Obturator nerve block

Anatomy

lumbar plexus
ventral rami of L2-4

The anterior and posterior branches of the obturator nerve, or the common obturator nerve, run between the pectineus and obturator externus muscles immediately after the nerve emerges from the obturator canal

considerable variability in where the division occurred in human dissections, noting 23% to be intrapelvic, 52% in the obturator canal, and 25% in the thigh

The anterior obturator nerve branch initially passes through the interfascial plane between the pectineus and adductor brevis muscles. Further caudad, it runs between the adductor longus and adductor brevis muscles, innervating the adductor longus, adductor brevis, and gracilis muscles

The posterior obturator nerve branch travels in the fascia between the adductor brevis and adductor magnus muscles

The articular branch supplying the hip joint is derived from the common obturator nerve or its branches at different levels in conjunction with the obturator canal

Vessel reach area:
medial circumflex femoral artery courses across the surface of the pectineus and at the plane of injection the obturator artery lies deep to the obturator nerve

Indication:

Motor

adductor spasm

TURBT

ONB prolonged time to recurrence of ca bladder

to ↓ obturator reflex

Surgical technique
- ?bipolar diathermy
- laser resector
- saline irrigation
- avoid over-distending the bladder
  - ∵ bring lateral bladder wall closer to ON
Anaes
- GA + NMBA
- SA + ONB

Sensory

Knee surgery

useful for gracilis tendon harvesting

Hip surgery

hip neurolysis

Testing the block

hip adduction: ↓ power by 40-50% defines successful block

may still have some power ∵

The adductor magnus muscle is innervated by both the posterior branch of the obturator nerve and the sciatic nerve
femoral nerve also innervates the pectineus muscle
pectineus muscle is occasionally (in 10%–30% of cases) innervated by the accessory obturator nerve, which arises from the anterior rami of the third and fourth lumbar nerves, descends along the medial border of the psoas major muscle, and passes above the superior pubic ramus

Approach

USG guided

Nerve stimulation guidance is not always used to perform proximal level ONB procedures because injection of local anesthetic into the interfascial plane between the pectineus and obturator externus muscles, which can be easily identified using ultrasound, provides successful ONB

Distal approach

Adv:

technically easier
more consistent superficial anatomy

after branching into anterior & posterior

some branch of ON may not be blocked

Use a nerve stimulator to confirm adductor twitch (typically 0.5–1.0 mA) if visualization is poor. Note that the neural stimulation of the adductor muscles by the anterior obturator nerve is more intense than the direct muscle stimulation that can be observed during advancement

anterior division: contraction of adductor longus + brevis
posterior division: contraction of adductor magnus

Proximal approach

adv: only need one injection (common ON)

Although the ON can branch proximal to this level and the posterior division can pass through the obturator externus, local anesthetic injection has been demonstrated to migrate proximally into the pelvis and reliably block both divisions

inject btwn pectineus & obturator externus muscles

Landmark technique

References

Obturator nerve: Anatomy, Pain, and Block. (youtube.com)

Ultrasound Guided Obturator Nerve Block (youtube.com)

USG Obturator Nerve Block (youtube.com)

Obturator Nerve Block - Landmarks and nerve stimulator technique - NYSORA

How I Do It Obturator Nerve Block

Ultrasound-Guided Obturator Nerve Block A Focused Review on Anatomy and

Ultrasound Standard for Obturator Nerve Block The Modified Taha's Approach