Patient Position: Supine with the thigh slightly externally rotated.
Indication: Surgery on the knee in conjunction with femoral nerve block.
Needle Size: 5-cm, 22-gauge insulated needle.
Surface Landmarks: Below the inguinal crease, 2 cm distal to pubic tubercle along the medial aspect of the thigh.
Ultrasound Landmarks: The lateral adductor brevis muscle. The anterior division of the obturator nerve usually lies superficial to the adductor brevis muscle. The posterior division of the nerve usually lies deep to the adductor brevis muscle. Both divisions produce broad, flat, hyperechoic images at the lateral border of the adductor brevis muscle. The obturator artery, which is sandwiched between the obturator internus and externus muscles.
Transducer Type: 5 to 8 MHz linear array.
Transducer Position: Axial or axial oblique along the medial aspect of the thigh, below the pubic ramus (Fig. 38-1).
Volume: 10 to 15 mL.
Technique: Sterile prep of the skin. The transducer is placed in the axial plane and the needle is inserted at a steep angle, in plane, at the lateral border of the transducer. The anterior division is identified superficial to the adductor brevis muscle and the posterior division is identified deep to the adductor brevis muscle. The needle is advanced under direct vision until a medial contraction is observed at a current of 0.5 mA. Injection proceeds until the posterior division is surrounded by a hypoechoic ring. Five mL of local anesthetic usually suffices. The needle is withdrawn and directed toward the anterior division. Injection of local anesthetic proceeds until the anterior division is surrounded by local anesthetic.
Figure 38-1. Photograph of the position of the ultrasound transducer for short axis (transverse cross section) imaging and in-plane needle placement for obturator nerve block. The patient is supine with the ipsilateral leg straight and slightly externally rotated. In this example, an insulated 5-cm, 22-gauge block needle is used to confirm nerve identity with evoked motor response.
Figure 38-2. Sonogram of the obturator nerve divisions in the medial thigh. The anterior and posterior divisions are seen in short axis view (transverse cross section) separated by the adductor brevis muscle. Large tick marks are 10 mm apart.
1. The femoral vein and lymphatics lie close to the needle path. Care should be taken to avoid these when possible.
2. Puncture of the obturator artery has led to intraperitoneal or retroperitoneal hemorrhage.
3. In some patients, the divisions can be traced proximally. The anterior division usually lies anterior to the obturator externus. The posterior division usually lies deep to the obturator externus muscle or pierces the obturator externus before running behind this muscle (Fig. 38-2).
Gray AT. Ultrasound-guided regional anesthesia: current state of the art. Anesthesiology 2006; 104:368–373.