Ultrasound guided injection inside the common sheath of the sciatic nerve at division level has a higher success rate than an injection outside the sheath

Background and objectiveThe recommendations for the level of injection and ideal placement of the needle tip required for successful ultrasound-guided sciatic popliteal block vary among authors. A hypothesis was made that, when the local anesthetic is injected at the division of the sciatic nerve within the common connective tissue sheath, the block has a higher success rate than an injection outside the sheath.MethodsThirty-four patients scheduled for hallux valgus repair surgery were randomized to receive either a sub-sheath block (n = 16) or a peri-sheath block (n = 18) at the level of the division of the sciatic nerve at the popliteal fossa. For the sub-sheath block, the needle was advanced out of plane until the tip was positioned between the tibial and peroneal nerves, and local anesthetic was then injected without moving the needle. For the peri-sheath block, the needle was advanced out of plane both sides of the sciatic nerve, to surround the sheath. Mepivacaine 1.5% and levobupivacaine 0.5% 30 mL were used in both groups. The progression of motor and sensory block was assessed at 5 min intervals. Duration of block was recorded.ResultsAdequate surgical block was achieved in all patients in the subsheath group (100%) compared to 12 patients (67%) in the peri-sheath group at 30 min. Sensory block was achieved faster in the subsheath than peri-sheath (9.1 ± 7.4 min vs. 19.0 ± 4.0; p < .001).ConclusionsOur study suggests that for successful sciatic popliteal block in less than 30 min, local anesthetic should be injected within the sheath.