"Trendelenburg sign" is positive in damage of the following nerve -

TRENDELENBURG's sign : This is a test for postural stability when the patient stands on one leg. In normal two-legged stance the body's centre of gravity is placed midway between the two feet. Normally, in one-legged stance, the pelvis is pulled up on the unsuppoed side and the centre of gravity is placed directly over the standing foot. If the weightbearing hip is unstable, the pelvis drops on the unsuppoed side; to avoid falling, the person has to throw his body towards the loaded side so that the centre of gravity is again over that foot. If the difference between the two hips is marked you can detect it by simply looking at the patient's stance. However, small differences are not so obvious. In the classical Trendelenburg test the examiner stands behind the patient and looks at the buttockfolds. Normally in one-legged stance the buttock on the opposite side rises as the person lifts that leg; in a positive (abnormal) test the opposite buttock-fold. The causes of a positive Trendelenburg sign are: (1) pain on weightbearing; (2) weakness of the hip ABDUCTORS. (3) shoening of the femoral neck; and (4) dislocation or subluxation of the hip. Gluteus medius and Gluteus minimus are the abductors of hip which is supplied by SUPERIOR GLUTEAL NERVE. REF:Apley&;s system of ohopaedics- 9th edn-pg no 494.