Factor of Risk for Fracture - Skeletal Aging and Osteoporosis: Biomechanics and Mechanobiology - page 136

Factor of risk analysis shows that both osteopenic and osteoporotic individuals
are likely to suffer a hip fracture in a fall. Based on analysis of 166 post-meno-
pausal women using QCT-based finite element analysis to estimate femoral
strength, 87% of osteoporotic (i.e femoral BMD t-score B-2.5) and 84% of
osteopenic (femoral BMD t-score between -1 and -2.5) women were likely to
have a hip fracture in a fall to the side, whereas only 35% of women with normal
BMD (femoral BMD t-score [ -1) were predicted to fracture during fall to the
side [ 34 ]. In addition, factor of risk was found to be only weakly correlated
(r 2 = 0.14) with total hip aBMD. In a somewhat different approach, Roberts et al.
[ 64 ] examined factor of risk in 73 cadaveric femurs, using the directly measured
strength value for a sideways fall configuration, and estimating fall loads from
subject height and weight. Nearly all osteoporotic femurs (femoral neck
t-score \ -2.5) and half of non-osteoporotic femurs, had a factor of risk [1.0,
indicating that they would be at high risk for a fracture in a sideways fall, whereas
only one in six femurs with femoral BMD t-score [ -1 had a factor of risk
value C1.0.

2.1.2 Association Between the Factor of Risk and Hip Fracture

Several studies have shown that factor of risk for hip fracture is increased in both
men and women with incident hip fractures. In an early retrospective study, Myers
et al. [ 50 ] reported a strong association between the factor of risk and hip fracture
in 231 elderly fallers, 98 cases with hip fracture and 138 controls without fracture.
Yang et al. [ 77 ] found 26 women with hip fractures to have higher factor of risk
than 85 healthy age-matched controls. In a prospective, nested case-control study,
21 postmenopausal women with incident hip fracture had reduced trochanteric soft
tissue thickness, reduced femoral aBMD and increased factor of risk compared to
42 age-matched controls [ 6 ]. The force applied to the femur was calculated both
with and without attenuation due to trochanteric soft tissue. Without soft tissue
attenuation the factor of risk was greater than one in both cases and controls, while
with soft tissue attenuation the factor of risk approached one in cases and was less
than one in controls (0.92 ± 0.44 and 0.62 ± 0.50, respectively). In this study, the
association with hip fracture was similar for femoral BMD (OR = 2.1, CI 1.2-3.5)
and the factor of risk (OR = 1.85, CI 0.96-3.6).
Orwoll et al. [ 56 ] studied the factor-of-risk for hip fracture in a prospective
study of community-dwelling men over age 65 with an average of 5.6 years of
followup. In a nested case-cohort analysis of 40 men who suffered a hip fracture
and 210 who did not, femoral strength was determined from baseline QCT scans
using finite element analysis and fall loads estimated using a fixed trochanteric soft
tissue thickness of 25 mm. Factor of risk was higher in fracture cases than non-
cases (1.13 ± 0.41 vs. 0.75 ± 0.24; p \ 0.01). Cox proportional hazards regres-
sion indicated that low femoral aBMD (hazard ratio, HR = 4.4, 95% CI 2.1-9.1),
low femoral strength (HR = 6.5; 95% CI 2.3-18.3) and high factor-of risk
(HR = 4.3, 95% CI 2.5-7.4) were significant risk factors for hip fracture,