What nerve dysfunction is implicated in the Trendelenburg sign?

Superior Gluteal Nerve Dysfunction is Implicated in Trendelenburg Sign

The Trendelenburg sign is primarily caused by dysfunction of the superior gluteal nerve, which innervates the hip abductor muscles (gluteus medius and minimus). When these muscles are weakened or paralyzed due to superior gluteal nerve damage, the patient cannot stabilize the pelvis during single-leg stance, resulting in the characteristic pelvic drop on the contralateral side 1, 2.

Anatomy and Function of the Superior Gluteal Nerve

The superior gluteal nerve:

• Originates from the posterior divisions of L4, L5, and S1 nerve roots 3
• Provides motor innervation to:
  • Gluteus medius
  • Gluteus minimus
  • Tensor fascia lata
• Provides sensory innervation to the superior part of the hip joint capsule 3

These muscles, particularly the gluteus medius and minimus, function as primary hip abductors and are essential for pelvic stabilization during gait.

Mechanism of Trendelenburg Sign

When a patient stands on one leg:

1. The hip abductors on the stance leg must contract to prevent the pelvis from dropping on the contralateral side
2. With superior gluteal nerve dysfunction, these muscles cannot generate sufficient force
3. The result is contralateral pelvic drop (positive Trendelenburg sign)
4. Patients may compensate with lateral trunk lean toward the affected side (compensated Trendelenburg gait)

Clinical Evidence

Research demonstrates a clear relationship between superior gluteal nerve damage and Trendelenburg sign:

• In a prospective study of patients undergoing hip surgery via the Hardinge approach, 23% showed evidence of superior gluteal nerve damage at two weeks post-operation 2
• Persistent damage to the superior gluteal nerve was strongly associated with a positive Trendelenburg test 2
• Another study found that 42.5% of patients had damage to the superior gluteal nerve after direct lateral approach to the hip, with some showing persistent changes at 6 months 1

Diagnostic Considerations

When evaluating a positive Trendelenburg sign:

1. Electromyography (EMG) can confirm superior gluteal nerve dysfunction 1, 2
2. MRI may be useful to evaluate the gluteal muscles and identify potential causes of nerve compression 4
3. Ultrasound can identify tendinopathy, partial tears, or complete tears/avulsion of the gluteus medius tendon 4

Clinical Significance

The threshold for hip abductor weakness to manifest as a Trendelenburg sign appears to be significant:

• Research suggests that hip abductor strength must fall below 30% of body weight before the Trendelenburg sign becomes apparent 5
• This explains why some patients with mild superior gluteal nerve dysfunction may not demonstrate the sign

Common Causes of Superior Gluteal Nerve Dysfunction

1. Iatrogenic injury during hip surgery, particularly with direct lateral approaches 1, 2
2. Trauma to the gluteal region or pelvis
3. Compressive neuropathy from masses or hematomas
4. Abductor tendon tears or avulsion from the greater trochanter 4, 6

Clinical Pitfalls and Caveats

1. A positive Trendelenburg sign does not always indicate nerve damage - it may also result from:

   • Primary muscle pathology
   • Tendon avulsion or tears
   • Pain inhibition of muscle function
   • Hip joint pathology

2. The most at-risk branch of the superior gluteal nerve during surgery is the muscular branch to the gluteus minimus muscle, as it runs closest to the greater trochanter 3

3. Not all patients with electromyographic evidence of superior gluteal nerve damage will demonstrate a positive Trendelenburg sign clinically 1

By understanding the relationship between superior gluteal nerve function and the Trendelenburg sign, clinicians can better diagnose and manage conditions affecting hip stability and gait.