Factor of Risk for Fracture - Skeletal Aging and Osteoporosis: Biomechanics and Mechanobiology - page 131

1 Introduction to the Factor of Risk
From the simplest mechanical perspective, a fracture occurs when an applied load
on a bone exceeds its strength, or failure load. Thus, both loading and strength
must be considered when examining the risk of fractures. The loading experienced
by bones in vivo varies considerably depending on the specific activity. For
example the forces on the proximal femur during a fall impact will naturally be
greater than those during quiet standing. The strength of bones also varies
considerably, both between individuals and within an individual throughout life.
Bone strength depends in general on the geometry and material properties of the
bone [ 3 , 68 ]. Of particular clinical relevance is bone mineral density (BMD),
which is the measure most commonly used to diagnose osteoporosis. Areal BMD
(aBMD) is defined as bone mineral content (BMC) divided by projected bone area,
typically measured using dual-energy X-ray absorptiometry (DXA). Because
BMD incorporates information about bone density and size, it provides a useful,
albeit imperfect, indirect measure of bone strength. aBMD accounts for about 50 to
80% of the variability in whole bone strength [ 4 , 8 , 37 , 38 , 48 ].
The importance of the interaction between skeletal loading and bone strength
has been demonstrated in several retrospective case-control studies. Nevitt and
Cummings [ 52 ] studied elderly women who fell and suffered a hip fracture
(n = 130), women who fell and suffered a wrist fracture (n = 294) and women
who fell and did not fracture (n = 467). They reported that among women who
fell on or near their hip, those who fell sideways or straight down were at fourfold
increased risk for hip fracture compared to those who fell in other directions,
whereas those who fell backward were less likely to suffer a hip fracture (odds
ratio 1 = 0.2). Those who fell forward were more likely to suffer a wrist fracture.
Among those who fell either on their hand or hip, there was a twofold higher risk
of fracture for every standard deviation decrease in BMD. In another study,
Greenspan et al. [ 24 ] compared community-dwelling elderly individuals who fell
and suffered a hip fracture (n = 72) to those who fell and did not fracture
(n = 77). They found that low hip BMD, low body mass index and characteristics
related to the fall itself were independent risk factors for hip fracture (Table 1 ).
Taken together, these studies confirm an important interaction between bone
strength (as reflected by BMD), skeletal loading, and fracture risk.
Insight into the relative contributions of skeletal strength and skeletal loading to
the etiology of fracture may be gained by evaluating the ratio of load applied to the
bone (applied load) to strength of bone (failure load). This comparison of applied
load versus failure load gives an estimate of how ''safe'' the structure is from
failure, and is termed the ''factor of risk'', U [ 3 , 26 , 34 , 68 ]:
U = Applied load/Failure load
1 odds ratio (OR) is defined as the odds of an event occurring in one group, divided by the odds
of the it occurring in another group.