Biology Reference
In-Depth Information
is different from the acute stress that creates injury and fractures, it is still another example of
how an applied understanding of biomechanics can aid an anthropologist in gathering infor-
mation from a skeleton.
Kennedy (1989)
is an authoritative reference on musculoskeletal
markers of stress, especially those considered to be correlated with certain occupations.
Smith (Chapter 7), this volume, also includes a discussion of MSM.
Material Properties of Bone
Both cortical and cancellous bones are anisotropic materials (for a review see
Antich, 1993;
Bonfield et al., 1985; Evans, 1973; Johnson, 1985; Keaveny and Hayes, 1993; Nordin and
Frankel, 1989; Turner and Burr, 1993
). Characteristically, anisotropic materials have different
material properties based on the different directions or axes of the bone (i.e., long axis, short
axis, etc.). For example, the human femur is designed to withstand the stress of weight
bearing. The femur is therefore much stronger and more resistant to an axial load, or a force
along the long axis of the bone, than a force that is applied perpendicularly. This differs from
isotropic materials that are more homogeneous, having the same material properties in all
directions.
Human cortical bone has a particular type of anisotropy referred to as transverse isotropy,
because it has the same resistance to force in all transverse directions, and a higher resistance
in the longitudinal direction (
Keaveny and Hayes, 1993
). For example, a femur shaft has the
same properties in all transverse or cross sections, but these properties are different from the
longitudinal axis. The histology of bone contributes to its anisotropy. Human bone is stronger
in the longitudinal dimension (the direction in which the osteons run) than in the transverse
direction. Human bone is also stronger under compression than under tension or shear in
most cases. Human limbs and bone have adapted to constant compressive stress from daily
activity and therefore have a higher resistance to compression than tension.
Human bone is also a viscoelastic material (for a review see
Bonfield et al., 1985; Piekarski,
1970; Turner and Burr, 1993
). The term viscoelastic means that the material can behave either
as an elastic material or as a more resistant material depending on the rate of strain applied. A
viscoelastic material behaves in different ways depending on the rate and the duration of
loading. Cortical bone is extremely sensitive to strain. Cortical bone absorbs a large amount
of energy from a normal activity such as running a mile. However, if less energy is applied all
at once, such as landing from a high fall, the failure level is reached and a fracture results.
Histologically, fractures induced by low load rate follow the interstitial bone around the
osteons, while at a higher load they travel indiscriminately through the bone (
Piekarski,
1970
).
The viscoelastic properties of bone also play an important role in trauma interpretation.
While the same basic principles of biomechanics and physics operate for both ballistic
trauma and blunt force trauma (i.e., when bone is impacted by another object) the resulting
fracture patterns are quite different (
Berryman and Symes, 1998
). This difference is due to
the rate of loading. Blunt force trauma impacts bone at
miles per hour
while a ballistic projec-
tile impacts bone at
feet per second
(
Symes et al., 1989, 2012
).
Keaveny and Hayes (1993)
state
that at high rates of loading, bone will behave like a brittle material (such as glass) and
therefore will skip the stage of plastic deformation and will fail quickly under sufficient
applied force.
