Biomedical Engineering Reference
In-Depth Information
particularly in bones, tendons and ligaments. Furthermore, there
is a cellular phase, which consists of three different types of cells:
osteoblasts, osteoclasts, and osteocytes; however, that is another
story. The inorganic to bioorganic ratio is approximately 75% to
25% by dry weight and about 65% to 35% by volume. This ratio not
only differs among animals, among bones in the same animal and
over time in the same animal but also it exerts a major control on the
material properties of bone, such as its toughness, ultimate strength
and stiffness. In general, load-bearing ability of bones depends on
not only architectural properties, such as cortical thickness and bone
diameter, but also intrinsic, size-independent, material properties
such as porosity, level of mineralization, crystal size and properties
derived from the organic phase of bone [558]. A higher mineral to
collagen ratio typically yields stronger, but more brittle, bones [559-
561]. For example, bone from the leg of a cow has a relatively high
concentration of calcium orthophosphates (for support), whereas
bone from the antler of a deer has a relatively high concentration
of collagen (for flexibility) [124]. It is interesting to note, that bone
exhibits several physical properties such as piezoelectricity [562]
and pyroelectricity [563].
Stability of the mineral composition of bones has a very long
history: calcium orthophosphates were found in dinosaur fossils [52,
99, 564-567]. Therefore, organisms have had a great deal of time to
exploit the feedback between composition and structure in apatite,
on the one hand, and benefit from its biological functionality, on the
other. Bones of modern animals is a relatively hard and lightweight
porous composite material, formed mostly of biological apatite (i.e.,
poorly crystalline CDHA with ionic substitutions). It has relatively
high compressive strength but poor tensile strength [568]. While
bone is essentially brittle, it has a degree of significant plasticity
contributed by its organic components.
The distribution of the inorganic and bioorganic phases depends
on a highly complex process that takes place during bone formation.
Each of these components may be assembled in different proportions
creating two different architectural structures depending on the bone
type and function. They are characterized by different structural
features that strongly correlate with the mechanical performance
of the tissue. These two types of bones are the following: the
cortical bone (or compact bone), which is a dense structure and
the cancellous bone (also known as trabecular or spongious bone),
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