Biomedical Engineering Reference
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Fig. 14 Age-related changes
in stiffness loss of human
cortical bone (diaphyseal
tibia) in tension. Stage I and
Stage II are the transition and
saturation region of bone
viscous responses,
respectively. e
y
indicates
the yield strain
4.2 Age-Related Changes in Mechanical Properties of Bone
4.2.1 Elastic Properties
Because of the well-documented increase in cortical porosity with age (reviewed
in the previous section), and the strong, negative relationship between elastic
modulus and porosity [
56
], it is expected that cortical bone elastic modulus
declines with age. Yet available data indicate either no change or, at most, a slight
decrease in the elastic modulus of human cortical bone with age. For example,
reports of age-related decreases in the elastic modulus of human femoral cortical
bone range between 1.5 and 2.3% per decade from a maximum value at *35 years
of age [
60
,
128
], although one study reported no significant decline from age 20 to
102 [
109
]. Results for tibial bone are mixed; one study reported an age-related
increase in tensile modulus of *1.5% per decade, but a decrease in compressive
modulus of *4.0% per decade [
60
]. Note that this result [
60
] is questionable,
because it is not consistent with the well-supported conclusion by the same
investigators that the tensile and compressive moduli of cortical bone do not
differ [
62
].
Elastic modulus loss for cortical bone with respect to applied strain is not
significantly correlated with age. For example, a study on human cadaveric
femora from middle aged (50-60 years old) and elderly ([70 years old) donors
indicates that no significant difference exists in the curve of elastic modulus
versus applied strain except that the elderly (old aged) bone breaks at much
smaller strain compared with middle aged bone (Fig.
14
). This also can be
verified by the elastic strain energy released by newly formed damage surfaces
as shown in Fig.
15
, indicating that the trend of released elastic strain energy
dissipation with respect to the applied strain is similar for both age groups
(i.e., middle aged vs. elderly).
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