Ultrasound-Guided Obturator Nerve Block: A Sonoanatomic Study of a New Methodologic Approach

BACKGROUND: Obturator nerve block is one of the most technically challenging regional anesthesia techniques. Recently, the characteristics of the nerve have been described using ultrasound. However, clinical application of proximal ultrasound-guided obturator nerve block on patients has not been reported. In this study, we used ultrasound to describe the anatomical localization of the obturator nerve and its two branches in cadavers, volunteers, and also patients. METHODS: A hyperechoic triangular shape formed by the superior pubic ramus, posterior margin of the pectineus muscle and anterior aspect of the external obturator muscle containing the obturator vessels and nerve was defined by ultrasound imaging in cadavers. In eight volunteers, bilateral obturator nerve images were obtained and the distances to specific landmarks (femoral artery, femoral vein, and pubic tubercle) were recorded. Ultrasound-guided obturator nerve block was further performed in 15 patients by using the previously defined approach. The final distance of the needle tip to the femoral artery, distances between the needle insertion point to the pubic tubercle and the depth of needle insertion were recorded. RESULTS: The rates of common obturator nerve, anterior and branching obturator nerve pattern visibility with ultrasound were determined in 12/16,13/16, and 7/16 sites in volunteers, respectively. Mean (SD) values of critical landmarks obtained from volunteers were obturator nerve-femoral vein 12.9 +/- 2.9 mm and obturator nerve-pubic tubercle 19.9 +/- 2.6 mm. Mean measurements obtained from patients were: femoral artery-needle tip 18.5 +/- 2.4 mm, needle depth 48.3 +/- 10.4 mm, pubic tubercle-needle insertion point (horizontal) 18.8 +/- 2.0 mm, and pubic tubercle-needle insertion point (vertical) 21.1 +/- 2.9 mm. Visual analog scale scores obtained from patients at 1 and 24 h were lower compared to baseline values (P < 0.001). Ninety-three percent (14 of 1.5) of the patients reported satisfaction from the block. CONCLUSIONS: Landmarks defined in this clinical trial can be used in patients for obturator nerve block with ultrasound guidance.