Biomedical Engineering Reference
In-Depth Information
strength using a power law. Bone mineral content is the ratio between the
mineral weight and the dry weight of the bone sample. The bone sample is
burnt to determine its mineral content or the ash fraction. Water content is
also important in the mechanical properties of cortical bone (Currey, 1988).
Wet bone, as found
in situ
, is less stiff, less strong, and less brittle than fully
dried bone.
4.2 Biomechanical properties of bone
Bone has a major advantage over engineering structural materials in that
it is self-repairing and can alter its properties and geometry in response to
changes in mechanical and metabolic demand. Bone properties also change
from species to species. Bone physical properties differ from one person to
another but also within one individual from one location to another. owing
to its different apparent porosity, mechanical properties of trabecular bone
and cortical bone are clearly different (Ascenzi, 1988; Reilly
et al
., 1974;
rho
et al
., 1996).
4.2.1 Cortical bone
Cortical bone accounts for approximatively 80% of the skeletal mass.
Cortical bone and relatively stiff trabecular bone have Young's moduli of
about 17 GPa and 1 GPa, respectively (it also depends on the species and
on the type of bone). Thus, most metals used in orthopedic applications are
an order of magnitude stiffer than cortical bone. Human cortical bone is
usually considered to be transversely isotropic with mechanical properties
substantially different in the longitudinal direction (parallel to the axis
of the osteons) than in the radial or circumferential directions but it has
similar properties in the radial and circumferential directions. The modulus
of cortical bone in the longitudinal direction is approximately 1.5 times its
modulus in the transverse direction. The material properties measured can
differ depending on the measuring techniques (rho
et al
., 1993). When
ultrasound is employed, various velocities are measured from which the
elastic coefficients are determined (Ashman
et al
., 1987; Bonfield and Tully,
1982). The technical constants are then found by matrix inversion.
cortical bone has a higher strength in compression than in tension and
is stronger in the longitudinal direction than in the transverse direction.
For longitudinal loading, cortical bone is a tough material because it can
absorb substantial energy before fracture. Furthermore, cortical bone can be
classified as a relatively ductile material for longitudinal loading since its
ultimate strain for longitudinal loading is substantially larger than its yield
strain. However, it is relatively brittle for transverse loading.
cortical bone exhibits a viscoelastic behaviour, that is, it is sensitive both
