Biomedical Engineering Reference
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increased with age in the primates, similar to humans. However, no change was
observed in osteonal area or percentage of osteonal bone, likely due to reduced
activation frequency and bone formation rate [ 82 ].
3.3 Collagen Orientation
At the microscopic level, two kinds of bone are evident, lamellar and woven bone.
Lamellar bone, by definition, is slowly formed and highly organized bone con-
sisting of parallel layers of collagen fibrils and mineral crystals with long axis
oriented along the collagen fibers [ 9 ]. In contrast, woven bone is quickly formed
and poorly organized bone with randomly arranged fibers and mineral crystals
[ 101 ]. Von Ebner [ 152 ] was the first to suggest about the orientation of collagen
fibers that appeared to change between successive lamellae of an osteon [ 74 ].
During the 60s and 70s, Ascenzi and Bonucci [ 8 ] and Frasca et al. [ 64 ] used
birefringence to study collagen orientation, where bone specimens are illuminated
and viewed through polarizing filters. Because of longitudinal and transverse
collagen fiber orientation, dark and bright fields are produced by the rotation of
plane of polarized light [ 8 , 64 , 153 ]. Later, Giraud-Guille, using transmission
electron microscopy (TEM), suggested two kinds of collagen organization. The
first kind of organization is analogous to orthogonal plywood, where within each
lamella fibers orient parallel, but the next lamella change direction by 90 at the
interface. Depending on the bone section considered, dark and bright images
are seen corresponding to transverse and longitudinal orientations, respectively, in
the TEM. The second kind is similar to twisted plywood, closely relating to the
'alternating' or 'intermediate' orientation suggested by Ascenzi, where collagen
fibers continuously change direction as a series of nested arcs [ 72 ]. Giraud-Guille
concluded that human compact bone consisted of both kinds. Contradicting the
classical models, Marotti et al. suggested another kind of organization where
osteons consist of collagen rich (densely packed) and collagen poor (loosely
packed) areas, producing bright and dark appearances under polarized light,
respectively [ 98 ]. However, Ascenzi's and Bonucci's model is widely accepted
and used to describe the birefringence pattern in cortical bone.
Spiraling collagen fibers were found in younger individuals when compared to
the old by Amprino et al., who suggested that elderly individuals tended to have
more transversely oriented collagen fibers [ 151 ]. Conversely, Smith demonstrated
light osteons (transverse) converted to dark osteons (longitudinal) with aging in
human femora and tibia [ 141 ]. It is still unclear what contributes to the orientation
of collagen fibers. Rearrangement of crystallites and fibers within a calcified and
matured osteons is highly unlikely but mechanical loading may be the determining
factor for fiber orientation at the time of osteon formation [ 151 ]. In support, more
transverse orientation was observed in newly formed osteons in older individuals
whereas newly formed osteons in younger individuals tended to have longitudinal
orientation.
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