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 mm4, 95% confidence interval [CI], 270-27,692 mg HA mm4; men: mean = 381.6-30,945 mg HA mm4, 95% CI, 358.2-32,853 mg HA mm4; 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 mm4, 95% CI, 209.3-24,539 mg HA mm4; men: mean=314.3-29,986 mg HA mm4, 95% CI, 297.3-31,331 mg HA mm4; 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 mm2, 95% CI, 41.45-367.52 mm2; men: mean=48.06-400.10 mm2, 95% CI, 46.67-408.72; percent difference=6%-25%, p=0.02-0.0001) and white women (women: mean=38.53-350.10 mm2, 95% CI, 36.99-359.80 mm2; men: mean=42.06-394.30 mm2, 95% CI, 40.95-402.10 mm2; 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 show a reduced cortical area that is 6% to 25% greater than expected for their external size, depending on the bone being considered.
Conclusions: The long bones of females are not just a more slender version of male long bones. Women have less cortical area than expected for their body size and bone size, which in part explains their reduced bone strength when compared with the more robust bones of men.
Clinical relevance: The outcome of this assessment may be clinically important for the development of diagnostics and treatment regimens used to combat fractures. Future work should look at how the relationship among parameters reported here translates to the more fracture-prone metaphyseal regions.