Relationship of femoral neck areal bone mineral density to volumetric bone mineral density, bone size, and femoral strength in men and women

Abstract

Using combined dual-energy X-ray absorptiometry (DXA) and quantitative computed tomography, we demonstrate that men matched with women for femoral neck (FN) areal bone mineral density (aBMD) have lower volumetric BMD (vBMD), higher bone cross-sectional area, and relatively similar values for finite element (FE)-derived bone strength.

Introduction: aBMD by DXA is widely used to identify patients at risk for osteoporotic fractures. aBMD is influenced by bone size (i.e., matched for vBMD, larger bones have higher aBMD), and increasing evidence indicates that absolute aBMD predicts a similar risk of fracture in men and women. Thus, we sought to define the relationships between FN aBMD (assessed by DXA) and vBMD, bone size, and FE-derived femoral strength obtained from quantitative computed tomography scans in men versus women.

Methods: We studied men and women aged 40 to 90 years and not on osteoporosis medications.

Results: In 114 men and 114 women matched for FN aBMD, FN total cross-sectional area was 38% higher (P < 0.0001) and vBMD was 16% lower (P < 0.0001) in the men. FE models constructed in a subset of 28 women and 28 men matched for FN aBMD showed relatively similar values for bone strength and the load-to-strength ratio in the two groups.

Conclusions: In this cohort of young and old men and women from Rochester, MN, USA who are matched by FN aBMD, because of the offsetting effects of bone size and vBMD, femoral strength and the load-to-strength ratio tended to be relatively similar across the sexes.

Figure 1

(A) Areal BMD (aBMD) of the femur neck (FN) by DXA; (B) FN total cross-sectional area by QCT; (C) FN total volumetric BMD (vBMD) by QCT; (D) FN trabecular vBMD by QCT; and (E) FN cortical vBMD by QCT in 116 women and 116 men matched for FN aBMD. To better reflect the relative differences between women and men for each of the parameters, all values are normalized to a value of 100 for women. Bars denote SDs. *P < 0.05, **P < 0.01, and ***P < 0.001 versus men.

Figure 2

Representative cross-sectional images of the FN in men and women matched for aBMD by DXA.

Figure 3

(A) Areal BMD (aBMD) of the femur neck (FN) by DXA; (B) FN total cross-sectional area by QCT; (C) FN total volumetric BMD (vBMD) by QCT; (D) FN trabecular vBMD by QCT; (E) FN cortical vBMD by QCT; (F) FE-derived bone strength; (G) estimated fall loads; and (H) load-to-strength ratio, Φ, in 28 women and 28 men matched for FN aBMD. To better reflect the relative differences between women and men for each of the parameters, all values are normalized to a value of 100 for women. Bars denote SDs. *P < 0.05, **P < 0.01, and ***P < 0.001 versus men.

Figure 4

Representative FE models in a male and female subject matched for FN aBMD. The color bar shows regions of relatively high to low bone material properties on a per-voxel basis.