Biomedical Engineering Reference
In-Depth Information
adjusting for age and BMI. Importantly, after additionally adjusting for femoral
aBMD, the factor of risk remained a significant risk factor for hip fracture
(HR = 3.1, 95% CI 1.6-6.1) while femoral strength did not (HR = 2.7, 95% CI
0.5-14.6).
2.2 Studies of the Factor of Risk for Vertebral Fractures
Vertebral fractures are the most common osteoporotic fracture, accounting for
about 27% of all osteoporotic fractures [ 7 ]. Although only a minority of radio-
graphically evident vertebral deformities come to clinical attention [ 11 , 16 , 53 ],
they are associated with significant morbidity and are strong predictors of future
fracture risk [ 15 , 36 , 53 ]. Unlike the clear association between hip fractures and
falls, the events and resultant spinal loading associated with vertebral fractures
remain unclear. In a hospital-based study, nearly 50% of acute, symptomatic
vertebral fractures in individuals over age 60 were associated with a fall, whereas
20% were associated with 'controlled' activities, such as bending, lifting, and
reaching [ 51 ]. In a prospective study of older men, approximately 57% of incident
clinically evident vertebral fractures were associated with a fall, but 21% occurred
in unknown circumstances [ 22 ]. In a retrospective study of circumstances asso-
ciated with clinically diagnosed vertebral fractures, a specific loading event was
reported for only about 50% of the total fractures [ 11 ]. Of this 50%, 10% of
fractures were associated with ''lifting a heavy object'', whereas nearly 40% were
associated with falling. Thus, while it appears that many vertebral fractures are due
to falls, many occur in unknown conditions. Since loads are applied to the spine
during every activity of daily living, it is crucial to distinguish which activities
are most strongly associated with increased risk of vertebral fractures.
Studies of factor of risk for vertebral fractures have primarily examined forward
flexion and/or forward flexion while lifting a weight [ 5 , 17 - 19 , 44 , 49 ]. Most of
these studies have estimated vertebral loads using simple biomechanical models
that include only a single spinal extensor muscle and have focused on the L3
vertebra. The exception is the work of Myers and Wilson [ 49 ], which examined the
L2 vertebra and used an optimization approach to estimate muscle forces and
vertebral compression. Such an approach may be more appropriate in estimating
vertebral loading, as it includes multiple trunk muscles and allows for other
activities to be modeled, such as twisting or asymmetric lifting, such as carrying a
suitcase. No studies have examined factor of risk for vertebral fracture during a
fall, likely due to the difficulty in estimating vertebral loading during a fall.
The majority of studies examining vertebral factor of risk have estimated ver-
tebral strength using empirical relationships with BMD. Myers and Wilson used a
linear relationship between strength and aBMD [ 49 ], while Duan and coworkers
used a power law relationship between strength and vBMD [ 17 - 19 ]. Bouxsein et al.
[ 5 ] determined strength from the vertebral body elastic modulus and cross-sectional
area, where elastic modulus was estimated from a linear relationship with vBMD.
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