What nerve is likely damaged when the left lower extremity is shortened and internally rotated after hearing a pop?

Sciatic Nerve Injury in Shortened and Internally Rotated Lower Extremity

A shortened and internally rotated left lower extremity after hearing a pop most likely indicates damage to the sciatic nerve, particularly when the injury occurs in the setting of hip dislocation or fracture. 1, 2

Mechanism of Injury

The clinical presentation of a shortened and internally rotated lower extremity strongly suggests:

• Hip dislocation or fracture causing direct trauma to the sciatic nerve
• The sciatic nerve is vulnerable to injury along its course from the pelvis through the buttock and posterior thigh
• The "pop" sound typically represents the femoral head dislocating from the acetabulum

Anatomical Considerations

The sciatic nerve is particularly susceptible to injury in this scenario because:

• It exits the pelvis through the greater sciatic foramen
• Passes beneath the piriformis muscle
• Courses along the posterior aspect of the hip joint
• Can be stretched, compressed, or directly injured during hip displacement 2, 3

Clinical Manifestations

When the sciatic nerve is damaged in this context, patients typically present with:

• Motor deficits: Weakness in hamstring muscles and all muscles below the knee
• Sensory deficits: Decreased sensation in the posterior thigh, lateral leg, and most of the foot
• Interestingly, the peroneal division of the sciatic nerve is more commonly affected than the tibial division 2

Diagnostic Approach

The American College of Radiology recommends:

• Initial radiographs of the hip and pelvis to assess for fracture or dislocation 1
• MRI for detailed evaluation of nerve injury and surrounding structures 4
• Electromyography (EMG) and Nerve Conduction Studies to confirm diagnosis and assess severity 4

Clinical Pearls and Pitfalls

• Sciatic nerve injuries can sometimes be mistaken for isolated peroneal nerve injuries due to the greater vulnerability of the peroneal division 2, 5
• The presence of a shortened, internally rotated limb is a classic sign of posterior hip dislocation or certain hip fractures 1
• Vascular assessment is critical as hip dislocations can also damage the femoral vessels 1
• Early recognition and prompt reduction of hip dislocations may minimize nerve injury 1

Management Considerations

• Urgent reduction of hip dislocation when present
• Surgical intervention for complete and persistent deficits 6
• Physical therapy and orthotic devices (ankle-foot orthosis) for foot drop 4
• Nerve action potential (NAP) recordings can guide whether neurolysis or resection/repair is needed 6

Prognosis

Recovery from sciatic nerve injury varies:

• Partial injuries have better prognosis than complete transections
• The tibial division generally shows better recovery than the peroneal division 6
• Approximately 36% of patients achieve significant recovery after surgical repair of the peroneal division 6
• Good-to-excellent outcomes are more common for the tibial division, even with proximal injuries 6

The sciatic nerve's vulnerability during hip trauma makes it the most likely nerve damaged when a shortened and internally rotated lower extremity presents after trauma with an audible pop.