Page 251 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

228 BIOMECHANICS OF THE HUMAN BODY

FIGURE 9.7 Reductions of human cortical bone mechanical properties with age. (a) Modulus is not reduced much,
if at all, whereas strength is reduced more, at a rate of about 2 percent per decade. (From Ref. 25.) (b) Ultimate strain
decreases markedly with age, at a rate of about 10 percent of its young value per decade. (From Ref. 10.)

5 percent strain at ages 20 to 30 years to a low of less than 1 percent strain above age 80 years 10
(see Fig. 9.7b). Thus the energy to fracture, given by the total area under the stress-strain curve
before fracture, is much less for old bone than for younger bone. As discussed below, fracture
mechanics studies also show a decrease in the fracture toughness with aging. For these reasons, old
cortical bone is more brittle than young bone. It is not currently clear if this age-related brittleness
arises from hypermineralization or collagen changes, although it appears that the latter is more
plausible, since mineralization does not change much in adult humans with aging. 10 Many of these
age-related changes in mechanical properties are to be expected, since porosity increases with age.
However, there are concurrent changes in other aspects of the tissue microstructure and composi-
tion such that porosity is not simply a surrogate measure of age. For example, although strength and
ductility clearly decrease with age in adults, there is controversy over whether elastic modulus
changes with age. 10,25,26
Although cortical bone is viscoelastic, the
effect of loading rate on modulus and strength is
only moderate. Over a 6 orders of magnitude
increase in strain rate, modulus only changes
by a factor of 2 and strength by a factor of 3
27
(Fig. 9.8). Thus, for the majority of physiological
activities that tend to occur in a relatively narrow
range of strain rates (0.01 to 1.0 percent strain
per second), the monotonic response of cortical
bone reasonably can be assumed to have minor
rate effects. Similarly, dynamic sinusoidal experi-
ments indicate that the loss tangent attains a
broad minimum (0.01 to 0.02) over the range of
physiological frequencies. 28,29 These values,
FIGURE 9.8 Strain-rate sensitivity of cortical bone for
longitudinal tensile loading. Typically, modulus and which are lower than those for polymers by a
strength increase only by factors 2 and 3, respectively, as factor of 10, indicate that significant mechanical
the loading rate is increased by 6 orders of magnitude. The damping does not occur within this frequency
higher strain rates shown here may occur in vehicular range. Clearly, in extraordinary situations such
accidents or gunshot wounds. (Data from Ref. 27.)
as high-speed trauma, strength properties can
increase by a factor of 2 to 3, and this should be
recognized. Additionally, it has been found that loading rate has a significant effect on the accumulation
of damage within the tissue. Slower loading rates produce higher numbers of acoustic emission
events, but these events are of lower amplitude than those emitted at faster rates. 30