Original Article Black–White Differences in Hip Geometry T. M. Theobald 1,2 , J. A. Cauley 1 , C. C. Gluer 3 , C. H. Bunker 1 , F. A. M. Ukoli 4 and H. K. Genant 5 for the Study of Osteoporotic Fractures Research Group* 1 Department of Epidemiology, University of Pittsburgh Graduate School of Public Health, Pittsburgh, Pennsylvania, USA; 2 Department of Medicine, Allegheny University of the Health Sciences, Pittsburgh, Pennsylvania, USA; 3 Research Group Medical Physics, Kiel, Germany; and 4 Department of Community Health, University of Berin Teaching Hospital, Berin City, Nigeria, 5 Department of Radiology–Skeletal Section, University of California, San Francisco, California, USA Abstract. Previous studies have demonstrated that reduced thickness of the femoral neck and shaft cortex, a wider intertrochanteric region and a longer hip axis length were predictive of hip fracture among Caucasian women. We hypothesized that racial differences in these features of hip geometry may contribute to explaining the differences in hip fracture incidence between women of African origin and Caucasian women. We measured the cortical thickness and bone widths in 132 African- American women and 43 Nigerian women who were pair-matched on height (+ 3 cm), age (+ 5 years) and weight (+ 3 kg) to 175 Caucasian women. Measures of cortical thickness were greater among women of African origin than Caucasian. Women of African origin had smaller bone widths and a shorter hip axis length than Caucasians. Several of these differences were indepen- dent of bone mineral density except for the cortical thickness of the femoral shaft. We conclude that women of African origin have thicker cortical bone of the hip, a shorter hip axis length and smaller intertrochanteric widths than Caucasians. Based on a model developed from hip fractures among Caucasian women, we predict that these observed racial differences could contribute to approximately a 25% decrease risk of hip fracture among blacks. Keywords: Black women; Cortical thickness; Hip frac- ture; Hip geometry; Nigerian women; Racial differences Introduction The risk of hip fracture in Caucasian women is twice that of black women [1–3]. The difference in the hip fracture incidence rates may partly be due to the higher bone mass in black women compared with Caucasians [4,5]. Quantitative ultrasound as a measure of the quality of bone has failed to demonstrate a racial difference between African-American and Caucasian-American women [6]. However, the geometry of the hip may play a role. A previous study found hip axis lengths to vary among Caucasian, African-American and Asian women [7], the African-American and Asian women having significantly shorter hip axis lengths than the Caucasians. These results are consistent with the association between hip axis length and hip fracture [8]. Additional measures of hip geometry may also differ by ethnic group. Simple measures from standard radiographs have been found to be predictive of hip fracture in Caucasian women [9]. Relative thinness of the cortical bone of the femoral shaft and neck, and a wider intertrochanteric region have been shown to be associated with an increased risk of hip fracture. To our knowledge, these hip geometry measures have not been studied in blacks. We hypothesize that women of African origin may have a more protective pattern of hip geometry measurements which may contribute to their lower incidence of hip fracture. Osteoporos Int (1998) 8:61–67 ß 1998 European Foundation for Osteoporosis and the National Osteoporosis Foundation Osteoporosis International *See Appendix for a list of members of the Study of Osteoporotic Fractures Research Group. Correspondence and offprint requests to: T. M. Theobald, Department of Medicine, Allegheny University of the Health Sciences, 320 East North Avenue, Pittsburgh, PA 15212, USA.