What is the procedure for performing an obturator nerve block?

Ultrasound-Guided Obturator Nerve Block Procedure

Ultrasound-guided obturator nerve block should be performed using a proximal interfascial approach with a single injection between the pectineus and obturator externus muscles for optimal efficacy and safety. 1, 2

Anatomical Considerations

• The obturator nerve and its branches (anterior, posterior, and hip articular) all pass through the obturator canal, making this location ideal for a single-injection technique 3
• A hyperechoic triangular shape formed by the superior pubic ramus, posterior margin of the pectineus muscle, and anterior aspect of the external obturator muscle contains the obturator nerve and vessels 4
• Important landmarks include the distance from the obturator nerve to the femoral vein (approximately 12.9 mm) and from the nerve to the pubic tubercle (approximately 19.9 mm) 4

Proximal Approach Technique

1. Patient Positioning

   • Position the patient supine with the leg slightly abducted 1, 2
   • For lithotomy procedures, the block can be performed with the patient in lithotomy position 3

2. Equipment Preparation

   • Use standard aseptic technique similar to neuraxial procedures 5
   • Calculate a safe dose of local anesthetic based on patient weight to prevent systemic toxicity 5
   • Use ultrasound guidance to reduce the risk of local anesthetic systemic toxicity 6, 5

3. Ultrasound Scanning

   • Place a linear ultrasound transducer on the anterior part of the genitofemoral sulcus 3
   • Identify the pectineus muscle and follow it cranially until the superior pubic ramus is visualized 1
   • Locate the external orifice of the obturator canal, which is immediately posterior to the inferior margin of the superior pubic ramus 3, 2

4. Needle Insertion and Injection

   • Use in-plane needling ALONG the visual axis for improved speed and accuracy 6
   • Guide the needle tip to the interfascial plane between the pectineus and obturator externus muscles 1
   • Inject 10 ml of local anesthetic into this space, which will spread into the obturator canal 3, 1
   • Median motor block onset is approximately 4 minutes 1

Alternative Distal Approach

• Can be performed by placing the transducer at the inguinal crease 2
• Requires two separate injections to block the anterior and posterior branches 2
• May require higher volumes of local anesthetic compared to the proximal approach 2
• Less likely to block the hip articular branch of the obturator nerve 2

Safety Considerations

• Monitor blood pressure and electrocardiogram, especially in patients with high cardiovascular risk 5
• Use dominant-hand needling for novices to improve accuracy 6
• Thoroughly test for block success before proceeding with surgery 6, 7
• Avoid excessive sedation to reduce the risk of respiratory compromise 6

Potential Complications and Precautions

• Avoid compression near the fibular head during patient positioning to prevent peroneal neuropathy 7
• Balance the benefit of perineural adjuvants against risks of immunosuppression (dexamethasone), sedation, bradycardia, and hypotension (clonidine and dexmedetomidine) 6
• Consider the risk-benefit ratio of continuous catheter techniques on a case-by-case basis 6

Ergonomic Considerations

• Position the ultrasound machine in front of the operator rather than perpendicular for improved accuracy 6
• Avoid turning the trunk, turning the head 45° or more, and non-dominant hand needling to prevent fatigue and suboptimal block performance 6
• Allow adequate space for proper positioning of both operator and equipment 6