Factor of Risk for Fracture - Skeletal Aging and Osteoporosis: Biomechanics and Mechanobiology - page 141

Factor of risk was 24% higher in cases as compared to controls (p \ 0.01).
However, vBMD (OR = 4.2; 95% CI 1.4-12), cortical thickness (OR = 4.0; 95%
CI 1.4-11), and bone axial rigidity (OR 3.8; 95% CI 1.4-10) were all similar
predictors of fracture compared to factor of risk (OR = 3.0; 95% CI 1.2-7.5). In
another cross-sectional case-control study, 100 post-menopausal women with
prevalent distal forearm fracture were compared to 105 without [ 46 ]. Factor of risk
was 15% higher in those with prevalent fractures (OR 1.9; 95% CI 1.4-2.6).
However, factor of risk did not offer an improvement over aBMD (OR 2.0; 95% CI
1.4-2.8) in terms of clinical assessment of fracture risk. In addition, the mean
factor of risk among controls was greater than one, indicating possible under-
estimation of bone strength and/or overestimation of fall loads. Another inter-
pretation would be that most post-menopausal women would be at high risk for
fracture when falling on the hand. Finally, factor of risk was reported for a cross-
sectional case-control study including 33 women with incident wrist fractures
and 33 controls matched for age, height, weight and age at menopause taken from
a prospective cohort examining determinants of bone loss [ 2 ]. Factor of risk was
about 16% higher in cases than in controls (1.08 ± 0.16 vs. 0.93 ± 0.19,

p \ 0.001).

3 Discussion

The factor of risk is a conceptually simple approach for estimating the risk of
fracture that considers both the strength of bone and loading applied to bone, and
thus offers a theoretical advantage compared to measures based only on bone
strength. However, studies examining factor of risk to date have had mixed results;
there has not been compelling evidence that factor of risk provides a major
improvement in predicting the risk of a fracture over common measures such as
BMD. Inaccuracies and uncertainties in estimates of loading and/or strength could
easily reduce the usefulness of the approach, as factor of risk does not account for
variability or uncertainty in either strength or loading estimates. An alternative,
albeit more complex, approach that can account for such variability is the use of
injury risk functions [ 29 ].
While its simplicity makes factor of risk an attractive approach, this may belie
the challenges associated with accurately determining loading and strength of
bone. Examinations of factor of risk to date have primarily used simple estimates
of loading. However, in vivo bone loading is complex, and may not always be
easily represented by a single applied load. For example, muscle contraction
during fall descent may help in absorbing energy, but muscle contraction during
impact can increase the impact load by 25-100% [ 28 ]. Muscle action may also act
to protect bone, as muscular contraction significantly increases the load and energy
required to fracture the tibia in rats [ 55 ]. Thus, muscle action may either increase
or reduce the loading applied to bone, and thereby affect the factor of risk.