OBTURATOR NERVE BLOCK AT A GLANCE
Indications: Relief of painful adductor muscle contractions, to prevent adduction of the thigh during transurethral bladder surgery, additional analgesia after major knee surgery, and may provide postoperative analgesia after hamstring tendon harvest for anterior cruciate ligament (ACL) reconstruction (Figure 33D–1).1,2
Transducer position: medial aspect of the proximal thigh.
Goal: Local anesthetic spread in the interfascial plane in which the nerves lie or around the anterior and posterior branches of the obturator nerve.
Local anesthetic: 5 mL into each interfascial space or around the branches of the obturator nerve.
Expected distribution of obturator nerve sensory and motor blockade.
Ultrasound (US)-guided obturator nerve block is simpler to perform and more reliable than surface landmark–based techniques. There are two approaches to performing a US-guided obturator nerve block. The interfascial injection technique relies on injecting local anesthetic solution into the fascial planes that contain the branches of the obturator nerve. With this technique, it is not important to identify the branches of the obturator nerve on the sonogram, but rather to identify the adductor muscles and the fascial boundaries within which the nerves lie. This is similar in concept to other fascial plane blocks (eg, the transversus abdominis plane [TAP] block in which local anesthetic solution is injected between the internal oblique and transverse abdominis muscles without the need to identify the nerves). Alternatively, the branches of the obturator nerve can be visualized with US imaging and blocked after eliciting a motor response.
The obturator nerve forms in the lumbar plexus from the anterior primary rami of the L2–L4 roots and descends to the pelvis on the medial side of the psoas muscle. In most individuals, the nerve divides into an anterior branch and posterior branch before exiting the pelvis through the obturator foramen. In the thigh, at the level of the femoral crease, the anterior branch is located between the fascia of the pectineus and adductor brevis muscles. The anterior branch lies further caudad between the adductor longus and adductor brevis muscles. The anterior branch provides motor fibers to the adductor longus, brevis and gracilis muscles; and cutaneous branches to the medial aspect of the thigh. The anterior branch has a great variability in the extent of sensory innervation of the medial thigh.
The posterior branch lies between the fascial planes of the adductor brevis and adductor magnus muscles (Figures 33D–2 and 33D–3). The posterior branch is primarily a motor nerve for the adductors of the thigh; however, it also may provide articular branches to the medial aspect of the knee joint. The articular branches to the hip joint usually arise from the obturator nerve, proximal to its division and only occasionally ...