Biomedical Engineering Reference
In-Depth Information
The structure used by Gebhardt (as well as by Ascenzi and Benucci) is known
as the
orthogonal plywood model
:onlytwofibrildirectionsexist,makinganangle
90
◦
. Next, in 1988, Giraud-Guille presented the twisted plywood model of collagen
fibril orientation within cortical bone lamellae. The twisted plywood model allows
for parallel collagen fibrils, which continuously rotate from one plane to another in
a helical structure [56].
Lying between or within the lamellae are special holes known as
lacunae
.Each
lacuna has an oblong ellipsoidal form and provides enough space for an individual
bone cell (osteocyte) to reside. In a microscopic section, viewed by transmitted light,
lacunae appear as fusiform opaque spots. Lacunae are connected to one another
by small canals called
canaliculi
. The osteocyte inside the lacuna is responsible for
secreting the bone salts surrounding it. Osteocytes are found between concentric
lamellae, within their cavelike lacunae, and connected to each other and the central
canal by cytoplasmic processes through the canals called
canaliculi
. Osteocytes
communicate with each other, and their network permits the exchange of nutrients
and metabolic waste. The human osteocyte under normal conditions lives for about
25 years. Thus, in the lifetime of a person there would be about four generations
of osteocytes.
Osteons are separated from each other by cement lines. Collagen fibers and
canaliculi do not cross cement lines. The space between separate osteons is occupied
by interstitial lamellae, which were formed by preexisting osteons that have since
been reabsorbed. Osteons are connected to each other and the periosteum by
oblique channels called
Volkmann's canals
.
Figures 1.15 and 1.16 are images of a sectioned bone. Cross section of a real
osteon is not perfectly circular and the lamellae are not perfectly concentric.
a
b
Figure 1.15
Compact bone - decalcified cross section. Os-
teonic structure is seen in the magnification: a - Haversian
canals; b - lacunar spaces. Courtesy of Litwin and Gajda.
