Extends the knee, flexes the hip and supplies sensation to the anteromedial thigh and medial leg
Mnemonic
“The femoral nerve extends the knee and flexes the hip”, from L2, L3 and L4:
- Motor - quadriceps femoris (knee extension), iliacus, sartorius, pectineus (hip flexion)
- Sensory - anterior thigh, and via the saphenous nerve the medial leg and foot down to the medial malleolus
- Reflex - the knee jerk (L3, L4)
“L3, 4 kick the door” for the reflex.
It enters the thigh lateral to the femoral artery and outside the femoral sheath, which is why it is the “N” of NAVEL but is not enclosed with the vessels.
Femoral nerve injury gives inability to extend the knee, a buckling gait, an absent knee jerk and anterior thigh numbness. Causes include pelvic surgery, lithotomy positioning, a psoas haematoma in anticoagulated patients, and diabetic amyotrophy.
The saphenous nerve is its only branch below the knee, and is purely sensory, which is why an adductor canal block preserves quadriceps power.
Expansion
- Motor - the quadriceps (knee extension) plus iliacus, sartorius and pectineus (hip flexion)
- Sensory - the anterior and medial thigh, and, through its saphenous branch, the medial leg and foot
- Roots - L2 to L4
A femoral nerve lesion causes a weak, buckling knee and loss of the knee jerk.