Factor of Risk for Fracture - Skeletal Aging and Osteoporosis: Biomechanics and Mechanobiology - page 135

of falls in the elderly are associated with a hip fracture [ 14 , 28 ]. This indicates that
the mechanics of the fall can have a major effect on the risk of fracture. For
example, falling to the side and impacting the hip or side of the leg increases the risk
of hip fracture approximately 20-fold relative to falling in any other direction [ 27 ].
A majority of hip fractures occur in individuals not classified as osteoporotic based
on BMD [ 67 , 69 - 71 , 76 ]. Thus, a factor of risk approach may be able to distinguish
the risk of hip fractures better than BMD.
To date, all studies of factor of risk for hip fracture have used a fall on the hip to
the side from standing height as the loading condition of interest. Loading has been
estimated based on laboratory studies of fall dynamics and impact mechanics
[ 65 , 66 , 73 , 74 ]. Thus, forces on the hip during a sideways fall are estimated using
subject-specific body height and weight, along with stiffness and damping
constants derived from these laboratory experiments [ 6 , 34 , 50 , 54 , 56 , 63 , 64 , 77 ].
The loading attenuation due to variation in the thickness of trochanteric soft tissues
was examined in two studies [ 6 , 54 ], whereas one study employed a uniform
trochanteric soft tissue thickness [ 56 ].
The denominator of the factor of risk, or load-bearing capacity of the proximal
femur, has been determined from linear regressions between BMD and femoral
failure loads in a fall configuration in several studies [ 6 , 50 , 54 , 77 ], whereas one
study used engineering beam-theory to predict femoral strength [ 63 ]. Two recent
studies estimated failure load using QCT-based finite element analysis [ 34 , 56 ],
and another measured failure load directly by mechanical testing of cadaveric
specimens [ 64 ].
2.1.1 Age- and Osteoporosis-Related Changes in the Factor
of Risk for Hip Fracture
A study of 788 (548 females, 240 males) healthy individuals aged 21-77 years in
Taiwan examined the change in factor of risk with age [ 77 ]. Factor of risk
increased with age in younger men (\50), but was relatively constant with age in
older men. However, in women factor of risk increased with age throughout life,
with the average factor of risk approaching one by age 80.
Riggs et al. [ 63 ] reported age- and sex-specific differences in the load to strength
ratio at the femoral neck in 700 men and women aged 21-97. They found that the
load to strength ratios for bending and axial loading were only marginally higher
(i.e, worse) in young women versus young men, but that the ratios increased
(i.e, worsened) nearly twofold more over life in women (+40-62%) compared
to men (+22-36%). This increase in the load to strength ratio was attributable to
greater declines in cortical and trabecular vBMD at the femoral neck in women than
men [ 62 ] as there was negligible age-related change in the load applied to the hip.
While this pattern explains some of the observed increase in hip fracture risk in
women, it did not fully explain the fourfold increase in hip fracture risk with age,
suggesting that additional factors not represented by the load to strength ratio, such
as greater incidence of falls, contribute to the increased risk of hip fracture with age.