Biomedical Engineering Reference
In-Depth Information
Figure 1.14
Examples of the osteon structure according to
Gebhardt, with different collagen fiber orientations in the
osteon lamellae. Helical course of fibers are noted at the
successive lamellae. After [104].
the preceding one. Collagen fibers in a lamellae run parallel to each other but the
orientation of collagen fibers across separate lamellae is oblique, cf. Figure 1.14.
The fiber density is also lower at the border between adjacent lamellae, which
accounts for the distinctive appearance of an osteon. In addition to blood vessels,
Haversian canals contain nerve fibers and bone cells called
bone lining cells
.Bone
lining cells are actually osteoblasts that have taken on a different shape following
the period in which they have formed bone.
In 1905, Gebhardt [104, 105] performed research on bone structure, in particular
on osteon architecture, using optical polarized microscope. Observations under
polarized light indicate preferable directions of fibers in the lamellae of osteon. As
a result, Gebhardt found that osteons are composed of a number of lamellae in
which collagen fibers lay in different directions, cf. Figure 1.14.
These observations were repeated half a century later by Ascenzi and Bonucci
[106-108]. They suggested that osteons that appear bright under polarized light are
composed of lamellae in which collagen fibers lay (in prevailing number) parallel
to the plane and perpendicular to the Haversian canal. The dark osteons under
polarized light in their model consist of lamellae in which collagen fibers are
oriented parallel to the long axis of the bone. In intermediate (alternating) osteons,
collagen fibers should in this classification alternate orientation from one lamella
to the other, having some lamella in which collagen fibers are orientated parallel
(dark bands) and some orientated perpendicular (bright bands) to the long axis of
the bone.
Ascenzi and Bonucci examined the mechanical properties of these three classes
of osteons. Dark osteons were found to be the strongest under tensile loading.
Bright osteons were stronger under compression. Intermediate osteons possess
intermediate properties between bright and dark osteons, cf. [106-112] (also
[113]).
