Obturator nerve block

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Dr Ahmed Taha and Michael Fredrickson

Introduction: Obturator nerve block is rarely used in isolation but usually combined with other blocks: e.g. central neuraxial block during transurethral bladder surgery, 1 and femoral/sciatic block for knee surgery. Neurostimulation-guided obturator block can be challenging due to the difficulty eliciting an adductor motor response. Ultrasound visualisation of the obturator nerve can also be difficult. Consequently, we perform obturator block using an ultrasound-guided interfascial injection technique (targeting the space between adjacent muscles rather than the nerves themselves) using one of two described techniques. 2, 3 The first technique involves blockade of the obturator nerve after division into its anterior and posterior branches approx. 2-3 cm distal to the inguinal crease. 2 This technique has been validated with a randomised controlled trial. 1 A more recently described technique, yet to be validated with a randomised trial involves a single injection at the level of the inguinal crease between the pectineus and obturator externus muscles. Our preference for obturaor block is for the latter proximal technique because of its simplicity and apparent reliability.

Skill level: simple

Procedure time: 2-5 mins

Common indications

Common – Knee surgery e.g. TKJR and ACLR. 4-6 Obturator nerve block is essential for knee surgery conducted under peripheral nerve block alone (femoral/sciatic). Obturator nerve block also improves postoperative pain control after hamstring graft ACLR, and after TKJR, it will eliminate the 10% of patients who report pain on emergence despite effective femoral and sciatic nerve blocks.

Less common – During transurethral bladder procedures to treat or prevent adductor spasm.

Sedation: e.g. midazolam 2 mg + alfentanil 250-500 mcg

Anatomy: The obturator nerve arises from the ventral divisions of the second, third, and fourth lumbar nerves. It descends into the pelvis through the medial side of the psoas muscle and enters the thigh through the obturator foramen. Initially, the nerve lies deep to the pectineus muscle (between pectineus and obturator externus muscles). In the medial thigh, it travels distally dividing into an anterior and posterior branch which are separated by the adductor brevis muscle (Fig. 1). The posterior division sits on top of the adductor magnus muscle while the anterior division lies deep to pectineus and adductor longus. The obturator nerve travels from lateral to medial and from deep to superficial so that proximally, the nerve lies deep to pectineus, and more distally, deep to adductor longus. Hence, 3 cm distal to the inguinal crease, the divisions are located:

                     1. Anterior branch – superficial to adductor brevis and deep to adductor longus.

                     2. Posterior branch – deep to adductor brevis and superficial to adductor magnus.

Innervation of the obturator nerve:

Sensory: medial thigh skin (cutaneous branch of anterior division) and posterior knee joint (articular branch of posterior division). NB. an articular branch also supplies the hip joint.

Motor: Adductor muscles (external obturator, adductor longus, adductor brevis, adductor magnus,gracilis)

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Fig. 1. Obturator nerve block: medial compartment of the thigh (adductor compartment) illustrating the course of the obturator nerve anterior division: superficial to adductor brevis and deep to pectineus (proximally) or adductor longus (distally).

Surface landmarks: Probe placement is just distal to the inguinal crease.

Needle: 22G short bevel or 18G Tuohy needle.

Setup:

1. High frequency linear probe

2. 22G short bevel or 18G Tuohy needle.

Procedure

Two ultrasound-guided techniques for obturator block are described:

Double injection obturator nerve block technique (distal to division) (Video 1)

1. Position patient supine with leg slightly abducted and externally rotated.

2. With the probe just distal to the inguinal crease and angled directly posterior, locate the femoral vessels then slide the probe medially over the adductor compartment. Identify the useful landmark of the "Y" shaped fascial convergence: adductor brevis is medial; adductor longus on top of the forks of the "Y"; pectineus is lateral. The obturator nerve anterior division travels down the adductor compartment from lateral to medial (initially at the foot of the "Y" between pectineus and brevis; more distally at the junction of the 3 forks of the "Y" and distal still between adductor longus and brevis). The posterior division lies between adductor brevis and magnus. The target injection points are between the respective muscles.

3. Under ultrasound guidance (OOP or IP), infiltrate with lidocaine 1% the anticipated needle puncture site and needle tract, then advance (OOP or IP) the block needle to the target fascial layers.

Single injection obturator nerve block technique (proximal to division) - our preferred technique (Video 2)

1. Position patient supine with leg slightly abducted and externally rotated.

2. With the probe on the medial aspect of the inguinal crease (Fig. 2), identify the pectineus muscle medial to the femoral vein (Fig. 3).

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Fig. 2. Obturator block: probe on the medial inguinal crease.

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Fig. 3. Obturator block: pectineus muscle medial to the femoral vein.

Note medial to pectineus is the adductor longus and adductor brevis muscles, which together with the pectineus form a tricompartmental configuration with the hyperechoic fascia inbetween forming a letter "Y" (the stem directed posteriorly): pectineus is lateral to the stem; adductor longus is superior to the forks, and adductor brevis is medial to the stem (Fig. 4).

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Fig. 4. Obturator block: characteristic letter "Y" corresponding to the tricompartmental configuration of the pectineus, adductor longus and brevis muscles.

3. Once the pectineus has been identified, angle the probe in a 50 degree cephalad direction (Fig. 5) until the superior pubic ramus is identified (= a hyperechoic structure deep/lateral to pectineus). Medial to the superior pubic ramus is a hyperechoic fascial layer separating the pectineus muscle from the more deeply located obturator extenus muscle (Fig. 6).  The target injection point is the interfascial space between pectineus and obturator externus, which contains the obturator nerve and its branches (Fig. 7). 

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Fig. 5. Obturator block: cephalad angulation of probe to visualise the superior pubic ramus.

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Fig. 6. Obturator block: superior pubic ramus, pectineus and the interfascial space separating pectineus from the more deeply located obturator externus muscle.

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Fig. 7. Obturator block: target point for LA injection.

4. Using out-of-plane ultrasound guidance, infiltrate with lidocaine 1% the anticipated needle puncture site and needle tract, then advance the block needle to the target interfascial space.

Local anesthetic regimen: 10 mL LA either at 2 separate points (2 divisions) or as one single injection (proximal to the division).

 

Specific Complications: None of note.

Clinical PEARLS

1. The in-plane technique (approaching from lateral) may result in trauma to the great saphenous or femoral vein because both veins can be inadvertently compressed by the probe. This is one reason why we prefer the out-of-plane approach.

2. Clinical assessment of successful obturator nerve block is primarily through evaluation of adductor muscle strength. When combined with femoral/sciatic blockade, obturator block results in complete loss of adduction. Obturator nerve cutaneous innervation is highly variable, and therefore, sensory testing has limited value.

 

Videos

Video 1. Obturator nerve block (distal dual injection technique).

 

Video 2. Obturator nerve block (proximal single injection technique)

References

1. Manessero A, Bossolasco M, Ugues S, Palmisano S, De Bonis U, Coletta G. Ultrasound-guided obturator nerve block: interfascial injection versus a neurostimulation-assisted technique. Reg Anesth Pain Med. 2012;37:67-71.

2. 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.

3. Taha AM. Brief reports: ultrasound-guided obturator nerve block: a proximal interfascial technique. Anesth Analg. 2012;114:236-9.

4. Macalou D, Trueck S, Meuret P, et al. Postoperative analgesia after total knee replacement: the effect of an obturator nerve block added to the femoral 3-in-1 nerve block. Anesth Analg. 2004;99:251-4.

5. Mcnamee DA, Parks L, Milligan KR. Post-operative analgesia following total knee replacement: an evaluation of the addition of an obturator nerve block to combined femoral and sciatic nerve block. Acta Anaesthesiol Scand. 2002;46:95-9.

6. Sakura S, Hara K, Ota J, Tadenuma S. Ultrasound-guided peripheral nerve blocks for anterior cruciate ligament reconstruction: effect of obturator nerve block during and after surgery. J Anesth. 2010;24:411-7.