Obturator nerve block
Proximal sinewy insertion of the long adductor muscle, femoral artery, anterior superior iliac spine, pubic tubercle
With the patient‘s leg rotated outwardly and abducted slightly, the sinewy origin of the long adductor muscle is felt. The puncture site is located 5 –10 cm beneath the pubic tubercle directly lateral to the tendon of the long adductor muscle. The needle is inserted in the craniolateral direction at an approx. 45° angle dorsal to the table pointing towards the anterior superior iliac spine. At a puncture depth of 4-6 cm, the anterior branch of the obturator nerve is reached and stimulus responses are visible from the adductor group. The amplitude is reduced in steps down to the threshold electrical current. Following a negative aspiration test, 10 – 15 ml of local anaesthetic are injected. Allow a reasonable period of approx. 10 – 15 min. for the anaesthetic to take effect. If an adequate adductor paresis is not achieved within this time, the patient presumably has an accessory obturator nerve that runs along with the femoral nerve through the lacuna musculorum. In such cases it is necessary to perform an additional femoral nerve block.
Ultrasound-guided obturator nerve block
For a block of the obturator nerve, the leg is positioned almost identically as for a femoral block. Nota bene: It often makes sense to combine the two blocks with one another, for example when a psoas block is impossible or undesirable. Starting from the „femoral position“, the transducer is advanced medially within the groin. The obturator nerve, whose ventral and dorsal branches lie respectively on top of and below the short adductor muscle, is covered by the pectineal and adductor longus muscles.