Obturator nerve block (anterior and posterior branches)

Intermediate. The muscle layers are easy to see, but the branches are small and their pattern varies. Injecting into the right fascial plane matters more than seeing each branch.

The obturator nerve comes from the lumbar plexus (anterior divisions of L2–L4). It emerges from the medial border of psoas inside the pelvis, runs along the lateral pelvic wall and leaves the pelvis through the obturator canal, in the upper part of the obturator foramen, with the obturator vessels.

It divides into an anterior and a posterior branch. Where it divides varies: inside the pelvis in about a quarter of people, within the obturator canal in about half, and in the thigh in the rest.

The anterior branch runs down in the fascial plane between pectineus and adductor longus in front and adductor brevis behind. It supplies adductor longus, adductor brevis and gracilis, sends a branch to the hip joint, and often ends as a small cutaneous branch to the lower medial thigh.

The posterior branch pierces obturator externus and runs between adductor brevis in front and adductor magnus behind. It supplies obturator externus and adductor magnus, and sends a genicular branch through the adductor hiatus to the back of the knee joint (part of the popliteal plexus).

The skin supplied by the obturator nerve is small and unpredictable: in many people it supplies no skin at all. So the block is tested by weakness of the adductors, not by numbness.

• Inguinal crease and the femoral artery pulse.
• Pubic tubercle.
• Adductor longus tendon, felt as a tight cord running down from the pubic tubercle when the leg is abducted.

• Transurethral resection of bladder tumours on the lateral bladder wall: stops the obturator (adductor) jerk, which can cause bladder perforation, when the resectoscope stimulates the nerve through the bladder wall. Spinal anaesthesia alone does not prevent this jerk.
• Knee surgery: added to a femoral, femoral triangle or adductor canal block to cover the back and inside of the knee.
• Hip surgery and hip fracture pain, as the obturator nerve supplies part of the hip joint (proximal approach).
• Adductor spasticity, for example in cerebral palsy, multiple sclerosis or after a stroke, to help hygiene, positioning and assessment before neurolysis or botulinum toxin.
• Diagnosis of obturator nerve entrapment and chronic medial thigh or groin pain.

• Anterior branch of the obturator nerve (L2–L4): adductor longus, adductor brevis, gracilis, an articular branch to the hip, and a variable patch of skin on the lower medial thigh.
• Posterior branch of the obturator nerve: obturator externus, adductor magnus (adductor part), and the genicular branch to the back of the knee.
• With the proximal approach, the common nerve or both branches near the obturator canal, including the articular branch to the hip.

Surface anatomy, relevant structures and landmarks

Classical (Labat) approach: 1.5 cm below and 1.5 cm lateral to the pubic tubercle. Inguinal (interadductor) approach: on the inguinal crease, halfway between the femoral artery and the adductor longus tendon.

Positioning

Supine, with the leg slightly abducted and externally rotated.

Block technique

Classical approach: the needle meets the inferior pubic ramus, then is walked off it laterally and slightly upwards towards the obturator canal. Inguinal approach: the needle is directed up and back from the crease, aiming at the two fascial planes between the adductors. Both rely on a nerve stimulator: landmarks alone are not reliable and the classical approach is near the obturator vessels.

Needle choice

22 G insulated short-bevel needle, 50–100 mm.

Neurostimulation technique

Look for contraction of the adductor muscles (the thigh moving inwards) at 0.3–0.5 mA. With the inguinal approach, find the anterior branch first (adductor longus and gracilis), then the posterior branch deeper (adductor magnus).

Choice of local anaesthetic (including volume)

5–7 mL per branch, or 10–15 mL with the classical approach. Lidocaine 1.5–2% for transurethral surgery; 0.5% ropivacaine or levobupivacaine for analgesia.

Positioning

Supine, leg slightly abducted and externally rotated. For bladder surgery the block can be done in the lithotomy position, before or after the spinal.

Scanning technique

• Distal (interadductor) approach. Put the probe across the inguinal crease and find the femoral vein. Slide medially: three muscle layers appear, adductor longus on top, adductor brevis in the middle and adductor magnus underneath, with pectineus laterally.
• The anterior branch is a small, flat, bright structure in the plane between adductor longus (or pectineus) and adductor brevis. The posterior branch lies in the plane between adductor brevis and adductor magnus.
• Proximal approach. From the same view, follow pectineus up towards the pubis and tilt the probe towards the head. Inject in the fascial plane between pectineus and obturator externus, just below the superior pubic ramus, where the nerve leaves the obturator canal.
• Use colour Doppler before injecting: the obturator artery and vein and branches of the medial circumflex femoral artery lie close by, especially with the proximal approach.

Structures to identify

• Femoral vein
• Pectineus
• Adductor longus
• Adductor brevis
• Adductor magnus
• Fascial planes between the adductors
• Anterior and posterior branches of the obturator nerve
• Obturator externus and the superior pubic ramus (proximal approach)

Position / ergonomics

Stand on the side to be blocked, facing the patient's head for the proximal approach, with the screen across the bed in line with your eyes and hands.

Probe

High-frequency linear probe.

Settings

Musculoskeletal or nerve preset, depth 4–6 cm, focus at the level of the fascial planes.

Depth

Anterior branch 1.5–3 cm, posterior branch 3–5 cm, proximal approach 3–6 cm.

Needle size

22 G, 50–80 mm short-bevel block needle. An insulated needle allows nerve stimulation.

Needling technique

In-plane, from medial or lateral (from medial keeps the needle away from the femoral vessels). For the distal approach, inject into the upper plane first, then advance the needle through adductor brevis into the lower plane.

Optimal needle-tip position

Within each fascial plane, so that the local anaesthetic spreads along the plane as a thin line, not as a swelling inside the muscle.

Drug choice

Lidocaine 1.5–2% (with adrenaline if wanted) for bladder surgery, where only a short block is needed. 0.5% ropivacaine or levobupivacaine for knee and hip analgesia.

Volume

Distal approach: 5–10 mL in each plane. Proximal approach: 8–15 mL in the plane deep to pectineus.

Optimal current if nerve stimulation is used

Adductor contraction at 0.3–0.5 mA (0.1 ms). Adding nerve stimulation to ultrasound makes the block more reliable for bladder surgery.

Muscle responses acceptable

Adductor longus, gracilis and adductor brevis (anterior branch); adductor magnus (posterior branch). A quadriceps twitch means the needle is too lateral, near the femoral nerve.

Testing block success

Weaker adduction: the patient squeezes a folded blood pressure cuff between the knees, and the pressure falls by 50% or more. Do not rely on skin numbness, because the cutaneous area is absent or variable.

Indication

Rarely used. A catheter can be considered for prolonged adductor spasticity or for knee or hip analgesia when the obturator nerve is the main source of pain.

Kit

Standard peripheral nerve catheter set.

Catheter

Place it in the plane between pectineus and obturator externus (proximal approach), 2–3 cm beyond the needle tip, and fix it well away from the groin crease.

Regime

0.2% ropivacaine at 4–6 mL per hour, counting the total local anaesthetic dose with any other blocks.

• Think of a sandwich: adductor longus on top, adductor brevis in the middle, adductor magnus underneath. The anterior branch is in the top layer of filling, the posterior branch in the bottom one.
• Test motor, not skin. The obturator nerve often supplies no skin at all, so a numb medial thigh proves nothing.
• For bladder surgery, block before the resection starts and use nerve stimulation as well as ultrasound if the planes are hard to see: a missed posterior branch can still cause a jerk.
• The proximal approach blocks the nerve before it divides, with one injection, and can also reach the articular branch to the hip.
• For knee surgery, a distal adductor canal injection can spread to the genicular branch of the posterior obturator nerve in the popliteal fossa, giving some of the benefit without adductor weakness.
• A femoral '3-in-1' block or a fascia iliaca block does not reliably block the obturator nerve.

• Bouaziz and colleagues (2002) found no cutaneous area after a successful obturator block in 57% of patients, and showed that adductor strength is the only reliable test of the block.
• Anagnostopoulou and colleagues (2009) dissected 84 cadavers: the nerve divided inside the pelvis in 23%, within the obturator canal in 52% and in the thigh in 25%, with many branching patterns.
• Helayel and colleagues (2007) first described ultrasound-guided obturator block with nerve stimulation in 22 patients: the nerve was found on the first attempt in 91%.
• Sinha and colleagues (2009) showed that injecting into the two interadductor fascial planes without nerve stimulation reduced adduction strength in all 30 patients, with 93% meeting their success criteria.
• Taha (2012) described the proximal interfascial approach, between pectineus and obturator externus, with both branches blocked in all patients.
• Manassero and colleagues (2012) compared interfascial injection with nerve stimulation-assisted blocks for bladder tumour resection: onset and success were similar, and interfascial injection was quicker to perform.
• Yoshida and colleagues (2016) described a pubic approach to the proximal block in the lithotomy position: all 20 blocks worked, and dye spread back through the obturator canal in cadavers.
• Runge and colleagues (2016) found that adding an obturator block to a femoral triangle block after knee replacement cut 24-hour morphine use from 20 mg to 2 mg, without affecting walking tests.
• A 2026 meta-analysis of five trials (451 patients) found that adding nerve stimulation to ultrasound prevented the adductor jerk during bladder tumour resection more reliably than ultrasound alone, at the cost of about 2 minutes longer to perform.

• Bouaziz H, Vial F, Jochum D, et al. An evaluation of the cutaneous distribution after obturator nerve block. Anesth Analg 2002;94:445–9. PubMed
• Anagnostopoulou S, Kostopanagiotou G, Paraskeuopoulos T, et al. Anatomic variations of the obturator nerve in the inguinal region: implications in conventional and ultrasound regional anesthesia techniques. Reg Anesth Pain Med 2009;34:33–9. PubMed
• Helayel PE, da Conceição DB, Pavei P, Knaesel JA, de Oliveira Filho GR. Ultrasound-guided obturator nerve block: a preliminary report of a case series. Reg Anesth Pain Med 2007;32:221–6. PubMed
• Sinha SK, Abrams JH, Houle TT, Weller RS. Ultrasound-guided obturator nerve block: an interfascial injection approach without nerve stimulation. Reg Anesth Pain Med 2009;34:261–4. PubMed
• Taha AM. Brief reports: ultrasound-guided obturator nerve block: a proximal interfascial technique. Anesth Analg 2012;114:236–9. PubMed
• Manassero A, Bossolasco M, Ugues S, et al. Ultrasound-guided obturator nerve block: interfascial injection versus a neurostimulation-assisted technique. Reg Anesth Pain Med 2012;37:67–71. PubMed
• Yoshida T, Onishi T, Furutani K, Baba H. A new ultrasound-guided pubic approach for proximal obturator nerve block: clinical study and cadaver evaluation. Anaesthesia 2016;71:291–7. PubMed
• Yoshida T, Nakamoto T, Kamibayashi T. Ultrasound-guided obturator nerve block: a focused review on anatomy and updated techniques. Biomed Res Int 2017;2017:7023750. PubMed
• Runge C, Børglum J, Jensen JM, et al. The analgesic effect of obturator nerve block added to a femoral triangle block after total knee arthroplasty: a randomized controlled trial. Reg Anesth Pain Med 2016;41:445–51. PubMed
• Runge C, Moriggl B, Børglum J, Bendtsen TF. The spread of ultrasound-guided injectate from the adductor canal to the genicular branch of the posterior obturator nerve and the popliteal plexus: a cadaveric study. Reg Anesth Pain Med 2017;42:725–730. PubMed
• Hu Q, Gan L, Ouyang C, et al. Ultrasound with nerve stimulation versus ultrasound alone for obturator nerve block in transurethral resection of bladder tumor: a systematic review and meta-analysis. Saudi J Anaesth 2026;20:788–794. PubMed