Trendelenburg sign

Pathophysiology: Normally, when a person stands on one leg, the gluteus medius and minimus on the stance leg contract to stabilize the pelvis and prevent it from dropping on the contralateral (swing) side. This action is mediated by the superior gluteal nerve (L4-S1). In a positive Trendelenburg sign, weakness or paralysis of these abductors on the stance leg causes the pelvis to sag toward the unsupported (swing) side, since the weak abductors cannot counteract the force of gravity pulling the pelvis down on the opposite side. The patient often compensates by leaning their trunk toward the affected (weak) side (Trendelenburg gait) to shift the center of gravity and reduce the biomechanical demand on the weakened abductors. Clinical significance: Causes of a positive Trendelenburg sign include: (1) Superior gluteal nerve injury - classically from iatrogenic injury during intramuscular injections in the wrong quadrant of the buttock, hip surgery (posterior approach to hip, total hip arthroplasty), or pelvic fractures. (2) Hip abductor muscle pathology - including muscular dystrophy, polio, or direct muscle damage. (3) L5 radiculopathy affecting the nerve roots that contribute to the superior gluteal nerve. (4) Chronic hip dislocation or developmental dysplasia of the hip (DDH) in children, where the abductors are mechanically disadvantaged due to altered hip anatomy. (5) Avascular necrosis of the femoral head. Testing: The patient is asked to stand on one leg while the examiner observes the pelvis from behind. A positive sign is when the pelvis drops on the side opposite the stance leg (i.e., if standing on the right leg, a drop of the left hip indicates right-sided superior gluteal nerve or abductor weakness). This is often confused with the Trendelenburg test for venous insufficiency, which is an entirely different test assessing saphenous vein valve competency - board questions should be read carefully to distinguish the two meanings.