What is the recommended approach for a motor sparing lower extremity block?

Motor Sparing Lower Extremity Block

For motor sparing lower extremity blocks, the recommended approach is to use low concentrations of local anesthetic (0.1-0.25% bupivacaine or ropivacaine) with careful technique selection to preserve motor function while providing adequate analgesia. 1

Recommended Techniques

• Use low-dose local anesthetic concentrations (0.1-0.25% bupivacaine or ropivacaine) to minimize motor block while maintaining sensory analgesia 1, 2
• Select anatomically targeted blocks that primarily affect sensory nerves while sparing motor function:
  • Modified adductor canal block combined with lateral femoral cutaneous nerve block for knee procedures 3
  • Pericapsular nerve group (PENG) block for hip procedures (25% lower incidence of quadriceps motor block compared to fascia iliaca block) 4
  • Hi-PAC (High-volume Proximal Adductor Canal) block for below-knee surgeries 5

Monitoring and Assessment

• Use straight-leg raising ability (raising heel off bed against gravity) as a screening tool to assess motor function 6
• If more detailed assessment is required, implement the Bromage scale to document the degree of motor block 6
• Monitor motor function regularly to detect any unexpected progression of block 6

Clinical Applications

For Knee Procedures

• Combine modified adductor canal block with lateral femoral cutaneous nerve block and posterior knee infiltration for longer analgesia (18.1 hours vs 9.25 hours with periarticular infiltration alone) while preserving quadriceps strength 3

For Hip Procedures

• Use PENG block with 20 mL of local anesthetic for significantly better preservation of motor function compared to fascia iliaca block (25% vs 85% incidence of quadriceps motor block at 6 hours) 4

For Below-Knee Procedures

• Implement Hi-PAC block targeting the saphenous nerve in the proximal adductor canal, which provides procedure-specific analgesia without affecting motor power 5

Important Considerations

• Motor sparing blocks are particularly important when monitoring for compartment syndrome, as they allow for continued assessment of leg strength as a critical monitor of spinal cord health 6
• For patients at risk of acute compartment syndrome, the ability to perform straight-leg raising serves as an important clinical indicator 6, 7
• Avoid high concentrations of local anesthetic (0.75% bupivacaine) as these produce complete motor block 2

Cautions and Pitfalls

• Do not attribute prolonged motor block (>24 hours) solely to expected effects of local anesthetic, as this may delay diagnosis of serious complications 1
• Be vigilant for unexpected extensive block, which may indicate misplaced catheter or developing complications 6
• If a patient is unable to straight-leg raise at 4 hours post-block, escalate to anesthesiologist assessment immediately 6
• Consider adding adjuncts cautiously as they can increase the density and duration of blocks 6

Algorithm for Motor Sparing Block Selection

1. Identify the surgical site and required dermatomes for analgesia 8
2. Select the most distal appropriate block to minimize motor involvement 9
3. Choose the lowest effective concentration of local anesthetic (0.1-0.25%) 1, 2
4. Monitor motor function regularly using straight-leg raising as a screening tool 6
5. Establish clear protocols for escalation if unexpected motor block occurs 6

Motor sparing blocks not only preserve mobility for early rehabilitation but also serve as a critical safety feature by allowing continued assessment of neurological function, particularly important for detecting potential complications such as epidural hematoma or compartment syndrome 6, 1.