Original articleEffects of gender, anthropometric variables, and aging on the evolution of hip strength in men and women aged over 65
Introduction
Osteoporosis is a disease characterized by increased skeletal fragility in elderly bones but fractures are more prevalent in women than in men. Gender differences in the rates of falling may play a role in the difference in fracture rates, but male bones may also be intrinsically stronger or may suffer less structural degradation with age than those of women. Investigations of gender differences in fragility most commonly use dual-energy X-ray absorptiometry scanners to measure bone mineral density (bone mineral content/projected area, or areal BMD, in g/cm2). Many population-based studies have firmly established that reduced BMD is associated with increased risk of hip fracture [1], [2], [3], [4]. Using that statistical link to provide a direct understanding of the mechanics of osteoporotic fragility is problematic because BMD does not describe a unique structural or material strength property. That is, a range of different and realistic bone configurations with very different mechanical strengths can have the same BMD [5], [6], [7]. The ability of bone to resist the tensile, compressive, and torsional forces placed upon it by life’s activities cannot be described by the amount of bone present, without taking into account how that tissue is distributed through the bone cross sections. Clearly that distribution is altered with aging and in osteoporotic fragility.
One important aspect of those changes is subperiosteal expansion. Based on principles enunciated by Euler 250 years ago and validated many times since, long bones gain structural rigidity as their diameter expands even for an unchanging bone mineral content (BMC). This type of expansion tends to reduce both areal and volumetric BMD due to the increase in area/volume, even without any bone being lost, but permits bending strength to be maintained. In light of this, it has been suggested that studying geometry alongside conventional bone mass measurements could lead to a better understanding of the processes leading to increased fracture risk with age as well as the gender differences in fracture risk. Recently, DXA-based techniques for estimating the structural strength of the proximal femur have been developed. Geometric information and absorptiometry data are integrated into calculations that apply engineering beam theory to regions such as the femoral neck, trochanter, and the proximal shaft [8], [9], [10]. These geometric properties have been shown to change with age [11], to differ between men and women [12], [13], to have the potential to improve hip fracture prediction [14], [15], [16], and to vary across diverse populations [17], [18].
We have undertaken a descriptive study of the evolution with age of hip structural parameters alongside BMD in a randomly selected population sample who were invited to participate in a study of diet and common disease endpoints in Eastern England. We used hip structural analysis (HSA) to study gender differences in hip geometry and bone mass as they evolved over time in elderly white men and women with the ultimate aim of identifying candidate biological pathways leading to a heightened risk of hip fracture in both genders. In this paper, we have explored the contributions of gender, age, weight, and height to measurements related to bone strength in the proximal femur. We have also explored the effects of these variables and their changes on hip structural strength over time.
Section snippets
Subjects and BMD measurement
We recruited 1511 men and women aged 65+ from a prospective population-based cohort study, the European Prospective Investigation of Cancer (EPIC) in Norfolk, UK, into a study of hip BMD loss. Hip BMD was measured on two occasions 2–5 years apart (mean = 2.7) by dual-energy X-ray absorptiometry (DXA) using a Hologic 1000 W bone densitometer. Particular attention was paid to achieving the same positioning on the second occasion as on the first and all scans were performed by the same operator.
Gender differences
Table 1 shows the cross-sectional gender differences at baseline for the measured bone variables. The differences were tested after adjusting for the effects of age, weight, and height. The magnitudes of the gender differences in all parameters reduced after adjustment, apart from the buckling ratio in which the differences slightly increased. The most noticeable effect of the adjustment was on differences in section modulus and cross-sectional area and indices of bending and axial strength,
Discussion
There are several key findings in this study with regard to gender differences in hip structure. More importantly there are both similarities and striking differences in the way that hip structure changed over time in these elderly men and women. These patterns should help to explain the higher incidence of hip fracture observed in white women compared to white men and should also illuminate the structural changes that underlie declining density in elderly individuals. As their higher hip
Conclusions
In conclusion, expansion of the proximal femur occurs in both sexes, being faster in women than in men over 65 years of age. This is sometimes accompanied by thinning of the cortices, due to faster expansion of the endosteal than of the periosteal envelope. Section modulus is lower in women than in men, but this may be due to proportionately lower muscle loads. However, section modulus does not decline at the same rate as BMD, implying that part of the effect of aging on BMD is due to expansion
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