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Osteoporosis International
Published in: Osteoporosis International 5/2011

01-05-2011 | Original Article

Ethnic differences in femur geometry in the women's health initiative observational study

Authors: D. A. Nelson, T. J. Beck, G. Wu, C. E. Lewis, T. Bassford, J. A. Cauley, M. S. LeBoff, S. B. Going, Z. Chen

Published in: Osteoporosis International | Issue 5/2011

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Abstract

Summary

Participants in the observational study of the Women's Health Initiative (WHI) were studied to determine if ethnic differences in femur geometry can help to explain differences in hip fracture rates. Structural differences in femurs of African and Mexican-American women appear to be consistent with lower rates of hip fractures vs. whites.

Introduction

Ethnic origin has a major influence on hip fractures, but the underlying etiology is unknown. We evaluated ethnic differences in hip fracture rates among 159,579 postmenopausal participants in the WHI then compared femur bone mineral density (BMD) and geometry among a subset with dual X-ray absorptiometry (DXA) scans of the hip and total body.

Methods

The subset included 8,206 non-Hispanic whites, 1,476 African-American (AA), 704 Mexican-American (MA), and 130 Native Americans (NA). Femur geometry derived from hip DXA using hip-structure analysis (HSA) in whites was compared to minority groups after adjustment for age, height, weight, percent lean mass, neck-shaft angle and neck length, hormone use, chronic disease (e.g., diabetes, rheumatoid arthritis, cancer), bone active medications (e.g., corticosteroids, osteoporosis therapies), and clinical center.

Results

Both AA and MA women suffered hip fractures at half the rate of whites while NA appeared to be similar to whites. The structural advantage among AA appears to be due to a slightly narrower femur that requires more bone tissue to achieve similar or lower section moduli (SM) vs. whites. This also underlies their higher BMD (reduces region area) and lower buckling ratios (buckling susceptibility). Both MA and NA women had similar advantages vs. whites at the intertrochanter region where cross-sectional area and SM were higher but with no differences at the neck. NA and MA had smaller bending moments vs. whites acting in a fall on the hip (not significant in small NA sample). Buckling ratios of MA did not differ from whites at any region although NA had 4% lower values at the IT region.

Conclusion

Differences in the geometry at the proximal femur are consistent with the lower hip fracture rates among AA and MA women compared to whites.
Appendix
Available only for authorised users
Glossary
AA
Women self identified as African-American
Centroid
Location of the center of mass of the hard tissue in the cross-section, measured by HSA from the mineral mass profile. Used in computing the CSMI and SM, but also useful in evaluating the symmetry of a cross-section since fragile NN and IT cross-sections are more asymmetric due to greater differences in cortical thickness on opposite margins
Cross-sectional area (CSA)
Total bone surface in a cross-section, exclusive of soft tissue spaces and pores. Forces directed along a long bone (axial forces) are distributed over the CSA, hence axial compressive stress varies inversely with CSA
Cross-sectional moment of inertia (CSMI)
The cross-sectional surface weighted by the square of distance from the center of mass of the cross-section. Bending stress within a cross-section depends inversely on the CSMI divided by the distance from the center of mass.
Geometry or structural geometry
The dimensions of the supporting material within an object, expressed in engineering terms especially within cross-sections
Geometric strength
That component of strength that governs stresses, not the ability of the material to withstand them, (currently cannot be measured by non-invasive methods). A method that evaluates only geometry (HSA and CT based methods) can only assess geometric strength.
HSA
Software program used to extract femur geometry from conventional DXA scan images of the hip at specific regions consisting of five parallel lines traversing the femur neck, separated by one pixel spacing. Measurements are computed for each line then averaged.
IT
Intertrochanter region HSA program, traversing the femoral neck at its narrowest point.
NN
Narrow-neck region used in HSA program, traversing the femoral neck at its narrowest point.
S
Shaft region located at a distance of 1.5× minimum neck outer diameter, distal to the intersection of the neck and shaft axes (ethnic differences in S region results were less remarkable and are not reported here).
MA
Women self identified as Mexican-American
NA
Women self-identified as Native American or American Indian
Local buckling
A failure mode where thin, relatively flat regions of a structure under compressive loads may bend (fold or crumple) locally leading to complete failure of a cross-section. Propensity to this failure mode is crudely estimated from DXA data with the buckling ratio (BR) where a higher value is associated with greater buckling susceptibility (see Methods).
Section modulus (SM)
Maximum bending stress in a cross-section is located at maximum distance from the center of mass (d max) and is thus inversely related to the SM, where SM = CSMI/d max.
Stress
force concentrations (per unit area) within an object from applied loads. Types of stress (i.e., tension, compression, torsion and shear) depend on how loads are applied, but magnitudes depend entirely on geometry of the object.
Strength
The loading force applied under a specific condition that causes internal stresses to exceed the material limits. Strength cannot actually be measured in vivo although it can be predicted using engineering information on the bone geometry, the loading conditions and the material strength.
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Metadata
Title
Ethnic differences in femur geometry in the women's health initiative observational study
Authors
D. A. Nelson
T. J. Beck
G. Wu
C. E. Lewis
T. Bassford
J. A. Cauley
M. S. LeBoff
S. B. Going
Z. Chen
Publication date
01-05-2011
Publisher
Springer-Verlag
Published in
Osteoporosis International / Issue 5/2011
Print ISSN: 0937-941X
Electronic ISSN: 1433-2965
DOI
https://doi.org/10.1007/s00198-010-1349-4

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