Ultrasound versus nerve stimulation-guided interadductor approach for obturator nerve block: evaluation of injectate spread into the obturator canal in a randomized controlled trial

BACKGROUND: This study aimed to compare the percentage of local anesthetic spread into the obturator canal between the nerve stimulation-guided interadductor approach (NS-INTAD) and the ultrasound-guided interadductor approach (US-INTAD), and to evaluate anatomical factors influencing the effectiveness of the obturator nerve block (ONB). METHODS: Eighty patients scheduled for transurethral resection of bladder tumor were randomized to receive either NS-INTAD (n = 40) or US-INTAD (n = 40). In the lithotomy position, a mixture of local anesthetic and contrast medium was injected via the interadductor approach from the medial thigh. Injectate spread into the obturator canal was assessed by fluoroscopy, and the success rate and injection volume were compared between groups. RESULTS: Enhancement in the obturator canal was observed in 75% of patients in the NS-INTAD group and 90% in the US-INTAD group. Although the percentage was higher in the US-INTAD group, the difference was not statistically significant (P = 0.139). The volume of local anesthetic used did not differ significantly between groups (P = 0.07). In patients where the posterior branch of the obturator nerve coursed between obturator externus muscle fascicles, local anesthetic propagation into the obturator canal appeared limited. CONCLUSION: The percentage of injectate spread into the obturator canal tended to be higher with US-INTAD than with NS-INTAD. Our findings suggest that the lithotomy-position approach facilitates more cephalad local anesthetic spread than the supine-position approach. Furthermore, the development of the superior fascicle of the external obturator muscle appears to be an anatomical factor that inhibits injectate spread into the obturator canal. TRIAL REGISTRATION: UMIN Clinical Trials Registry (UMIN-CTR) UMIN000027762; registered 15 June 2017.