Background and Objectives Interfascial injection of local anesthetic under ultrasound guidance has been proposed as a new technique for performing an obturator nerve block. We hypothesized that interfascial needle placement could supplant nerve stimulation as the end point for local anesthetic injection during ultrasound-guided obturator nerve block after the division of the obturator nerve.

Methods Fifty spinal anesthesia patients who had experienced unilateral adductor muscle spasm during transurethral bladder tumor resection were randomly allocated to receive either 5 mL of lidocaine 2% injected under ultrasound guidance into the interfascial plane between the adductor longus and the adductor brevis and between the adductor brevis and the magnus muscles (US group) or an injection of 5 mL of lidocaine 2% in combination with nerve stimulation after identification of the divisions of the obturator nerve (USENS group). At 5, 10, and 15 minutes after block placement, muscle spasm was assessed by an independent observer masked to treatment allocation. The primary outcome was motor block onset time. Secondary outcomes were block performance time, total anesthesia-related time, motor block success at 15 minutes, and number of needle passes.

Results Motor block onset time did not differ between the 2 groups (6.2 minutes for USENS versus 7.2 minutes for US group, P = 0.225), block performance time was longer in the USENS than in the US group (3.0 versus 1.6 minutes, P < 0.001), and total anesthesia-related time did not differ between the 2 groups (9.2 versus 8.9 minutes, P = 0.71). Block success rate at 15 minutes was 100% in the USENS group and 88% in the US group (P = 0.23). There was no difference in the number of needle passes (2.3 versus 2.1, P = 0.28).

Conclusions In ultrasound-guided obturator nerve block performed after the division of the nerve, injection of local anesthetic between the planes of the adductor muscles is comparable to nerve stimulation.