Evaluation of hip fracture risk in relation to fall direction.

The purpose of this study is to evaluate hip fracture risk in relation to fall direction, and to elucidate factors that influence the impact force in falls on the hip. Eight healthy volunteers performed deliberate falls in three directions (lateral, posterolateral and posterior) on a force platform covered by a mattress of 13 cm thickness. Fall descent motions and impact postures were examined by a three-dimensional analyzer. The maximum ground force reaction, velocity of the greater trochanter at impact, and activity of quadriceps and gluteus medius were measured. In all trials of lateral and posterolateral falls, but not of posterior falls, the subjects hit their greater trochanter directly on the mattress. The impact forces were between 2,000 N and 4,000 N. Posterolateral falls showed significantly higher velocity at impact than did posterior falls. The height and the lower limb length exhibited positive correlations with the impact force in all directions of fall. In the lateral fall, there was a positive correlation between the activity of quadriceps and the impact force. In view of the impact point, force, and velocity, the posterolateral fall seemed to carry the highest risk of hip fracture.