 50 9 25 nm 9 *1.5-4.0 nm, respectively [ 9 - 11 ].
Early X-ray diffraction studies indicate that the bone mineral is similar to
hydroxyapatite, Ca 5 (PO 4 ) 3 (OH) [ 12 , 13 ], but with some distinctions compared
to synthetic hydroxyapatite [ 14 ]. Such distinctions are usually considered due to
non-stoichiometric ratio of calcium to phosphorous, presence of strongly bound
water, and deposition of amorphous mineral (tricalcium or octacalcium phosphate)
[ 8 ]. Some studies suggest that the mineral phase may be classified as a carbonated
apatite Ca 5 (PO 4 CO 3 ) 3 since hydroxyl groups are not observed when bone is
analyzed

with Fourier Transform Infrared Spectroscopy (FTIR) or Nuclear

Magnetic Resonance (NMR) [ 15 , 16 ].

2.1.2 Organic Matrix

The organic matrix consists of collagen and non-collagenous proteins, with type I
collagen being the major part ([90%). Secreted by bone forming cells known as
osteoblasts, procollagen is a triple helical rod of three intertwining polypeptide
chains (two identical a1 helices and one different a2 helix), each containing
approximately 1,000 amino acids, in which every third residue is glycine and
positioned toward the center of the super-coil [ 17 ]. Proline typically occupies the
next position, and there is an abundance of hydroxyproline in the third position.
Hydroxylysine is a unique residue of bone collagen and gives rise to cross-linking.
Upon enzymatic cleavage of the non-helical, amino (N) and carboxyl (C)
terminals, procollagen forms collagen fibrils by self-assembling via crosslinks
into a staggered arrangement. In general, collagen fibrils are 30-80 nm in
diameter [ 18 ]. There are no data currently available regarding the length distri-
bution of collagen fibrils in bone. However, it is anticipated that they are longer