Superior gluteal nerve injury and the Trendelenburg sign
The superior gluteal nerve supplies gluteus medius and minimus, hip abductors (muscles that move the leg out to the side).
Remember
When the superior gluteal nerve is injured, gluteus medius and minimus are weak, so the pelvis drops on the unsupported side (the side of the lifted leg) when standing on the affected leg, which is a positive Trendelenburg sign.
Mnemonic
Sassy Irish Sailors Pester Parrots
- S Superior gluteal
- I Inferior gluteal
- S Sciatic
- P Posterior femoral cutaneous
- P Pudendal
It lists the five main nerves of the sacral plexus in order, and the first, the superior gluteal nerve, is the one that keeps the pelvis level.
The superior gluteal nerve is a major branch of the sacral plexus (a network of spinal nerves lying beside the sacrum). It has three spinal nerve contributions: L4, L5 and S1. It leaves the pelvis by the greater sciatic foramen (a large opening at the back of the pelvis), above the piriformis muscle (a deep buttock muscle that acts as a landmark). It has no sensory function, so there is no sensory loss after injury.
Its motor supply is gluteus medius, gluteus minimus and tensor fascia lata (a small muscle on the outer hip). Gluteus medius and minimus are hip abductors, and when you stand on one leg they contract on the standing side to keep the pelvis level.
If the nerve is damaged, gluteus medius and minimus are weak, so the pelvis drops on the unsupported side (the side of the lifted leg) when standing on the weak leg. This is a positive Trendelenburg sign. A pelvis that drops on the unsupported side points to the superior gluteal nerve on the standing side.
Doctors test for the sign by asking the patient to stand on each leg in turn while watching the level of the iliac crests (the upper bony rims of the pelvis).
The inferior gluteal nerve is a nearby nerve. It leaves below piriformis and supplies gluteus maximus, the main hip extensor, and it affects hip extension, not pelvic stability.
Key facts
- Spinal nerves
- the superior gluteal nerve has three spinal nerve contributions: L4, L5 and S1.
- Leaves pelvis
- by the greater sciatic foramen, superior to the piriformis muscle.
- Muscles
- it supplies gluteus medius, gluteus minimus and tensor fascia lata.
- Sensation
- it has no sensory function.
- Sign
- in a positive Trendelenburg sign the pelvis drops on the unsupported side when standing on the weak leg.
- Contrast
- the inferior gluteal nerve supplies gluteus maximus and leaves below piriformis.
Exam traps
- A Trendelenburg sign points to the superior gluteal nerve, not the inferior gluteal nerve, which supplies gluteus maximus, the main hip extensor, and so affects hip extension.
- Loss of hip extension power is not the effect of superior gluteal nerve injury, because gluteus maximus, the main hip extensor, is supplied by the inferior gluteal nerve.
- The obturator nerve does not supply the abductors that level the pelvis; it supplies muscles that adduct the hip (pull the thigh towards the midline), the opposite action to abduction.
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