Hip axis length in mid-life Japanese and Caucasian US residents: no evidence for an ethnic difference

The rate of hip fracture in Japanese women is approximately 50% lower than that of Caucasian women. Shorter hip axis length (HAL) in Japanese women is one possible explanation underlying the observed ethnic difference in fracture risk. We measured HAL in a community-based sample of Japanese (n=267) and Caucasian (n=199) women, aged 46 years on average, residing in Los Angeles, California. We hypothesized that HAL would be shorter in the Japanese women compared with the Caucasian women, and that this ethnic difference would persist after height adjustment. Unadjusted mean HAL was 101.45 mm in Japanese women and 105.98 mm in Caucasians (p<0.0001). Average height-adjusted HAL was 103.24 and 103.58 mm in Japanese and Caucasians, respectively (p=0.52). The relation between HAL and height was linear across the range of values observed in each ethnicity. Results were similar in analyses that compared Caucasians to Japanese women born in the US (n=142) or outside the US (n=125). We conclude that there was no Japanese-Caucasian difference in height-adjusted HAL in our study sample.