This study is based upon observations made upon the hip joints of 12 embalmed cadavers. Windows were made into the joints and the effects of extension and flexion of the femur, produced both actively and passively, were noted. Particular attention was paid to associated movements. Use was made of x-rays as well as direct observation.
It was determined that, as the femur extends it abducts and medially rotates; a terminal medial rotation takes up the last of the slack in the iliofemoral and ischiofemoral ligaments. The locked position therefore is brought about by these three movements. There is no evidence of any medial displacement or translation of the femoral head on extension, as is commonly stated to occur. The passively extended femur also abducts and tends to rotate medially whereas the flexed femur laterally rotates and falls into a position about midway between abduction and adduction. The same general effects are reproducibile in the skeleton articulated in such a way as to leave hip motion practically unhindered. This would indicate that the ligaments play a passive role, restricting motion within physiological limits. The shape of the acetabulum and the direction toward which it faces, together with the topography of the articular surface of the head of the femur, are no doubt responsible for the associated movements.