How can I safely perform a landmark‑guided obturator nerve block in an adult patient when ultrasound is unavailable, for knee surgery analgesia, reduction of adductor spasm, or assistance with bladder catheterisation?

Landmark-Guided Obturator Nerve Block: Practical Technique When Ultrasound is Unavailable

Landmark-guided obturator nerve block should only be performed when ultrasound is truly unavailable, as ultrasound guidance significantly improves success rates, reduces complications, and allows for lower local anesthetic volumes. However, when ultrasound is not accessible, a systematic anatomical approach can achieve successful blockade.

Understanding the Anatomical Landmarks

The obturator nerve emerges from the obturator foramen and divides into anterior and posterior branches as it passes between the adductor muscles of the thigh. For landmark technique, you must identify:

• The pubic tubercle - palpate along the superior pubic ramus to locate this bony prominence 1
• The femoral artery pulse - identify at the inguinal crease, as the obturator nerve lies approximately 12-20 mm medial to the femoral vein 1
• The inguinal crease - marks the level where the nerve branches are most accessible 2

Patient Positioning and Preparation

• Position the patient supine with the operative leg slightly abducted and externally rotated to relax the adductor muscles 2
• Prepare the inguinal area with strict aseptic technique comparable to neuraxial procedures 3
• Have resuscitation equipment immediately available at bedside 3
• Monitor blood pressure and electrocardiogram continuously, especially in patients with cardiovascular risk 3

Landmark-Based Injection Technique (Distal Approach)

The distal approach requires two separate injections to block both the anterior and posterior branches:

• Identify the pubic tubercle and mark a point 1.5-2 cm lateral and 1.5-2 cm inferior to it 1
• Insert a 22-gauge, 8-10 cm insulated needle perpendicular to the skin at this point 1
• Advance the needle until you contact the inferior pubic ramus (typically 2-4 cm depth) 1
• For the anterior branch: Walk the needle laterally off the bone and advance 1-2 cm deeper into the plane between adductor longus and adductor brevis muscles 4
• Inject 5-7 mL of local anesthetic (ropivacaine 0.5% or bupivacaine 0.5%) after negative aspiration 4
• For the posterior branch: Redirect the needle more posteriorly and advance into the plane between adductor brevis and adductor magnus muscles 4
• Inject an additional 5-7 mL of local anesthetic after negative aspiration 4

Critical Safety Considerations

• Calculate maximum safe local anesthetic dose based on patient weight before beginning the procedure 3
• The needle insertion point should be approximately 18-19 mm horizontal and 21 mm vertical from the pubic tubercle 1
• Expected needle depth is 48 mm (range 38-68 mm) to reach the obturator nerve 1
• Maintain needle tip at least 18 mm away from the femoral artery to avoid vascular injury 1

Advantages and Limitations of Landmark Technique

Advantages over neuraxial techniques:

• No sympathectomy-induced hypotension occurs 3
• Lower incidence of urinary retention compared to spinal or epidural 3
• Classified as low-risk for hemorrhagic complications because bleeding is readily compressible 5

Critical limitations compared to ultrasound guidance:

• Higher failure rates - ultrasound techniques achieve 88-100% success versus variable success with landmarks 4, 6
• Requires larger volumes of local anesthetic (10-14 mL total versus 5-10 mL with ultrasound) 4, 6
• Cannot visualize nerve anatomy variations or the actual nerve divisions 2
• Increased risk of multiple needle passes and longer performance times 4

Assessing Block Success

• Test adductor muscle strength at 5,10, and 15 minutes after injection 4
• Successful block produces at least 50% reduction in thigh adduction strength by 15 minutes 6
• Mean onset time is 6-7 minutes for motor blockade 4
• Sensory distribution is highly variable - 32% of patients have no cutaneous sensory block despite successful motor block 7

Common Pitfalls and How to Avoid Them

• Do not attempt this block without proper training in the anatomical relationships - the proximity to femoral vessels (12-13 mm from femoral vein) creates significant vascular injury risk 1
• Never inject without negative aspiration - the obturator vessels run alongside the nerve and inadvertent intravascular injection can cause systemic toxicity 1
• Do not use this technique if ultrasound is available - modern practice standards strongly favor ultrasound guidance for improved safety and efficacy 5
• Avoid in patients requiring anticoagulation unless the benefit-risk ratio clearly favors the block, as landmark technique has higher complication rates than ultrasound-guided approaches 5

Integration with Multimodal Analgesia

• Combine obturator nerve block with femoral and sciatic blocks for complete knee surgery analgesia 2
• Add scheduled acetaminophen and NSAIDs to enhance overall analgesic effectiveness 3
• The opioid-sparing effect reduces respiratory complications and delirium in high-risk patients 3

When Landmark Technique is Justified

Landmark-guided obturator nerve block should only be performed when:

• Ultrasound equipment is genuinely unavailable (not merely inconvenient) 5
• The clinical indication is compelling (prevention of adductor spasm during bladder tumor resection, knee surgery analgesia, or hip adductor spasticity treatment) 2
• The operator has adequate training in the anatomical technique 5
• The patient has been informed that ultrasound guidance would be preferred if available 5