Biomedical Engineering Reference
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generated QCT-based FEA models of the proximal femur from 362 women and
317 men (aged 21-89 years) from the Rochester, MN study group of Riggs et al.
[ 50 ], and reported that estimated femoral strength (fall configuration) was strongly
age dependent. Femoral strength declined by 55 % in women and 39 % in men.
The age-dependence was roughly bi-modal, with negligible changes until the mid-
40s for women and the mid-50s for men, followed by sharp declines (Fig. 3 a).
Notably, the declines in strength accelerated with increasing age and were much
greater than the corresponding declines in areal BMD (Fig. 3 b). The authors noted
that the deficit in strength in older women compared to older men was due mostly
to a delay in the onset of decline for men rather than differences in initial values or
rates of decline. We note that the strong, linear decline in strength after middle-age
is at odds with the experimental findings from Roberts et al. [ 59 ], who found a
non-significant decline after middle-age. This discrepancy may be due to differ-
ences in techniques (experimental vs. FEA) or to differences in samples (cadavera
from donors of unknown racial/ethnic makeup vs. a random population sample
[98 % white] from Rochester, MN). Additional experimental work is needed to
clarify this discrepancy.
In a recent longitudinal study, Lang et al. [ 61 ] used QCT-based FEA to estimate
changes in failure force of the proximal femur over a 5-year period in 112 elderly
women and 111 elderly men (avg. age at baseline 77 years). Loading conditions
were applied to stimulate stance and fall configurations, using methods previously
validated by Keyak and co-workers [ 62 ]. At baseline, the failure load in a fall
configuration was 29 % lower for women than men, and in a stance configuration
was 25 % less for women than men. These sex differences are essentially the same
as the sex differences in proximal femur size described in the previous section.
Women had significantly greater declines in trabecular BMD and strength than
men. In the fall configuration, the rate of change in strength was -25 %/decade in
women compared to -14 %/decade in men. These declines can be attributed to
loss of both trabecular and cortical bone, although the rates of trabecular loss were
much greater.
The difference in average bone strength between women and men has been
noted in all the studies we reviewed, and is consistently attributed to size differ-
ences. But do women and men ''build'' bones of similar structure? A recent study
by Srinivasan et al. [ 63 ] used QCT-based FEA models to test whether women and
men who have similar femoral neck areal BMD values have similar femoral
strengths. They reported that in 114 women and 114 men with matched aBMD, the
equivalence of aBMD was the result, on average, of greater bone size in men
(38 % greater bone area) combined with lower volumetric bone density (16 %
lower vBMD). Estimated femoral bone strength in 28 women and 28 men matched
for aBMD showed similar values of bone strength and load-to-strength ratio in the
two groups. Thus, based on this relatively small sample, it appears that the
proximal femur of women and men have different structural ''designs'' that can
accomplish similar function (strength).
Compared to the steep declines in estimated strength of the proximal femur,
declines in strength of the distal tibia are much less. MacDonald et al. [ 51 ]
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