The traditional pubic approach to obturator nerve block (ONB) was first described by Pauchet [1]. In 2005, an inguinal approach to obturator nerve block was described [2]. This technique with nerve stimulation is easy, successful, less painful and more comfortable than the conventional approach. Recently, ultrasound guidance has gained popularity in the field of peripheral nerve block, but there is little report about ultrasound-guided obturator nerve block [3,4,5]. We describe the inguinal approach to ONB with ultrasound guidance.

Relevant anatomy

The adductor (medial) compartment of the thigh consists of the adductor longus, brevis, magnus, and the gracilis muscles, mainly innervated by the obturator nerve.

Ultrasound identification of these muscles is essential to achievement of this block.

The obturator nerve crosses the obturator foramen with the obturator vessels behind pectineus and in front of obturator externus muscles, and gives two divisions (anterior and posterior) for the medial side of the thigh. The anterior branch runs in front of the adductor brevis muscle and behind the adductor longus, and contributes to the cutaneous posterior and / or medial aspect of the knee. The posterior branch runs over adductor magnus and behind the adductor brevis, and contributes to the articular capsule of the knee.

The aim of the technique is to detect the nerves in the aponeurotic septum of the adductor muscles under real-time ultrasound guidance and to surround them with local anesthetic.

Positioning of the patient

The patient is placed supine and the hip slightly abducted and externally rotated as for a femoral block.

Equipment

A high-frequency probe (>10-MHz) placed transversally at the level of the inguinal crease, medial to the femoral vessels with an imaging depth of 4 cm;
A nerve stimulator;
A 50-mm short bevel needle near the pubis; a 100-mm at the inguinal fold, or in overweight patients;
A 10-ml syringe of local anesthetic.

Ultrasound scanning

The ultrasound probe is positioned at a 90° angle to the skin at the ipsilateral inguinal crease. The transducer is maintained in a transversal orientation to image the obturator nerve and its divisions in short-axis (transverse) view (figure 1).

The femoral artery and vein are initially identified. From this point, the probe is sled medially until the 3 muscle layers consisting of the adductor longus, adductor brevis, and adductor magnus are identified (figure 2).

The probe is then swept proximally and/or distally and tilted to a 90° to 60° angle to the skin to visualize divisions of the obturator nerve in the fascia investing the muscles. The anterior branch is visualized as a hypoechoic ovoid structure (fascicle) in the layer between adductor longus and brevis muscles; the posterior branch exhibit lower visibility in the layer between adductor brevis and magnus muscles (figure 3).

When neural components are identified, the nerve divisions are traced proximally. The anterior division characteristically converges toward the posterior division along the lateral border of the adductor brevis muscle to form the common obturator nerve (figure 4).

The obturator nerve is visualized as a hyperechoic flat or lip-shaped structure with internal hypoechoic dots between pectineus and obturator externus muscles. Adjacent arteries running in the fascia are identified with the nerve using color-power Doppler (figure 5).

The insertion point is determined to bring the needle to the common nerve in the intermuscular aponeurotic septum of the pectineus (and adductor longus) muscle (anteriorly), obturator externus muscles (medially) and adductor brevis muscle (posteriorly). A more caudal insertion point may be chosen to bring the needle to the nerve divisions between the adductors muscles.

Puncture

The needle, attached to a nerve stimulator using a current intensity of 0.3 mA (1 Hz, 0.1 ms), is inserted through the skin laterally to the probe via in plane approach, at an angle of 30-45° to the transducer (figure 1). The needle tip is directly guided to the vicinity of the nerve.

The current may be slowly increased to confirm nerve identification by elicitation of adductor muscles contractions. The muscles twitch is easily recognized on real-time ultrasound image and in the inner part of the thigh. Then, 5 to 10 mL of local anesthetic (LA) is slowly injected in the fascial plane containing the nerve (figure 6).

Local-anesthetic solution spread is monitored under realtime visualization. The distribution of the solution is aimed around the nerve.

When the typical hypoechoic structure is not visualized, local anesthetic solution is injected to divides the fascial plane containing the nerve (figure 7).
If this spread pattern is not obtained, the needle is repositioned.

In case of caudal insertion point, the needle tip is first avanced into the layer between adductor brevis and magnus muscles toward the posterior division of the obturator nerve and 4-ml of LA is injected in the fascial plane (figure 8).

The needle is then withdrawn and placed between adductor longus and brevis muscles and additional 4-ml of LA is injected too surround the anterior division (figure 9).

Tips

The pectineus muscle is visualized above the adductor longus muscle, separated from it by a thin aponeurotic septum.
Optimal nerve imaging requires soft-tissue compression with the transducer, the obturator veins are coapted and therefore not visible.
Obturator arteries can be detected by visible pulsation or by color-power Doppler.
The obturator nerve is frequently positioned superiorly to the obturator vessels.
The divisions are smaller than the common nerve.
The saphenous and femoral veins are localized to avoid vascular puncture.
After the injection, the nerve structure is more easily seen surrounded by an anechoic ellipse of local-anesthetic solution.

Block assessment

One may be unable to demonstrate cutaneous loss of sensation following ONB.
The only way to effectively evaluate obturator nerve function is to assess the adductor strength asking the patient to squeeze his (her) knees.
With a complete combined femoral, sciatic and obturator nerve block, the patient only adduct the contralateral leg toward the midline.