Bone that it is a tissue that is hard,

Bone is a complex, highly organized and
specialized connective tissue. It is characterized physically by the fact that
it is a tissue that is hard, rigid and strong, and microscopically by the
presence of relatively few cells and much intercellular substance formed of
collagen fibers and stiffening substances 1,2.

Bone consists of 65% mineral, 35% organic
matrix, cells, and water. The bone mineral is in the form of small crystals in
the shape of needles, plates, and rods located within and between collagen
fibers. The mineral is largely impure hydroxyapatite, Ca10 (PO4)6
(OH)2, containing constituents such as carbonate, citrate,
magnesium, fluoride, and strontium incorporated into the crystal lattice or
absorbed onto the crystal surface. Foreign substances such as tetracyclines,
polyphosphates, bisphosphonates, and bone-seeking radionuclides can also be
incorporated with high affinity. The organic matrix consists of 90% collagen
and about 10% of various non-collagenous proteins4,8.

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During skeletal growth, removal and
replacement of bone proceeds at a rapid pace. The rate of turnover of the
skeleton approaches 100% per year in the first year of life, declining to about
10% per year in late childhood, and then usually continues at approximately
this rate or more slowly throughout life. Much of the turnover of bone during
growth results from bone-modeling, but presumably at least some remodeling also
occurs. After the completion of skeletal growth, the turnover of bone results
primarily fromremodeling. Modeling and remodeling result from coordinated
resorption and formation of bone over extensive regions of the tissue, over
prolonged periods of time.

The discipline of mechanics is the physical
science that deals with the effects of forces on objects. The concern here is
with the mechanics of deformable objects, in particular, bone. Bones are
physical objects that obey the laws of mechanics. The primary laws of mechanics
that concern deformable objects like bone are the three laws of motion by Sir
Isaac Newton in 1687 and the law of elasticity of solid materials described by
Robert Hooke in 1678. The following three Newtonian laws are the basis of
classical mechanics:

1-      A body remains at rest or moves at a
constant speed in a straight line unless acted on by a force. This is a
statement of the principle of inertia.

2-      The total force acting on a body is equal
to the mass of the body times its acceleration; that is, f = ma, where f and a
are vectors oriented in the same direction.

3-      If a body exerts a force on a second body,
the second body exerts a force on the first body that is equal in magnitude and
opposite in direction to the first force. This is the law of action and
reaction.

Hooke’s law states that there is a linear
relation between the force and deformation of a solid object. The laws of
Newton and Hooke form the foundation of the mechanics of elastic objects. The
mechanical behavior of bone in normal physiological situations is quite similar
to the mechanical behavior of an elastic object4.

It is obvious that mechanical forces have a
major influence on the bone modelingand remodeling processes in both cortical
and trabecular bone, since their effects on bonemorphology are obvious (Wolff,
1892). The pathways by which mechanical forces areexpressed in osteoclast and
osteoblast activity is currently one of the main unresolvedissues in bone
mechanobiology. The current concept is that the bone architecture iscontrolled
by a local regulatory mechanism. This idea originates from Roux (1881),
whoproposed that bone remodeling is a self-organizing process. Frost captured
these conceptsin his ‘mechanostat’ theory (Frost, 1964, 1987). It assumes that
local strains regulatebone mass. If strain levels exceed a so-called mechanical
‘set-point’, new bone is formed.If strain levels are below this set-point, bone
is removed. It is a qualitative theory, but itforms the theoretical basis for
several mathematical and computational theories that weredeveloped to study
bone adaptation.

The
mechanostat does not specify the cellular level mechanisms behind
the(re)modeling process. In other words, it does not describe how local
mechanical signalsare detected, nor how they are translated to bone formation
and resorption. Osteocytesmay play an important role here. Several studies
revealed that these cells respond tomechanical stimulation. Together with
thelining cells they form a system that seems well equipped for signal
transduction. It could be that mechanically induced osteocyte signals are
transferredthrough the canaliculi to the bone surface where they control
osteoclast and osteoblastactivity. Whether this is true remains to be proven
9.

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