Biology Reference
In-Depth Information
osteocytes act as the mechanosensory cells or strain receptors. The process of mechanotrans-
duction is possible because the bone cells have a connected cellular network (CCN) and this
network functions like its own nervous system (
Aarden et al., 1994; Pearson and Lieberman,
2004
). When the bone begins to fail as a result of a minor injury, microfractures form and fluid
is forced through the canaliculi and across the gap junctions, which is sensed and interpreted
by the cells themselves (
Aarden et al., 1994
). How the cells interpret this information and
respond is unknown (
Pearson and Lieberman, 2004
). Each skeletal tissue has a microdamage
threshold in which normal repair can keep up with the need, but fatigue fracture occurs when
the damage overwhelms the repair, which happens when there are repetitive small fractures
(
Frost, 1993
).
During growth and development, bones are extremely plastic to forces of load bearing due
to their more elastic material properties (greater percentage of collagen in youth). These
forces alter the resultant shape of the bone shaft and its articulations. Endochondral ossifi-
cation
2
occurs primarily through modeling or the depositing of lamellar bone. Because of
the higher percentage of collagen present, young bone typically adapts efficiently to normal
activities and to the environment (
Carter et al., 1991
). Mechanical loading early in develop-
ment may guide endochondral ossification, so that the skeletal form is well designed for its
mechanical function (
Carter et al., 1991
).
As an individual ages, the material properties of bone change from being elastic to more
stiff as collagen is gradually lost (
Rubin et al., 1990; Frost, 1993
). Adult bone therefore
responds slightly differently to forces of loading over time. In adult bone, the epiphyses
are fused and there is increased mineralization compared to juvenile bone. This gives greater
strength and stiffness to adult bone, but it inevitably becomes more brittle. Young's modulus
(a measure of bone's stiffness) is used to explain material properties of bone (and other mate-
rials) in terms of resistance to stress and strain. Bone biomechanics are presented in terms of
traumatic bone injury by Kroman and Symes in the chapter on trauma in this volume
(Chapter 8).
Long bones can be interpreted as analogous to engineering beams (e.g., in a bridge or
a building) when examining the cross-sectional shape properties of the beam (i.e., bone
shaft). The cross-sectional cortical area reflects the bone's strength to axial compression
(for example, the downward force of gravity acting on body mass). The area moments of
inertia are the cross-sectional property of a beam cross-sectional property (e.g., bone) used
to predict its resistance to bending (i.e., bending strength). The area moments of inertia
are a measure of the distance of the bone surface to the center of the bone (centroid) and
are measured in different directions. The direction of greatest bending is represented with
the symbol I
max
, which is perpendicular to the minimum area moment of inertia is signified
by I
min
. The polar moments of area are the cross-sectional property of a beam proportional to
the torsional rigidity to predict a bone's resistance to twisting or torsion (i.e., torsional
strength, calculated as J is divided by the distance from the subperiosteal surface to the
centroid) (
Frankel and Nordin, 1980
; Ruff, 2000). The symbol “J” is typically used to represent
the polar moments of area and equals the sum of any two perpendicular area moments of
inertia. Increased surface area provides greater resistance to axial compression, which is
2
Endochondral ossification occurs mostly postcranially in the long bones. In contrast, intramembranous
ossification occurs mostly in the skull. See Uhl (Chapter 3) this volume, for more on aging.
