Biomedical Engineering Reference
In-Depth Information
Regardless of its relatively small volume and high apparent porosity, trabecular
bone is well adapted to resist and conduct compressive loads, as can be seen by the
vertebrae, handing over to the cortical bone other structural functions, as resisting
bending and torsional stresses.
6.1.3.4 Trabecular Bone Remodelling
The border of the trabecula is filled with osteblasts and osteoclasts, ready and
active. During the growth or remodelling of the trabecula the deposition of new
bone by osteblast activity is counterbalanced by the removal of bone by osteoclast
activity in the opposite surface of the trabecula, remodelling and reshaping the
trabeculae configuration to a better stress path configuration or simply to repair
micro-fractures. The diminishing of the bone mass with age is the result of the
change in the balance between the rate of bone formation and the rate of bone
reabsorption. Pathologic fractures occur when the bone density becomes so low
that the skeleton can no longer withstand the mechanical stresses of everyday life.
In a experimental study [
11
] it was showed that the compressive strength of bone is
proportional to the square of its apparent density, e.g., if the bone density decreases
by a factor of 3, its compressive strength decreases by a factor of 9. Both cortical
and trabecular bone present analogous microscopic physical properties and bio-
chemical composition. However, in order to suit local physical requirements, due
to loads and to stresses distribution, the bone macroscopic structure is organized in
order to maximize the strength and stiffness. For example, at the ends of long
bones the thin cortical shell, inner supported by trabecular bone, is the optimal
configuration to hold and distribute in the bone structure the concentrated loads
applied. While the tubular cortical diaphysis is optimized to support the large
torsional and bending loads. Despite being cortical or a trabecular structure, all
normal adult bone is lamellar bone. In adults, only due to normal fracture healing,
or in pathologic conditions, immature woven bone, or fibre bone, is seen.
6.2 Bone Tissue Mechanical Properties
As previously mentioned, bone tissue can be classified as cortical bone, highly
densified bone tissue, and trabecular bone, which shows a considerably smaller
apparent density. Although both types show the same molecular arrangement the
mechanical behaviour is different. Many experimental studies show that the bone
mechanical properties depend on the bone composition and on the bone porosity
(directly related with the bone density) [
12
-
17
].
In the work of Carter and Hayes [
12
] it is stated that for trabecular and cortical
bone the elastic modulus is closely related to the cube of the bone apparent density
and that the strength is closely related to the square of the bone apparent density.
