F racture incidence varies markedly among racial and ethnic groups. Hip and nonvertebral fractures are consistently highest among whites, lowest among those of African descent and intermediate among Asians. Specifically, age-adjusted hip fracture incidence ranges from a low of 1 per 100,000 personyears in black women in Nigeria to a high of 421 per 100,000 person-years in white women in Norway, respectively. Variation in vertebral fracture incidence by ethnicity is less well established, although the incidence of clinical vertebral fractures is at least as high in Asians as whites. An improved understanding of the mechanisms underlying these variations in fracture rates may provide insight for identifying those at greatest risk for fracture and potentially identify novel targets for intervention.
Some of the ethnic-related variation in fracture rates is explained by differences in bone mineral density (BMD). However, even after accounting for differences in BMD by ethnicity, fracture rates differ. For example, at every level of BMD, black women have a lower risk of nonvertebral fracture than white women. (3) Furthermore, despite having the lowest BMD values, postmenopausal Asian Americans have a lower risk of nonvertebral fracture than white, Hispanic, black, and Native American women. All together, these observations suggest that mechanisms other than differences in BMD are responsible for differences in fracture rates. This may be so because BMD measurements do not quantify some of the factors that contribute to bone strength, such as bone tissue properties, morphology, and microarchitecture. Indeed, relatively little is known about race-and ethnicity-related differences in these characteristics because until recently, there were no tools to assess bone structure adequately in vivo. In this month's JBMR, Walker and colleagues use highresolution peripheral computed tomography (HR-pQCT) to compare bone microarchitecture at the distal radius and tibia in postmenopausal Chinese-American and white women. These results extend previous work showing that premenopausal Chinese women have smaller bone cross-sectional area but