Trendelenburg sign and the hip abductors
Trendelenburg sign: standing on one leg, the pelvis drops on the raised-leg side, usually from weak abductors (muscles moving the thigh away from the midline) on the standing side.
Remember
Work out which leg is on the ground first: the weak muscle is on that side, and the pelvis drops on the other side, the side of the raised leg.
When someone stands on one leg, body weight tends to tip the unsupported side of the pelvis downwards. The gluteus medius (a fan-shaped muscle on the outer surface of the pelvis, lying between gluteus maximus and gluteus minimus) contracts on the standing side to hold the pelvis level. The smaller gluteus minimus lies directly beneath it and works with it.
Both muscles are hip abductors. Abduction means moving the thigh away from the midline. Both are supplied by the superior gluteal nerve, so an injury to that nerve weakens them together.
If the muscles are weak, or the nerve is damaged, the unsupported side sags. That is a positive Trendelenburg sign. The sign usually indicates weakness of these two hip abductors.
Work out first which leg is on the ground. The weak muscle is on that side, and the pelvis drops on the other side, the side of the raised leg. If the pelvis drops on the left when a woman stands on her right leg, the right gluteus medius is the weak one. If instead the left abductors are weak, the right side drops when she stands on her left leg.
Gluteus maximus and iliopsoas do not hold the pelvis level when standing on one leg. Gluteus maximus extends the hip, and iliopsoas is the main flexor of the hip, lifting the thigh forwards.
Key facts
- The sign
- the pelvis drops on the unsupported side when standing on one leg.
- Where the weakness is
- on the standing side, opposite the drop.
- The muscles
- gluteus medius and gluteus minimus, with the minimus lying directly beneath the medius.
- Their nerve
- the superior gluteal nerve supplies both, so one injury weakens both.
- Their action
- abduction of the hip, and medial rotation (turning the thigh inwards).
- Spared in superior gluteal nerve injury
- gluteus maximus extends the hip and is supplied by the inferior gluteal nerve, so it is spared.
Exam traps
- Gluteus medius on the swing side (the raised leg) is not the weak muscle, because it is the right muscle on the wrong side: the one that stops the drop is on the leg bearing the weight.
- Gluteus maximus on the standing side does not hold the pelvis level, because it is the powerful hip extender; gluteus medius, supported by gluteus minimus, does that job.
- Iliopsoas does not abduct the hip or hold the pelvis level, because it is the main flexor of the hip; its weakness would affect lifting the leg rather than cause a pelvic drop.
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