After hip surgery, a patient has lost sensation in the anterior thigh, which other area is also likely to experience loss of sensation?

Femoral Nerve Injury After Hip Surgery: Expected Sensory Distribution

The correct answer is A. Medial calf, as the saphenous nerve (a terminal sensory branch of the femoral nerve) provides sensation to both the anterior thigh and medial calf/medial leg extending down to the medial malleolus. 1

Anatomical Basis

When a patient loses sensation in the anterior thigh after hip surgery, this indicates femoral nerve injury. The femoral nerve has a predictable sensory distribution pattern:

• The femoral nerve provides sensory innervation to both the anterior and medial aspects of the thigh through its terminal branches 1
• The saphenous nerve, which is the terminal sensory branch of the femoral nerve, continues distally to supply sensation to the medial calf and extends down to the medial malleolus 1
• Therefore, femoral nerve injury affecting the anterior thigh will also affect the medial calf region through saphenous nerve involvement 1

Mechanism of Injury in Hip Surgery

Femoral nerve injury during hip surgery occurs through several mechanisms:

• The anterior acetabular retractor placement is the most common cause, with the femoral nerve being closest to the acetabular rim at 90° (only 16.6 mm away), making this the highest risk area 2
• Motor-evoked potential studies show that 77% of patients experience significant reduction in femoral nerve amplitude during retractor placement on the anterior acetabular wall 3
• Excessive leg lengthening during hip arthroplasty can also cause femoral nerve palsy 4
• Regional anesthesia techniques (femoral blocks, fascia iliaca blocks, or lumbar plexus blocks) can cause temporary femoral nerve dysfunction through local anesthetic spread 1

Clinical Presentation

The sensory loss pattern helps confirm femoral nerve injury:

• Anterior thigh sensory loss combined with medial calf/leg sensory loss indicates saphenous nerve involvement as part of femoral nerve injury 1
• Motor weakness of the quadriceps femoris (knee extension weakness) typically accompanies the sensory deficits 4
• The incidence of femoral nerve palsy following hip arthroplasty via the direct anterior approach is approximately 1.1%, with most cases recovering completely within one year 4

Why Other Options Are Incorrect

• Little toe (fifth digit): This is innervated by the sural nerve (from the sciatic nerve distribution), not the femoral nerve
• Medial malleolus alone: While the saphenous nerve does extend to this area, the medial calf is the more characteristic and extensive area of sensory loss that would be noticed clinically