Course
The femoral nerve runs from the lumbar spine to the front of the thigh in three stages: the iliac fossa, the groin and the femoral triangle.
Origin
It forms inside psoas major from the posterior divisions of the ventral rami of L2, L3 and L4. The obturator nerve takes the anterior divisions of the same rami, so the two nerves share roots but supply opposing muscle groups: extensors of the knee versus adductors of the hip.
In the iliac fossa
The nerve emerges from the lateral border of psoas major and descends in the gutter between psoas and iliacus, deep to the iliac fascia. Here it gives branches to iliacus. Psoas major itself receives direct twigs from the ventral rami of L1–L3, not from the femoral nerve.
At the inguinal ligament
It passes deep to the inguinal ligament through the muscular compartment (lacuna musculorum), lateral to the femoral artery. It lies outside the femoral sheath, separated from the artery by the iliopsoas fascia.
In the femoral triangle
Within a few centimetres of the ligament the trunk breaks into a spray of anterior and posterior divisions. The lateral circumflex femoral artery passes between the two divisions, a reliable landmark when dissecting the triangle.
Branches and distribution
The femoral nerve supplies the flexors of the hip that cross the front of the joint and all four heads of the knee extensor, plus a long strip of skin from groin to foot.
| Level | Motor | Cutaneous | Articular |
|---|---|---|---|
| Iliac fossa | Iliacus | None | None |
| Anterior division | Sartorius, pectineus | Medial and intermediate cutaneous nerves of thigh | None of note |
| Posterior division | Rectus femoris, vastus lateralis, vastus intermedius, vastus medialis | Saphenous nerve | Hip (via nerve to rectus femoris), knee (via nerves to the vasti) |
Pectineus often has a dual supply, with a second twig from the obturator nerve or the accessory obturator nerve when present.
Saphenous nerve
The saphenous nerve is the longest branch of the femoral nerve and the only one to pass below the knee. It runs lateral to the femoral artery in the femoral triangle, enters the adductor canal and crosses in front of the artery from lateral to medial. At the lower end of the canal it pierces the fascial roof, becomes subcutaneous between the tendons of sartorius and gracilis, and gives an infrapatellar branch to the skin in front of the knee. It then accompanies the great saphenous vein down the medial leg, passes in front of the medial malleolus and ends on the medial border of the foot.
Relations
The key relation of the femoral nerve is the femoral artery, which lies immediately medial to it at the inguinal ligament.
- In the pelvis: the nerve lies lateral to the external iliac artery, separated from it by psoas major.
- At the groin: from lateral to medial the order is femoral nerve, femoral artery, femoral vein, then the femoral canal containing lymphatics. The artery, vein and canal are inside the femoral sheath; the nerve is not.
- In the femoral triangle: the nerve lies on iliacus, deep to the fascia lata, with sartorius forming the lateral boundary of the triangle.
- At the apex: only two branches continue into the adductor canal, the saphenous nerve and the nerve to vastus medialis.
Clinical relevance
Femoral nerve injury weakens knee extension, reduces the knee jerk (L3, L4) and numbs the front of the thigh and the medial leg.
Causes
- Retroperitoneal or iliacus haematoma, especially on anticoagulation or in haemophilia, compressing the nerve under the iliac fascia.
- Pelvic and hip surgery: self-retaining retractors, anterior approaches to the hip and prolonged lithotomy positioning.
- Femoral arterial puncture and catheterisation, or a femoral pseudoaneurysm.
- Psoas abscess and pelvic tumours.
Signs
The patient reports the knee giving way, especially on stairs or walking downhill. Hip flexion is only mildly weak because psoas major is spared. Hip adduction is normal, which separates a femoral nerve lesion from an L3 or L4 radiculopathy or a lumbar plexopathy, both of which also weaken the obturator-supplied adductors.
Procedures
Femoral and fascia iliaca blocks are used for analgesia after fractured neck of femur; the injection is placed just lateral to the femoral pulse. The saphenous nerve is at risk during great saphenous vein stripping or harvest, and its infrapatellar branch can be cut by medial knee incisions, leaving an area of numbness below the patella.