Dual hip bone mineral density in postmenopausal women: geometry and effect of physical activity.

The objectives of this study were to (1) obtain both femoral neck strength (FNS) and hip axis length (HAL) values from left and right femurs (regardless of hip dominance) measured by DXA and evaluate their relationship with BMD of all hip regions including total hip, (2) determine if there is a difference between dominant and nondominant hip BMD in any of the hip regions, and (3) determine how physical activity influences hip BMD. Participants were 136, generally healthy Caucasian women (57.4-88.6 years). BMD was measured by DPX-MD. Past and present activity was assessed by the Allied Dunbar National Fitness Survey for older adults and normal/brisk walking pace was measured in a straight hallway. FNS analysis uses femoral geometry to calculate stresses at the femoral neck for two loading conditions: Safety Factor Index (SF) indicates risk of fracture for forces generated during a one-legged stance, and Fall Index (FI) indicates risk of fracture for forces generated during a fall on the greater trochanter. Simple and multiple regression analyses were used to determine predictive ability of HAL, SF, and FI for respective hip BMD values. There was no statistical difference in BMD between two hips in any of the measured regions, however, the nondominant hip correlated better with other skeletal sites. Subjects with a faster normal walking speed had higher neck BMD in the nondominant hip, 0.832 +/- 0.12 vs. 0.791 +/- 0.10 g/cm2 (P < 0.05). Longer HAL of the left hip was negatively related to neck, trochanter, shaft, and total hip BMD. FI was significantly associated with all sites of the hip BMD, while SF was associated only with neck and wards BMC (P < 0.05). In summary: (1) a longer HAL is associated with lower BMD and a higher FI with higher BMD, (2) it might be sufficient to measure BMD in only the nondominant hip, and (3) walking at a faster pace may positively benefit femoral neck BMD. Therefore, it appears that HAL, SF, and FI all play important roles in estimating fracture risk and should be assessed along with BMD when using DXA.