Racial differences in hip axis lengths might explain racial differences in rates of hip fracture. Study of Osteoporotic Fractures Research Group.

Compared with white women, Asian women have about a 40%-50% and blacks a 50%-60% lower risk of hip fracture, but the reason for this racial difference is not known. Women with a shorter hip axis have a lower risk of hip fracture. To test the hypothesis that a shorter hip axis length could account for the lower risk of hip fracture among Asian and black women, we measured hip axis length in 135 Caucasian, 74 Asian and 50 black women. The mean hip axis lengths of Asian and black women were significantly shorter (1.2 and 0.7 standard deviations, respectively) than that of the whites (p < 0.0001). We estimate that, compared with white women, Asians would have a 47% lower risk (95% confidence interval: 32%-63%) and blacks would have a 32% (15%-45%) lower risk of hip fracture because of their shorter hip axis. We conclude that a shorter hip axis length might be a major factor accounting for Asian women's lower risk of hip fracture and might contribute to the lower risk in black women.