Trendelenburg sign is positive in damage of the following nerve -

Trendelenberg test : It tests the stability of the hip and particularly of the abductor mechanism of the limb on which the patient is standing. The principle of the test : Normally when one leg is raised from the ground the pelvis tilts upwards on that side, through the action of the hip abductors of the standing limb. If the abductors are inefficient they are unable to sustain the pelvis against the body weight and it tilts downwards instead of rising up on the side of the lifted leg. Note that the test is performed with the patients standing on the affected leg. Abductor mechanism : It can be compared with the lever, with the body weight is trying to tilt the pelvis down; the hip joint being the fulcrum and the abductor muscles being the power acting through the lever arm (from the head of the femur to attachment of the abductor's muscles on the greater trochanter - the neck of the femur) Causes of the positive Trendelenburg test : Any failure of the abductor mechanism causes dipping of the pelvic (ASIS) on the normal side. There are three fundamental causes : 1)   Paralysis of the abductor's muscles eg. Polio. Paralysis of superior gluteral nerve (Supplies gluteus medius & minimus) 2) Marked approximation of the insertion of the abductor muscles to their origin by upward displacement of the greater trochanter ; so that the muscles are slack. eg. Severe coxa vara, dislocation of hip 3) The absence of stable fulcrum & lever arm about which the abductor muscles can act Neck femur fracture Dislocation of hip, Destructruction of head as in Perthe's diseases.