Cortical Bone Mechanics and Composition: Effects of Age and Gender - Skeletal Aging and Osteoporosis: Biomechanics and Mechanobiology - page 72

Fig. 15 Age-related changes
in the released elastic strain
energy of human cortical
bone (diaphyseal tibia) in
tension. No significant
changes exist except that
elderly bone breaks at a
significantly reduced applied
strain
4.2.2 Strength
There is clear evidence that human cortical bone strength deteriorates with age by
*2-5% per decade. For example, a study on bone obtained from donors ranging
from 20 to 98 years old reported that femoral yield strength (tension) decreases
2.2% per decade, while tibial yield strength decreases 0.5% per decade [ 60 ]. In the
same report, the ultimate tensile strength of femoral and tibial cortical bone
decrease at rates of 2.1 and 1.2% per decade, respectively, while the compressive
strengths decrease at 2.5 and 2.0% per decade, respectively [ 60 ]. In addition, a
study on cadaveric femurs ranging between 20 to 102 years old reported that
tensile ultimate stress and ultimate strain decrease by 5 and 9% per decade,
respectively [ 109 ]. These authors also reported that changes in porosity account
for *76% of the reduction in bone strength, whereas changes in calcium content
(a measure of mineralization) play a minor role in age-related changes. Similarly,
another study reported that the tensile strength of cortical bone decreases by 3.7%
per decade with increasing age [ 128 ]. In bending, cortical bone strength is
diminished by about 15-20% between the ages of 35 and 70, equivalent to
4.3-5.7% reduction per decade [ 129 ].
4.2.3 Toughness
Measures of cortical bone toughness decline with age at a faster rate than measures
of strength. For example, toughness (energy absorption to failure) was reported to
drop 12% per decade from 20 to 102 years old [ 109 ]. Similarly, it is reported that
the critical stress intensity factor (K C ) falls by 4.1% per decade, J-integral by 3%
per decade, and the work to fracture (W f ) by 8.7% per decade [ 128 ]. Such dramatic
age-related changes in bone toughness are more determined by the final failure
strain rather than the mechanism of plastic energy dissipation as a function of