Femoral Nerve Injury with Medial Thigh Sensory Loss
If a patient has lost sensation in the anterior thigh after hip surgery, you should expect sensory loss in the medial thigh as well, indicating femoral nerve injury.
Anatomical Distribution of Femoral Nerve Injury
The femoral nerve provides sensory innervation to both the anterior and medial aspects of the thigh through its terminal branches 1:
- Anterior thigh sensation: Supplied by anterior cutaneous branches of the femoral nerve
- Medial thigh sensation: Supplied by the saphenous nerve (terminal sensory branch of femoral nerve) and medial cutaneous branches 1
When the femoral nerve is injured during hip surgery, both anterior and medial thigh sensory loss typically occur together because these are sequential distributions of the same nerve 1.
Motor Deficits to Assess
Beyond sensory loss, femoral nerve injury presents with characteristic motor deficits 2, 3:
- Quadriceps weakness (manual muscle test <3) is the hallmark finding 2
- Inability to extend the knee against resistance 3
- Difficulty with straight leg raising 2
Mechanisms of Femoral Nerve Injury in Hip Surgery
The femoral nerve is at risk during hip arthroplasty through several mechanisms 2, 4:
- Improper anterior acetabular retractor placement: The most common cause, with retractors placed too medially compressing the nerve 2
- Excessive leg lengthening: Stretches the nerve beyond its tolerance 2
- Hip extension and traction: Places maximum tension on the femoral nerve during femoral preparation 4
- Spread of local anesthetic: From regional blocks (femoral, fascia iliaca, or lumbar plexus blocks) can cause temporary femoral nerve dysfunction 1, 3
Distinguishing from Lateral Femoral Cutaneous Nerve Injury
Critical pitfall: Do not confuse this with lateral femoral cutaneous nerve (LFCN) injury, which is far more common in anterior approach hip surgery 5, 6:
- LFCN injury: Causes numbness/paresthesias over the lateral thigh only, with no motor deficit 5, 6
- Femoral nerve injury: Causes anterior AND medial thigh sensory loss PLUS quadriceps weakness 2, 3
- LFCN injury occurs in 16-36% of direct anterior approach cases, while femoral nerve palsy occurs in only 1.1% 5, 6, 2
Prognosis and Recovery
Femoral nerve palsy following hip surgery typically has favorable outcomes 2:
- Complete motor recovery occurs within one year in most cases 2
- Sensory symptoms may persist longer but generally improve with time 2
- No specific treatment beyond observation and physical therapy is typically required 2, 3
Regional Anesthesia Considerations
If the sensory loss appeared immediately postoperatively, consider iatrogenic causes from regional blocks 1, 7:
- Femoral nerve blocks cause significant quadriceps weakness and should be avoided in favor of fascia iliaca blocks 1, 8
- Lumbar plexus blocks can affect femoral, obturator, and lateral cutaneous nerves simultaneously 1
- Temporary femoral nerve palsy from local anesthetic spread typically resolves within 8-24 hours 3