How Does Bone Strength Compare Across Sex, Site, and Ethnicity?

Background The risk of fragility fractures in the United States is approximately 2.5 times greater among black and white women compared with their male counterparts. On average, men of both ethnicities have wider bones of greater cortical mass compared with the narrower bones of lower cortical mass among women. However, it remains uncertain whether the low cortical area observed in the long bones of women is consistent with their narrower bone diameter or if their cortical area is reduced beyond that which is expected for the sex differences in body size and external bone size. Questions/purposes We asked (1) do black and white women consistently have narrower bones of less strength across long bones compared with black and white men; and (2) do all long bones of black and white women have reduced cortical area compared with black and white men? Methods Peripheral quantitative CT was used to quantify bone strength and cross-sectional morphology from the major long bones of 125 white and 115 black adult men and women (20–35 years of age). Regression analyses were used to test for differences in bone strength and cortical area after for adjusting for either body size, bone size, or both. Results After adjusting bone strength for body size, regression analyses showed that black women had lower bone strength compared with black men (women: mean = 298.7–25,522 mg HA mm 4 , 95% confidence interval [CI], 270–27,692 mg HA mm 4 ; men: mean = 381.6–30,945 mg HA mm 4 , 95% CI, 358.2–32,853 mg HA mm 4 ; percent difference = 12%–38%, p = 0.06–0.0001). Similarly, white women also had lower bone strength compared with white men (women: mean = 229.5–22,892 mg HA mm 4 , 95% CI, 209.3–24,539 mg HA mm 4 ; men: mean = 314.3–29,986 mg HA mm 4 , 95% CI, 297.3–31,331 mg HA mm 4 ; percent difference = 27%–49%, p = 0.0001). All long bones of women for both ethnicities showed lower cortical area compared with men. After accounting for both body size and external bone size, black women (women: mean = 43.25–357.70 mm 2 , 95% CI, 41.45–367.52 mm 2 ; men: mean = 48.06–400.10 mm 2 , 95% CI, 46.67–408.72; percent difference = 6%–25%, p = 0.02–0.0001) and white women (women: mean = 38.53–350.10 mm 2 , 95% CI, 36.99–359.80 mm 2 ; men: mean = 42.06–394.30 mm 2 , 95% CI, 40.95–402.10 mm 2 ; percent difference = 6%–22%, p = 0.02–0.0001) were shown to have lower cortical area than their male counterparts. Therefore, the long bones of women are not only more slender than those of men, but also