Biomedical Engineering Reference
In-Depth Information
where
a
,
b
and
c
are constants that depend on the architecture of the tissue (rice
et al
., 1988). In general, the exponent
c
has a value of approximately 2.
Trabecular bone under compression has similar characteristics to porous
foam materials such as wood. In the first stage, bone deforms in the linear
elastic region, in which individual trabeculae bend and compress as the bulk
tissue is compressed (Fig. 4.6). in the second stage, some trabeculae fail or
buckle without an increase in loading (Turner, 1989). As more and more
trabeculae fail, the strain increases until broken trabeculae begin to fill the
pores, causing the specimen to stiffen. Thus, trabecular bone has a unique
ability to resist large compressive load for a minimal mass. This energy
absorption allows compressive strains of over 50% (Linde
et al
., 1989). As in
fiber-reinforced concrete, the tensile behavior of trabecular bone is poorer.
4.2.3 Bone poroelasticity
a poroelastic formulation can be used to describe the mechanical properties of
bone. Fluid-filled porosities of bone can affect the mechanical response of the
tissue, depending on the loading rate applied to the bone and the subsequent
pressure that develops within the fluid. The interstitial fluid within bone has an
important role in sustaining the load applied in the bone. Pore fluid pressure
contributes to the total stress in the porous matrix medium and pore fluid
pressure can also strain the porous matrix medium in addition to the strain
developed from the loading. The presence of interstitial fluid is useful for
transporting nutrients from the vasculature to the cells in the tissue and to
transport waste products for removal. This fluid movement is created from
the differences in pore fluid pressure developed by different pore volume
strains associated with mechanical loading of the porous medium.
200
Cortical bone
Trabecular bone
150
100
50
0
0
5
10
15
20
25
Strain (%)
4.6
Example of typical compressive stress-strain behaviors of
trabecular and cortical bone for different apparent densities (adapted
from Keaveny and Hayes, 1993).
