Biology Reference
In-Depth Information
Bone Deformation
Bone under stress and strain reacts in a predictable manner, as outlined extensively by
Keaveny and Hayes (1993), Nordin and Frankel (1989)
, and
Turner and Burr (1993)
. The
deformation of the material has a direct relationship to the force of the load exerted upon
it. This relationship is depicted as a stress
e
strain or load
e
deformation curve (
Figure 8.1
).
Load
e
deformation curves depict the stages that bone undergoes throughout loading until
complete failure. The elastic deformation region is the first area of the load
e
deformation
curve. When bone is in elastic deformation and the load is removed the bone will return
to its former shape with no visual structural alteration. Bone enters the plastic deformation
stage when a tolerance threshold has been reached. After release of the force, bone in the
plastic deformation stage cannot return to its original shape even though visible fracture
may not be evident.
Load
e
deformation curves provide information on the amount of energy absorbed, load
sustained, and deformation achieved before failure (
Nordin and Frankel, 1989
). The amount
of energy absorbed is calculated by the area underneath the curve, and by the load and defor-
mation sustained at failure.
The overall structure stiffness is demonstrated by the slope of the curve. Stiffness is
calculated by using the modulus of elasticity or Young's modulus. The stiffer the material,
the higher the moduli value. Young's modulus is important in bone fracture mechanics to
demonstrate stiffness or ductility (ability to bend or deform), which has a great influence
on fracture mechanics. Brittle materials (glass) deform very little before failure, while
ductile materials (rubber band) can withstand a great deal of elastic deformation. Brittle
materials do not undergo plastic deformation when force is applied, and often require
little force to reach failure. As we will see later, this principle and the amount of
plastic deformation become important when assessing whether an insult was ballistic or
blunt trauma.
Reaction to Tension
When equal loads are applied in a direction outward from the bone surface, tension is
created. Maximum tensile stress occurs in a direction perpendicular from the applied force
FIGURE 8.1
Stress
e
strain curve illus-
trating the stages of elastic and plastic
deformation that bone undergoes before
failure.
