High-yield overview
The Nerve of the Thigh Adductors
L2-L4Root origin (anterior div.)
Obturator canalKey passage
AdductorsMotor function
Medial thighSensory territory
Divisions in the Thigh
Anterior branch
PatternAdductor longus, adductor brevis, gracilis; cutaneous branch to medial thigh.
Treatment
Posterior branch
PatternAdductor magnus (adductor part), obturator externus; articular branch to the knee.
Treatment
Critical Must-Knows
- The obturator nerve arises from the ANTERIOR divisions of the anterior rami of L2, L3 and L4 (contrast: the femoral nerve is from the POSTERIOR divisions).
- It emerges from the MEDIAL border of psoas major, runs along the lateral pelvic wall, and enters the thigh through the OBTURATOR CANAL.
- It supplies the thigh ADDUCTOR muscles (adductor longus/brevis/magnus adductor part, gracilis, obturator externus).
- It carries sensation from a small area of the MEDIAL THIGH.
- It also gives an articular branch to the hip and (via the posterior branch) the knee - the anatomical basis of referred hip-to-knee pain.
- The Howship-Romberg sign (medial thigh/knee pain on hip extension/adduction/internal rotation) suggests obturator nerve compression, classically by an obturator hernia.
Clinical Pearls
- “Anterior vs posterior divisions: obturator = anterior division; femoral = posterior division (both L2-L4).
- “It supplies BOTH the hip and knee with articular branches - hence obturator pathology can refer pain to the medial knee.
- “Proximal obturator nerve is the highest-risk segment during pelvic lymph node dissection.
The Howship-Romberg Sign
What it is
Medial thigh/knee pain and paraesthesia provoked by hip extension, adduction or internal rotation, relieved by hip flexion. It reflects obturator nerve compression - classically by an obturator hernia (typically a thin, elderly woman with bowel obstruction).
Why it matters
It is an easily-missed cause of bowel obstruction and groin/medial-knee pain. Recognising the sign points to the obturator canal and prompts the right imaging - a high-yield exam association.
Origin & Course
Origin
- The obturator nerve arises from the anterior divisions of the anterior rami of L2, L3 and L4.
- This contrasts with the femoral nerve (same roots, but posterior divisions).
- It forms within psoas major and emerges from its medial border (the femoral nerve emerges from the lateral border - a classic comparison).
Motor & Sensory Supply
Motor: the Adductors
The obturator nerve is the motor nerve of the thigh adductor compartment: adductor longus, adductor brevis, gracilis, obturator externus, and the adductor part of adductor magnus (the hamstring part of adductor magnus is supplied by the sciatic/tibial division).
- Action: adduction of the thigh (and gracilis assists knee flexion/medial rotation).
- Sensory: a variable patch of medial thigh skin via the cutaneous branch of the anterior division (the accessory obturator nerve, when present, is a minor variant).
- Articular: branches to both the hip and the knee, explaining why hip pathology can refer to the medial knee.
Clinical Correlations
Causes
- Pelvic surgery: pelvic lymph node dissection (e.g. during radical prostatectomy), gynaecological surgery, and acetabular/quadrilateral-plate fracture fixation - the proximal nerve is the highest-risk segment.
- Obturator hernia: compression in the obturator canal producing the Howship-Romberg sign.
- Hip surgery / arthroscopy and pelvic masses.
- Adductor spasticity is treated with obturator nerve blocks/neurectomy in cerebral palsy.
Mnemonic
OAFPObturator vs Femoral Nerve
O
Obturator = Anterior
Anterior divisions of L2-L4; medial psoas border.
A
Adductors
Obturator supplies the adductor compartment.
F
Femoral = Posterior
Posterior divisions of L2-L4; lateral psoas border.
P
Posterior = quadriceps
Femoral supplies the knee extensors.
Hook:Obturator = Anterior divisions/Adductors; Femoral = Posterior divisions/quadriceps.
Evidence Base
Evidence
Obturator Nerve Injury Risk Map in Pelvic Surgery
Gozen AS, Aktoz T, Akin Y, Klein J, Rieker P, Rassweiler J • J Laparoendosc Adv Surg Tech A (2015)
Key Findings:
- Series of 3558 laparoscopic/robotic radical prostatectomies; obturator nerve injury in 0.1% during pelvic lymph node dissection
- Across the literature the PROXIMAL part of the obturator nerve accounts for ~77.8% of reported injuries
- Recognising injury intra-operatively allowed successful simultaneous repair (clip removal or microsuture)
- No long-term adductor or neurologic deficit after timely repair
Clinical implication: Defines the proximal obturator nerve as the danger zone in pelvic dissection and shows that recognised injuries can be repaired immediately with good outcomes.
Evidence
Obturator-to-Femoral Nerve Transfer (Feasibility)
Goubier JN, Teboul F, Yeo S • Microsurgery (2012)
Key Findings:
- Cadaveric feasibility study transferring obturator motor branches (to gracilis/adductor longus) to the motor portion of the femoral nerve
- Tension-free direct neurorrhaphy was possible in all specimens (mean overlap 26 mm; matched nerve diameters)
- Access in the thigh was straightforward compared with pelvic transfer
- Proposed as a reproducible reconstruction for proximal femoral nerve injury
Clinical implication: Illustrates the surgical use of obturator motor branches and the close functional relationship between the obturator and femoral nerves.
Viva Scenarios
Practise clinical reasoning and management decisions out loud
Viva scenarioStandard
Scenario 1: Medial thigh pain and bowel obstruction
Clinical prompt
“A thin, elderly woman presents with bowel obstruction and intermittent medial thigh and knee pain that worsens when she extends and internally rotates the hip. What sign is this and what is the likely cause?”
Practical approach
This is the Howship-Romberg sign - medial thigh/knee pain and paraesthesia provoked by hip extension, adduction or internal rotation and relieved by flexion - which reflects obturator nerve compression in the obturator canal. In a thin elderly woman with bowel obstruction, the classic cause is an obturator hernia trapping bowel and compressing the nerve. I would recognise this as a surgical emergency, resuscitate, and obtain CT of the abdomen and pelvis to confirm the obturator hernia and bowel involvement, then arrange surgical repair. The pain refers to the medial knee because the obturator nerve gives an articular branch to the knee.
Key clinical points
Name the Howship-Romberg sign and its provocation/relief
Localise to the obturator nerve / obturator canal
Obturator hernia in a thin elderly woman with obstruction
Explain medial-knee referral via the articular branch
Common pitfalls
Missing the hernia as a cause of obstruction
Attributing medial knee pain to knee pathology rather than referred obturator pain
Further questions
“From which divisions does the obturator nerve arise?”
“The anterior divisions of L2-L4 (the femoral nerve arises from the posterior divisions of the same roots).”
Guidelines, Registries & Global Practice
Global Practice Picture
Obturator nerve anatomy is foundational for pelvic and acetabular surgery and for regional anaesthesia. Internationally consistent teaching points are: protect the proximal nerve during pelvic lymph node dissection and quadrilateral-plate fixation; recognise the Howship-Romberg sign of obturator hernia; and use the obturator block for adductor spasticity and to prevent the adductor jerk in bladder surgery.
Side-by-Side Synthesis
Roots/division
- Obturator nerve
- L2-L4 anterior divisions
- Femoral nerve (for contrast)
- L2-L4 posterior divisions
Exit from psoas
- Obturator nerve
- Medial border
- Femoral nerve (for contrast)
- Lateral border
Pelvic course
- Obturator nerve
- Obturator canal
- Femoral nerve (for contrast)
- Under inguinal ligament
Motor
- Obturator nerve
- Thigh adductors
- Femoral nerve (for contrast)
- Quadriceps
Sensory
- Obturator nerve
- Medial thigh
- Femoral nerve (for contrast)
- Anterior thigh + saphenous
Classic lesion
- Obturator nerve
- Obturator hernia (Howship-Romberg), PLND
- Femoral nerve (for contrast)
- Anterior THA retractor, iliacus haematoma
Exam day cheat sheet
Anatomy
- L2-L4 anterior divisions
- Medial border of psoas
- Through the obturator canal
- Anterior + posterior branches
Supply
- Motor: thigh adductors + obturator externus
- Sensory: medial thigh
- Articular branches to hip AND knee
- Adductor magnus = dual (obturator + sciatic)
Clinical
- Howship-Romberg = obturator hernia
- Proximal nerve at risk in pelvic LN dissection
- Deficit: weak adduction, medial thigh numbness
- Obturator block for adductor spasticity