Do variations in hip geometry explain differences in hip fracture risk between Japanese and white Americans?

Despite lower femoral neck bone mass, Japanese women have a substantially lower incidence of hip fracture than North American whites. Reasons for this discrepancy were sought in a study of 57 Japanese and 119 white American women aged 50-79. All women were in good health. Bone mineral content (BMC) in the femoral neck, femoral neck length (NL), femoral neck angle (theta), cross-sectional moment of inertia (CSMI), safety factor (SF), and fall index (FI) were calculated using dual x-ray absorptiometry. Height and weight were greater in Americans than in Japanese (1.62 versus 1.52 m; p < 0.0001 and 66.0 versus 49.4 kg; p < 0.0001, respectively). Mean BMC in the femoral neck and CSMI were greater in Americans than in Japanese (3.91 versus 3.02 g; p < 0.0001 and 0.99 versus 0.57 cm4; p < 0.0001, respectively). NL was longer in Americans (5.6 versus 4.4 cm; p < 0.0001) and theta was larger in Americans (130 versus 128 degrees; p < 0.01), whereas SF and FI were less in Americans than in Japanese (3.41 versus 5.12; p < 0.0001 and 1.00 versus 1.40; p < 0.0001, respectively). These results indicate that despite lower bone mass, Japanese women have lower risks of structural failure in the femoral neck, attributable primarily to shorter femoral necks and, to a lesser degree, a smaller femoral neck angle. Geometric characteristics of the femoral neck in Japanese women are associated with their lower hip fracture risk, and the measurement of proximal femoral geometry, combined with bone mass, may provide further clinical information about the risk of hip fracture.