ArticlesRelation between age, femoral neck cortical stability, and hip fracture risk
Introduction
The risk of hip fractures increases ten-fold with every 20 years of age.1 When tubular structures such as long bones are bent, they often fracture through mechanical failure beginning in the cortex under tension. If they have thin walls they can instead break through local buckling of the compression cortex. Galileo pointed out that resistance to a bone's bending (measured by engineers as section modulus, Z) can be maintained with less material as its diameter is widened.2 However, without increasing the amount of bone tissue, cortical thinning will result, making buckling more likely.
Loss of bending resistance with normal ageing is modest.3 Ageing is unlikely to influence the risk of failure in tension because the thick inferomedial cortex bears this load in most dangerous falls.4 Bone mineral density declines with age, but age has an independent and strong effect on fracture risk after adjustment for bone mineral density. Growing asymmetry of the femur's internal structure might reduce the ability of the superior cortex either to resist crushing in compression,5 or to increase its tendency to develop local buckling or elastic instability as is generally thought to contribute to other types of fracture (eg, in lytic cancers or Paget's disease). We aimed to look for a large effect of ageing that, unlike the moderate rise in risk of falling,6 could be primarily responsible for the steeply exponential rise in hip fractures.
The sideways falls that lead to hip fracture7 compress the posterior part of the thin, superolateral cortex of the femoral neck (figure 1).4 This region is very lightly loaded in walking,8 the main physical activity of middle-aged and elderly people. So, in a sample of normal proximal femurs spanning a wide age range, we have investigated whether the superolateral cortex develops with age a geometry that is structurally unstable. We compared the sample against a large healthy population measured with clinical densitometry to establish that our cadaveric material was representative and also studied material from hip fracture cases.
Section snippets
Procedures
The Victorian Institute of Forensic Medicine obtained the proximal third of the femur under strict ethical regulation from 81 people older than 20 years who died suddenly. Relatives gave permission (initially verbal, later confirmed in writing) for use of the part femurs (66% compliance) and brief medical history data. After dual-energy X-ray absorptiometry scanning to measure the neck shaft angle and distance from head to mid-neck,5 we scanned the 77 (35 female) proximal femurs9 that showed no
Results
The table shows characteristics of individuals in the study. After adjustment to mean height and weight, areal femoral neck bone mineral density declined in women by a mean of 32% (SD 7) from age 20 to 80 years (adjusted r2 0·37, p<0·0001), equivalent to a reduction in T score from 0·0 to −2·3 (0·2 SD units above WHO's diagnostic threshold for osteoporosis), and similar to the decrease in NHANES 3. For the superior 2 cortical sectors, the distance from the centroid increased by 20% (95% CI
Discussion
We have shown that there is substantial loss of elastic stability with age, such that tissue toughness—the capacity to absorb energy through microscopic damage—might become unable to contribute to fracture prevention. This loss is mainly due to thinning of the superolateral cortex. These findings could profitably redirect the search for the real cause of the steep increase in hip fragility with age. In young people, a sideways fall will only fracture the femoral neck if the applied load is
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2022, Journal of Clinical DensitometryImpaired bone quality in the superolateral femoral neck occurs independent of hip geometry and bone mineral density
2022, Acta BiomaterialiaCitation Excerpt :Fractures of the proximal femur primarily initiate at the superolateral site, subsequently migrating in the inferomedial direction [32], which has previously been explained by nano-structural alterations in the lateral femoral neck [33]. Additionally, these observations are in line with reports by several groups indicating the mechanical and structural heterogeneity within the femoral neck, which reflect the site-specific loading pattern [34–39]. The superolateral site is believed to direct less mechanical stress during habitual loading compared to the inferomedial site, which is consistent with our previous observation that reductions in microstructure and osteocyte properties may promote fracture propagation [40].
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These authors contributed equally to this work