High-yield overview
Largest Branch of the Lumbar Plexus
L2-L4Root origin (posterior div.)
LateralTo femoral artery (NAVEL)
QuadricepsKey motor function
SaphenousTerminal sensory branch
Branches
In the pelvis/abdomen
PatternBranches to iliacus and pectineus.
Treatment
Anterior division
PatternMotor to sartorius; sensory medial & intermediate cutaneous nerves of thigh.
Treatment
Posterior division
PatternMotor to quadriceps femoris; sensory saphenous nerve (medial leg/foot).
Treatment
Critical Must-Knows
- The femoral nerve arises from the posterior divisions of the anterior rami of L2, L3 and L4 - the largest branch of the lumbar plexus.
- It descends in the groove between psoas major and iliacus, then passes UNDER the inguinal ligament LATERAL to the femoral artery (outside the femoral sheath).
- Lateral-to-medial at the groin: NAVEL - Nerve, Artery, Vein, Empty space (canal), Lymphatics. The NERVE is most lateral.
- Motor: iliacus, pectineus, sartorius and the quadriceps femoris (the knee extensor).
- Sensory: anterior thigh (cutaneous branches) and, via the saphenous nerve, the medial leg, ankle and foot.
- Femoral nerve palsy causes quadriceps weakness (knee buckling), a reduced/absent patellar reflex, and anteromedial sensory loss.
Clinical Pearls
- “NAVEL from lateral to medial places the femoral nerve outside (lateral to) the femoral sheath - which contains only artery, vein and canal.
- “In direct anterior approach THA, the femoral nerve is closest to the acetabular rim at the 'three o'clock' (90°) position - avoid a retractor there.
- “An iliacus/retroperitoneal haematoma (anticoagulation, haemophilia) is a classic non-surgical cause of femoral nerve palsy.
Femoral Nerve at Risk
Surgical risk
The nerve is vulnerable in anterior-approach THA (retractor over the anterior acetabular rim), acetabular/pelvic fracture surgery, inguinal and pelvic procedures, and from excessive leg lengthening. Cadaveric work shows it lies closest to the anterior rim at the 90° position.
Non-surgical risk
Iliacus or retroperitoneal haematoma (anticoagulation, haemophilia) compresses the nerve in the iliopsoas groove; lithotomy positioning and prolonged hip hyperflexion can also stretch it. These are classic, examinable non-operative causes.
Origin & Course
Origin
- The femoral nerve forms from the posterior divisions of the anterior rami of L2, L3 and L4.
- It is the largest branch of the lumbar plexus.
- It forms within the substance of psoas major and emerges from its lateral border.
Motor & Sensory Supply
Motor: the Knee Extensor Nerve
The femoral nerve supplies the muscles that extend the knee and assist hip flexion: iliacus, pectineus, sartorius, and the quadriceps femoris (rectus femoris, vastus lateralis, medialis and intermedius).
- Hip flexion is assisted (iliacus, pectineus, sartorius, rectus femoris) - but psoas major also contributes via direct lumbar plexus branches, so hip flexion is only partly affected by a femoral lesion below the inguinal ligament.
- Knee extension depends on the quadriceps - the key deficit in femoral nerve palsy (knee gives way).
- Sensory: anterior thigh via the medial and intermediate cutaneous nerves of the thigh; and the saphenous nerve (the terminal sensory branch of the posterior division) supplies the medial leg, ankle and medial foot.
Clinical Correlations
Deficit
- Weak knee extension (quadriceps) - the patient describes the knee buckling/giving way, especially on stairs.
- Reduced or absent patellar (knee-jerk) reflex.
- Sensory loss over the anterior thigh and the medial leg (saphenous territory).
- Hip flexion is relatively preserved (psoas spared if the lesion is distal).
Mnemonic
NAVELFemoral Nerve at the Groin
N
Nerve (femoral)
Most LATERAL - outside the femoral sheath.
A
Artery
Femoral artery.
V
Vein
Femoral vein.
E
Empty space
Femoral canal (node of Cloquet).
L
Lymphatics / Lacunar lig.
Most medial.
Hook:Lateral to medial: NAVEL - the femoral Nerve is most lateral.
Evidence Base
Evidence
Femoral Nerve & the Anterior Acetabular Rim (Cadaveric)
Yoshino K, Nakamura J, Hagiwara S, Suzuki T, Kawasaki Y, Ohtori S • J Bone Joint Surg Am (2020)
Key Findings:
- Cadaveric study of 84 hips mapping the femoral nerve's distance from the anterior acetabular rim
- Minimum distance ranged 16.6-33.2 mm; the nerve was closest to the rim at the 90 degree (anterior, 'three o'clock') position
- Iliopsoas thickness and femoral length correlated with the distance at 90 degrees
- Recommends avoiding retractor placement at 90 degrees to the anterior rim to reduce femoral nerve injury
Clinical implication: Gives the anatomical basis for avoiding anterior-rim retractor injury during direct anterior approach THA - the nerve is most exposed at the anterior (90°) point.
Evidence
Femoral Nerve Palsy after Direct Anterior THA
Hoshino C, Koga D, Koyano G, Yamauchi Y, Sakai T, Okawa A, Jinno T • PLoS One (2019)
Key Findings:
- Retrospective review of 273 primary direct-anterior-approach THAs; femoral nerve palsy incidence 1.1%
- Suspected causes: improper anterior acetabular retractor positioning and excessive leg lengthening
- All three palsies recovered completely within a year
- No significant relationship between palsy and the surgeon's direct-anterior experience
Clinical implication: Quantifies femoral nerve palsy after anterior THA (~1%), links it to retractor position and lengthening, and notes that most recover.
Viva Scenarios
Practise clinical reasoning and management decisions out loud
Viva scenarioStandard
Scenario 1: Knee buckling after anterior THA
Clinical prompt
“Two days after a direct anterior approach THA, a patient's knee gives way and they cannot extend it; there is numbness over the anteromedial thigh and medial leg. What is the diagnosis and your management?”
Practical approach
This is a femoral nerve palsy. The motor pattern (quadriceps weakness with knee buckling) plus sensory loss over the anterior thigh and saphenous (medial leg) territory localises to the femoral nerve, which is at risk in the direct anterior approach - typically from an anterior acetabular retractor (the nerve lies closest to the rim at the 90° position) or from excessive leg lengthening. I would confirm with examination (quadriceps power, absent patellar reflex, sensory mapping), exclude a compressive haematoma (the nerve runs in the iliopsoas groove - check for an enlarging mass, especially if anticoagulated), and review the operative note and limb-length change. Most retractor-related neuropraxic palsies recover, often within a year, so management is largely supportive - a knee extension orthosis/locked brace to prevent falls, physiotherapy, and protected mobilisation - with electrodiagnostics and surgical review if it fails to recover or if a haematoma needs decompression.
Key clinical points
Localise to femoral nerve (quadriceps + saphenous sensory)
Mechanism: anterior retractor at the rim / leg lengthening
Exclude compressive iliacus haematoma (esp. anticoagulated)
Mostly supportive (extension brace) - good recovery
Common pitfalls
Missing an expanding retroperitoneal haematoma as the cause
Failing to brace the knee - fall risk from buckling
Further questions
“What is the order of structures at the groin from lateral to medial?”
“NAVEL: Nerve, Artery, Vein, Empty space (canal), Lymphatics - the femoral nerve is most lateral, outside the sheath.”
Guidelines, Registries & Global Practice
Global Practice Picture
Femoral nerve anatomy underpins safe anterior-based hip exposure worldwide. With the international growth of the direct anterior approach, awareness of femoral nerve palsy (incidence around 1% in reported series) and the anatomical "danger zone" at the anterior acetabular rim has become standard teaching, alongside recognition of iliacus haematoma as a non-operative cause.
Side-by-Side Synthesis
Origin
- Detail
- L2-L4 posterior divisions (largest lumbar plexus branch)
Course
- Detail
- Psoas-iliacus groove → under inguinal ligament, lateral to artery
Groin relations
- Detail
- NAVEL (nerve most lateral, outside sheath)
Motor
- Detail
- Iliacus, pectineus, sartorius, quadriceps (knee extension)
Sensory
- Detail
- Anterior thigh + saphenous (medial leg/foot)
Palsy signs
- Detail
- Weak knee extension, absent patellar reflex, anteromedial sensory loss
Key risks
- Detail
- Anterior THA retractor / lengthening; iliacus haematoma; lithotomy
Exam day cheat sheet
Anatomy
- L2-L4 posterior divisions
- Largest lumbar plexus branch
- Psoas-iliacus groove → under inguinal ligament
- Lateral to femoral artery (NAVEL), outside sheath
Supply
- Motor: iliacus, pectineus, sartorius, quadriceps
- Knee extension = key function
- Sensory: anterior thigh + saphenous (medial leg)
- Saphenous = terminal sensory branch
Clinical
- Palsy: weak knee extension, knee buckling
- Reduced/absent patellar reflex
- Risks: anterior THA retractor, leg lengthening
- Iliacus haematoma (anticoagulation)