Sex- and race-related differences in cross-sectional geometry and bone density of the femoral mid-shaft in older adults.

BACKGROUND: Femoral shaft fracture incidence increases in older adults and is associated with low-energy trauma. Apart from bone density, the distribution and size of bone contributes to its strength.

AIM: To examine if bone geometry and density of the femoral mid-shaft in older adults differs by sex and race, we studied 197 White women, 225 Black women, 242 White men, and 148 Black men aged 70-79 years participating in the Health, Aging, and Body Composition study; a prospective cohort study in the USA. A secondary purpose of the study was to examine the association of site-specific muscle and fat to bone geometry and density.

SUBJECTS AND METHODS: Subjects were community-dwelling and reported no difficulty walking one-quarter of a mile or climbing stairs. Mid-femoral volumetric bone mineral density (vBMD, mg cm(-3)), total area (TA), cortical area (CA), medullary area (MA), cross-sectional moments of inertia (CSMI: I(x), I(y), J), and muscle and fat areas (cm(2)) were determined by computed tomography (CT; GE CT-9800, 10 mm slice thickness).

RESULTS: vBMD was greater in men than women with no difference by race (p < 0.001). Bone areas and area moments of inertia were also greater in men than women (p < 0.001), with Black women having higher values than White women for TA and CA. Standardizing geometric parameters for body size differences by dividing by powers of femur length did not negate the sex difference for TA and MA. Significant differences (p < 0.05) among the four groups also remained for I(x) and J. Mid-thigh muscle area was an independent contributor to TA in all groups (Std beta = 0.181-0.351, p < 0.05) as well as CA in women (Std beta = 0.246-0.254, p < 0.01) and CSMI in White women (Std beta = 0.175-0.185, p < 0.05). Further, muscle area was a significant contributor to vBMD in Black women.

CONCLUSION: These results indicate that bone geometry and density of the femoral diaphysis differs primarily by sex, rather than race, in older well-functioning adults. In addition, site-specific muscle area appears to have a potential contributory role to bone geometry parameters, especially in women.