After obtaining approval from the Gachon Gil Hospital Ethical Committee, written informed consent was obtained from all patients anticipating TURB with American Society of Anesthesiologists physical status (ASA) I or II. Sixty two patients anticipating transurethral resection of the bladder tumors (TURB) under spinal anesthesia were enrolled in this randomized controlled trial from December 2014 to May 2015. Exclusion criteria included patients with diabetes or peripheral neuropathy; motor or sensory deficits in the lower extremities, ASA greater than III, coagulation disorders, anticoagulant medication, known allergy to local anesthetics, contraindications for spinal anesthesia (infection at injection site, severe scoliosis or fusion operation), uncooperative patients and patients' refusal. Routine monitoring was begun and Spinal anesthesia with hyperbaric 0.5% bupivacaine 12-15 mg was performed to reach the level of anesthesia, at least T10 to T4 in all patients.

Patients were then randomized into 1 of 2 groups (US-NS vs US-IFI) to receive ONB in the inguinal crease level. Randomization was performed by a computer-generated random numbers table. We counted one side obturator nerve as one, the number. Whether the right or left ONB was done, was decided by surgeon according to the tumor location. If obturator block was done in both sides, let it belong to the same group, and we counted it as two, the number.

Patients were then positioned supine, and the affected leg was slightly abducted and rotated externally without knee flexion, and the inguinal region was prepared with a povidone iodine solution. The 10 MHz linear probe (Zonare Medical Systems, California, USA) equipped with a sterile plastic cover and gel. The transducer was positioned parallel to the inguinal crease with 90° to the skin with the image depth of 4-5 cm. The inguinal region was examined laterally from the femoral vein until the 3 muscle layers consisting of the adductor longus, adductor brevis, and adductor magnus were identified with pectineus muscle medially along the inguinal crease. We allowed transducer tilting cranially 0°-20° angle until a fascial planes of the pectineus and adductor muscles were identified. A thick hyperechoic fascia was used as a targeting landmark for ONB, not focus on the obturator nerve itself (One between adductor longus and adductor brevis muscles for anterior branch, another between adductor brevis and adductor magnus muscles for posterior branch of obturator nerve). We excluded the case, in which three muscle layers were not identified definitely within the range.

In the US-NS group, 22-gauge, 120-mm stimulating needle (Stimuplex insulated needle; D Plus B. Braun, Melsungen, Germany) attached to a nerve stimulator (Stimuplex HNS12;B. Braun, Melsungen, Germany) advanced via an ultrasound in-plane approach from lateral to medical direction to position the needle tip on the anterior branch. The nerve stimulator was then turned on, and the stimulation current started at 0.5 mA. If adductor muscle twitching was observed on the sonogram even at the stimulation current 0.3mA, 10mL of local anesthetics (LA; 1.5% lidocaine + epi 1:200,000) were slowly injected within the muscle interface after negative aspiration, resulting in separation of these two muscles under real-time visualization. If there was misdistribution of LA spread (eg, spread into the muscle tissue), the needle was redirected until the correct spread of LA was visualized. The needle was withdrawn to the skin and reinserted to position the needle tip on the posterior branch. The stimulation current started at 0.5 mA. If adductor muscle twitching was visualized on the sonogram even at 0.3mA, another 5mL of LA was injected. After injection to the anterior and posterior branch, the stimulation current increased 1.0 mA, and needle was re-advanced as like Fig 1d for searching the residual muscle twitching. If muscle twitching was observed, then current was decreased to 0.5 mA, and confirming the muscle twitching on the sonogram at that current, 5ml of LA injected. We wrote that muscle twitching was occurred in what kind of muscles or fascias.

In the US-IFI group, 22-gauge, 120-mm stimulating needle (Stimuplex insulated needle; D Plus B. Braun, Melsungen, Germany) without nerve stimulator advanced via an ultrasound in-plane approach from lateral to medical direction to position the needle tip on anterior branch. After negative aspiration, 10mL of LA were slowly injected within the muscle interface, resulting in separation of these two muscles under real-time visualization. The needle was withdrawn to the skin and reinserted to position the needle tip on posterior branch, another 5mL of LA was injected. After that, the needle was withdrawn to the skin and reinserted to the same spots, attached with nerve stimulator at the stimulation current 1.0 mA for confirming the block of anterior and posterior branch. If adductor muscle twitching was shown on sonogram, another 5mL of LA was injected, and it was documented as 'fail'. After that, needle was re-advanced as like US-NS group for searching the residual muscle twitching. It then went through the same process as the US-NS group, and we wrote that muscle twitching was occurred in what kind of muscles or fascias. We used only the nerve stimulator for confirming the success or fail of the ONB before the TURB surgery.

Any needle redirection after withdrawing to the skin, was recorded as an additional needle pass. Before the injection, the image was captured as static, and the depth of anterior and posterior branch was measured by use of the built-in caliper of the ultrasound machine. We recorded ONB time, and time from the start of the sonographic examination until muscle layers identification was included. All blocks were performed by one anesthesiologist with more than 60 ONB experiences.

After performing the ONB, patients were positioned in lithotomy position. Endoscopic resection of the neoplasm was started using a bipolar resectoscope (electrical current: 280 W) and endovesical irrigation with a normal saline solution. The surgery was performed by six surgeons randomly. We requested two assistants of urology, who didn't know the study groups (US-NS vs US-IFI) for obturator reflex grading. Obturator reflex grade was assessed by the scale of Lee's paper. Gr I: no movement or palpable muscle twitching, Gr II: palpable muscle twitching without movement, Gr III: slight movement of the thigh not interfering with the surgical procedure, and Gr IV: vigorous movement interfering with the surgical procedure.