What are the pros and cons of femoral (fem) nerve block versus adductor canal block for pain management in a patient with a patellar fracture?

Femoral Nerve Block vs Adductor Canal Block for Patellar Fracture

Direct Answer

For patellar fractures specifically, femoral nerve block is superior to adductor canal block because the patella receives its primary sensory innervation from the femoral nerve's anterior branches, which are not adequately covered by the adductor canal block's predominantly distal saphenous nerve distribution. However, this analgesic superiority comes at the significant cost of quadriceps weakness, which may delay mobilization and increase fall risk 1, 2.

Anatomical Rationale

The key distinction lies in nerve coverage:

• Femoral nerve block provides comprehensive blockade of the femoral nerve and its branches, including the anterior cutaneous branches that supply the patella and anterior knee 1, 3
• Adductor canal block primarily targets the saphenous nerve (a terminal sensory branch) and provides predominantly medial knee and lower leg analgesia, with minimal coverage of the anterior patella 3, 4

For patellar fractures, the anterior knee and peripatellar region are the primary pain generators, making femoral nerve block anatomically more appropriate 1.

Pros and Cons Comparison

Femoral Nerve Block

Advantages:

• Superior analgesia for anterior knee and patellar pain with significantly lower pain scores and reduced opioid consumption 1, 5, 2
• Comprehensive coverage of femoral nerve distribution including patella 1, 3
• Earlier PACU discharge criteria achievement with lower initial pain scores 6, 1
• Well-established technique with extensive evidence base 7

Disadvantages:

• Significant quadriceps weakness (median strength reduced to 0-18% of baseline at 6-8 hours), which is the critical limitation 8, 2, 4
• Increased risk of falls and delayed mobilization due to motor blockade 6, 1
• Higher rate of knee extension ROM loss (21.1% vs 5.0% with ACB in one study) 2
• More pronounced motor blockade compared to local infiltration analgesia 6, 1

Adductor Canal Block

Advantages:

• Preserved quadriceps strength (52% of baseline vs 18% with FNB at 6-8 hours) 8, 4
• Allows earlier ambulation without assistive devices (100% vs 84.2% at 4 weeks) 2
• Lower complication rates including less extension ROM loss 2
• Predominantly sensory block with minimal motor impairment 4

Disadvantages:

• Inadequate analgesia for patellar fractures due to limited anterior knee coverage 3, 4
• Higher pain scores at 18-24 hours postoperatively 5
• Increased morphine consumption compared to femoral block 5, 2
• Does not reliably block the anterior cutaneous branches supplying the patella 3

Clinical Recommendation for Patellar Fractures

For acute patellar fracture pain management, use femoral nerve block as the primary regional technique, but implement strict fall precautions and delay weight-bearing mobilization until quadriceps function returns 1, 2. The American Society of Anesthesiologists recommends 20-30 mL of 0.25-0.5% bupivacaine, 0.25% levobupivacaine, or 0.2% ropivacaine for single-shot femoral blocks 1.

Critical Implementation Points:

• Always combine with multimodal analgesia including baseline paracetamol and NSAIDs/COX-2 inhibitors unless contraindicated—never use nerve blocks as monotherapy 1
• Consider continuous femoral catheter (0.2% ropivacaine at 8 mL/h) for extended analgesia beyond 24 hours 1, 4
• Monitor for quadriceps weakness and implement fall prevention protocols 1, 2
• Assess motor function before allowing ambulation 8, 2

When to Consider Alternatives:

• If early mobilization is the priority over analgesia, fascia iliaca block may be preferable to femoral block (though still superior to adductor canal for patellar coverage) 6, 1
• For patients at high fall risk, consider local infiltration analgesia or neuraxial techniques instead 6
• Adductor canal block should be avoided for patellar fractures as it provides insufficient anterior knee analgesia 3, 4

Common Pitfall

The most critical error is choosing adductor canal block for patellar fractures based solely on its quadriceps-sparing properties, which results in inadequate pain control for the primary injury site 5, 2, 4. While adductor canal block is preferred for knee arthroplasty and ACL reconstruction to facilitate early mobilization 1, 8, patellar fractures require the more comprehensive anterior knee coverage that only femoral nerve block provides 1, 3.