Root Origin
- Spinal nerve roots: L2, L3, L4
- Plexus: Lumbar plexus (forms within the psoas major muscle)
- Type: Mixed (motor and sensory)
Course
- Lumbar plexus. The nerve forms within the substance of the psoas major muscle from the posterior divisions of L2, L3, and L4 ventral rami. This intramuscular formation means the psoas itself can compress the nerve — relevant in psoas abscess, hematoma, or marked hypertrophy.
- Iliacus groove. The nerve emerges from the lateral border of the psoas major and descends between the psoas and iliacus in the iliac fossa, deep to the iliac fascia. It lies in the groove between the two muscles — lateral to the psoas, medial to the iliacus.
- Inguinal ligament. The nerve passes beneath the inguinal ligament to enter the anterior thigh. It lies lateral to the femoral artery (the relationship is Nerve-Artery-Vein-Empty space-Lymphatics, lateral to medial — the "NAVEL" mnemonic). The nerve is separated from the femoral vessels by the iliopectineal arch. The inguinal ligament region is a potential compression site.
- Femoral triangle. Immediately below the inguinal ligament, the nerve enters the femoral triangle (bounded by the inguinal ligament superiorly, sartorius laterally, and adductor longus medially). The nerve divides rapidly into anterior and posterior divisions within or just below the femoral triangle — this division occurs only 2-4 cm below the inguinal ligament, which is why femoral nerve lesions are almost always "high" and affect multiple branches.
- Anterior division. Supplies the sartorius and gives off the intermediate and medial cutaneous nerves of the thigh (anterior thigh sensation).
- Posterior division. Supplies the quadriceps (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius) and gives off the saphenous nerve — the longest sensory branch, which continues into the leg and foot.
Motor Distribution
Anterior Division
| Muscle | Action | Notes |
|---|---|---|
| anatomy/muscles/sartorius | Hip flexion, external rotation, knee flexion | The longest muscle in the body; part of the pes anserine group at the knee |
Posterior Division
| Muscle | Action | Notes |
|---|---|---|
| anatomy/muscles/rectus-femoris | Knee extension, hip flexion | The only quadriceps muscle that crosses the hip; two-joint muscle |
| anatomy/muscles/vastus-lateralis | Knee extension | Largest of the quadriceps group |
| anatomy/muscles/vastus-medialis | Knee extension | The VMO (vastus medialis oblique) is critical for patellar tracking; first quad component to atrophy with disuse |
| anatomy/muscles/vastus-intermedius | Knee extension | Deepest quad — lies directly on the femoral shaft |
| anatomy/muscles/iliacus | Hip flexion | Often included; the nerve supplies iliacus before passing under the inguinal ligament in many individuals |
| anatomy/muscles/pectineus | Hip flexion, adduction | May receive femoral or obturator innervation (or both) depending on the individual |
Sensory Distribution
- Anterior cutaneous branches (intermediate and medial cutaneous nerves of the thigh). Supply the anterior thigh from the inguinal ligament to the knee.
- Saphenous nerve (terminal sensory branch). The longest branch of the femoral nerve. It descends through the adductor canal (Hunter's canal) with the femoral artery, emerging subcutaneously at the medial knee. It then continues down the medial leg alongside the great saphenous vein to the medial foot.
- Territory: Medial knee, medial leg, medial ankle, medial foot (to the level of the first metatarsophalangeal joint)
- Clinical significance: The saphenous nerve is the ONLY nerve below the knee that is NOT from the sciatic nerve. If a patient has medial leg numbness with otherwise normal lower extremity function, the saphenous nerve (femoral territory) is the source — not the sciatic nerve.
Entrapment Sites
1. Within the Psoas Major
- Location: Where the nerve forms within the psoas muscle belly
- Structure: Psoas abscess (from spinal infection), retroperitoneal hematoma (anticoagulant use), or psoas hypertrophy can compress the nerve within the muscle
- Presentation: Anterior thigh pain and weakness of knee extension and hip flexion. Pain worsens with hip extension (which stretches the psoas over the entrapped nerve). May present with a flexed hip posture — the patient avoids extension.
- MT relevance: Psoas abscess is a medical emergency. If a patient presents with acute anterior thigh pain, fever, and a flexed hip posture, refer immediately. Psoas hematoma in patients on anticoagulants produces a similar picture without fever.
2. Inguinal Ligament
- Location: Where the nerve passes beneath the inguinal ligament
- Structure: The nerve can be compressed by the inguinal ligament during prolonged hip flexion (lithotomy position during surgery, prolonged cycling with aggressive hip flexion angle), tight clothing, or after inguinal hernia repair.
- Presentation: Anterior thigh numbness, quadriceps weakness, diminished patellar reflex. Post-surgical femoral neuropathy is one of the most common iatrogenic nerve injuries — particularly after gynecological or abdominal procedures performed in the lithotomy position.
- MT relevance: Post-surgical quadriceps weakness with anterior thigh numbness in a patient who recently had abdominal or pelvic surgery should raise suspicion for femoral nerve compression at the inguinal ligament. Recovery depends on the severity — most neurapraxia cases recover in weeks to months.
3. Adductor Canal (Saphenous Nerve Branch)
- Location: The saphenous nerve can be compressed within the adductor canal (Hunter's canal), a fascial tunnel in the mid-thigh between the vastus medialis, adductor longus/magnus, and the sartorius
- Structure: The subsartorial fascia roofing the canal can compress the saphenous nerve. Also compressed by the vasto-adductor membrane at the canal exit.
- Condition: Saphenous nerve entrapment (see anatomy/nerves/saphenous-nerve for details)
- Presentation: Medial knee and leg pain or numbness without motor weakness — the saphenous nerve is purely sensory at this level.
Clinical Tests
| Test | Procedure | Positive Finding | What It Tells You |
|---|---|---|---|
| Patellar reflex (L3-L4) | Tap the patellar tendon with the patient sitting, legs dangling. | Diminished or absent reflex compared to the opposite side. | Femoral nerve or L3-L4 nerve root. The patellar reflex is the most reliable single test for femoral nerve integrity — it tests the quadriceps arc directly. |
| Resisted knee extension | Patient extends the knee against resistance from a seated position. | Weakness (grade 0-5) compared to the opposite side. | Quadriceps motor function. Femoral nerve palsy produces inability to extend the knee against gravity (grade <3), causing the knee to buckle during walking. |
| Femoral nerve stretch test (reverse SLR) | Patient prone. Passively flex the knee, then extend the hip. | Reproduction of anterior thigh pain radiating along the femoral nerve distribution. | Tension on the femoral nerve and L2-L4 nerve roots. The lower extremity equivalent of SLR but for the anterior thigh. Positive in femoral nerve entrapment and L2-L4 disc herniations. |
| Hip flexion MMT | Patient supine, actively flexes the hip against resistance. | Weakness of hip flexion. | Iliopsoas function (femoral nerve to iliacus; L1-L3 direct branches to psoas). Combined with knee extension weakness, confirms a proximal femoral nerve lesion. |
| Anterior thigh sensation | Light touch and pinprick across the anterior thigh. | Decreased sensation compared to the opposite side or adjacent dermatomes. | Sensory function of the anterior cutaneous branches of the femoral nerve. Loss of anterior thigh sensation combined with quadriceps weakness localizes to the femoral nerve. |
Clinical Notes
- The patellar reflex is your screening tool. A diminished patellar reflex is the single fastest test for femoral nerve integrity. It takes three seconds and directly assesses the quadriceps reflex arc through L3-L4. If the patellar reflex is brisk and symmetric, the femoral nerve is almost certainly intact.
- Knee buckling is the functional hallmark. Without quadriceps function, the knee cannot be locked in extension during stance phase of gait. The patient reports the knee "giving way" — particularly going downstairs (eccentric quad demand is highest). This is different from knee instability due to ligament damage — instability from femoral nerve palsy is directional (into flexion) and consistent, without the "shifting" quality of ligamentous laxity.
- VMO atrophies first. The vastus medialis oblique (VMO) is the first quadriceps component to weaken in femoral nerve compromise and the first to atrophy in disuse. Visible VMO wasting above the medial knee is an early sign. This is also why patellar tracking problems develop in femoral nerve lesions — the VMO is the primary medial stabilizer of the patella.
- Diabetic amyotrophy targets the femoral nerve. Diabetic lumbosacral radiculoplexus neuropathy (diabetic amyotrophy) commonly presents with acute onset of severe thigh pain followed by quadriceps weakness and wasting. It preferentially affects the femoral nerve distribution. In a diabetic patient with acute thigh pain and quad weakness, consider this diagnosis — it is inflammatory, not compressive, and often improves over months.
- Do not confuse femoral nerve stretch test with hip flexor tightness. The femoral nerve stretch test (prone, knee flexion + hip extension) also stretches the rectus femoris and iliopsoas. Muscle tightness produces anterior thigh pulling or hip pain. Neural stretch reproduces burning, shooting, or electric pain along the nerve distribution. The quality of pain differentiates — dull stretching is muscular, sharp shooting is neural.
Related Nerves
- anatomy/nerves/saphenous-nerve — The terminal sensory branch of the femoral nerve. Continues distally after the motor branches have terminated, providing the only non-sciatic sensation below the knee (medial leg and foot).
- anatomy/nerves/obturator-nerve — L2-L4, lumbar plexus. Supplies the medial thigh (adductors). The femoral and obturator nerves share the same root levels (L2-L4) but innervate opposite compartments — anterior (femoral) versus medial (obturator). A lumbar plexus lesion would affect both.
- anatomy/nerves/lateral-femoral-cutaneous-nerve — L2-L3, lumbar plexus. Purely sensory nerve supplying the lateral thigh. Entrapment produces meralgia paresthetica — lateral thigh burning that is often confused with femoral nerve or L2-L3 radiculopathy but has no motor component.
Key Takeaways
- Supplies all knee extensors (quadriceps) and anterior thigh sensation — loss produces knee buckling and a diminished patellar reflex, which is the fastest screening test.
- Forms within the psoas major — psoas pathology (abscess, hematoma) can compress the nerve at its origin, presenting with anterior thigh pain and a flexed hip posture.
- The saphenous nerve (terminal branch) is the only non-sciatic sensation below the knee — medial leg numbness with normal strength points to saphenous, not sciatic involvement.
- Post-surgical femoral neuropathy (lithotomy position compression) is one of the most common iatrogenic nerve injuries.