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Femoral Nerve (L2–L4)
Anatomy and Origin

The femoral nerve is one of the major nerves of the lumbar plexus, arising from the posterior divisions of the L2, L3, and L4 roots. It emerges from the lateral border of the psoas major muscle, descends between psoas and iliacus, and passes beneath the inguinal ligament to enter the thigh.

Motor Supply: the femoral nerve supplies:
Quadriceps femoris – the primary extensor of the knee
Iliacus – assists hip flexion.
Pectineus – weak hip adduction and flexion
Sartorius – flexes, abducts, and externally rotates the hip, and flexes the knee

Injury of the femoral nerve leads to loss of quadriceps function, which results in difficulty climbing stairs, rising from a chair, and extending the knee

Reflex: The femoral nerve mediates the patellar (knee jerk) reflex. Injury to the nerve leads to absent or diminished patellar reflex.

Sensory Supply
The femoral nerve provides cutaneous sensation to:
The anterior thigh (via anterior cutaneous branches)
The medial leg and foot through its largest cutaneous branch, the saphenous nerve

Saphenous Nerve
The saphenous nerve is the longest cutaneous branch of the femoral nerve and is purely sensory.
Course
Travels in the adductor canal beneath the sartorius muscle, alongside the femoral artery and vein.
Becomes superficial between the sartorius and gracilis muscles in the distal thigh.
Descends with the great saphenous vein to supply the medial leg, ankle, and foot.

Branches
Infrapatellar Branch
Leaves near the knee, often piercing the sartorius and fascia.
Provides sensation to the anteromedial knee.
Frequently injured during knee surgeries or arthroscopy, causing numbness or neuropathic pain in front of the knee.
Descending Medial Crural Branch (sometimes called the sartorial branch)
The main continuation of the nerve.
Runs distally with the great saphenous vein.
Provides cutaneous sensation to the medial leg, ankle, and foot.
Clinical Relevance
At the level of the pes anserinus, the saphenous nerve lies anterior to the gracilis and semitendinosus tendons. It is particularly vulnerable during anteromedial arthroscopy portals, medial meniscus repair, and hamstring tendon harvest. Safe dissection should remain anterior to the sartorius muscle to minimize injury.
Injury leads to numbness or a painful neuroma along the anteromedial knee.

Clinical Scenarios
1. Differentiation from L3 Radiculopathy
Femoral nerve palsy → Weakness of quadriceps, but adductor muscles remain intact.
L3 radiculopathy → Weakness of both quadriceps and adductors (obturator nerve from L3 supplies adductors).
Testing hip adduction helps distinguish between the two.

2. Iliopsoas Hematoma Compression
In patients with hemophilia or on anticoagulation, bleeding into the iliopsoas muscle may compress the femoral nerve. This produces:
Severe groin or anterior thigh pain
Weakness of quadriceps → loss of knee extension
Patients may also complain of pain over the anteromedial knee because of saphenous involvement.

3. Surgical Exposure
The femoral nerve runs within the fascial sheath of the iliopsoas muscle, lateral to the femoral vessels. It is separated from them by the iliopectineal fascia, which may need to be divided to access the true pelvis. The nerve is vulnerable during pelvic and vascular surgeries.

4. Femoral Nerve Block and Fall Risk

The femoral nerve block is used for pain control in orthopedic procedures involving the knee. While it provides excellent analgesia, it temporarily paralyzes the quadriceps muscle, eliminating active knee extension. This weakness significantly increases the risk of falls during ambulation. Patients who receive a femoral nerve block must be assisted in walking until strength returns.
Summary
The femoral nerve (L2–L4) provides motor innervation to key anterior thigh muscles, mediates the patellar reflex, and carries sensation from the anterior thigh and medial leg. Its largest cutaneous branch, the saphenous nerve, supplies the medial leg, ankle, and foot, and gives the infrapatellar and descending medial crural branches that are often at risk during knee surgery. Clinically, femoral neuropathy must be distinguished from L3 radiculopathy, which can result from iliopsoas hematoma, may be endangered in pelvic surgery, and may also be temporarily weakened by femoral nerve blocks used in orthopedic anesthesia, with fall risk being an important consideration.
Quiz
1) A patient complains of numbness over the medial leg and foot after knee surgery. Which nerve is most likely injured?
A) Common peroneal nerve
B) Tibial nerve
C) Obturator nerve
D) Saphenous nerve
Answer: D) Saphenous nerve
2) Which muscle is NOT supplied by the femoral nerve?
A) Sartorius
B) Iliacus
C) Quadriceps femoris
D) Adductor longus
Answer: D) Adductor longus
3) How can femoral nerve palsy be differentiated from L3 radiculopathy?
By testing hip adduction