Ultrasound Guided Popliteal Fossa Block. Does Blockade Distal to Sciatic Nerve Bifurcation Speed Onset Time? A Prospective, Randomized Trial

Overview

Sciatic nerve block is used routinely in ankle and foot surgery. It is applied often by a posterior approach at the popliteal fossa, near where the nerve divides into the common peroneal and tibial nerves. Due to the size of the sciatic nerve, the largest in the body, infiltration of anesthetic and onset of anesthesia can take a significant amount of time. This study hypothesizes that selectively blocking both the common peroneal and the tibial nerves distal to the bifurcation may result in a faster block onset than blockade of the sciatic nerve proximal to its bifurcation.

Sciatic nerve block (SNB) has been shown to reduce postoperative pain and opioid requirements, as well as improve patient satisfaction after outpatient foot and ankle surgery. The sciatic nerve divides into two terminal branches (the common peroneal and tibial nerves) in the popliteal fossa. Through these two branches it innervates the entire leg, ankle and foot except for the skin over the anterior-medial aspect of the leg, which is supplied by the saphenous nerve, a branch of the femoral nerve. Many approaches have been described to sciatic nerve blockade. High in the thigh or buttock are the classical posterior approach, the supine approach, and the anterior approach. In the popliteal fossa, a posterior, a lateral and a medial approach have been described. The popliteal fossa approaches are most commonly used to provide regional anesthesia and/or analgesia for major foot and ankle surgery. Traditional nerve localization techniques include seeking paresthesiae on needle to nerve contact, or alternatively seeking a muscle response to electrical nerve stimulation using a blunt insulated needle and a peripheral nerve stimulator. However, block results with traditional nerve localization techniques are highly variable, often requiring high local anesthetic doses, and a prolonged onset time. When a distal approach in the popliteal fossa is used (5-7 cm above the knee crease) with a nerve stimulator technique, a double motor response (both tibial and peroneal components) may be required to achieve consistent success. Recently, ultrasound guided techniques for SNB at the popliteal fossa have been described. Ultrasound guidance offers the unique advantages of real time anatomic assessment of the sciatic nerve itself, its site of bifurcation, and its relationship to surrounding muscles and the popliteal vessels. In a recent study we have shown that ultrasound guidance significantly improves the success rate of SNB at the popliteal fossa via a posterior approach. Despite these advantages, a prolonged onset time for complete blockade of up to 30-60 minutes (depending on the choice and dose of local anesthetic) is common. This may be a limitation to the widespread use of popliteal sciatic nerve blockade in a peri-operative environment, where a faster block onset may be preferable. This delayed onset has been attributed to the large size of the sciatic nerve. Therefore logically it should be faster to induce a block by depositing the local anesthetic after the sciatic nerve divides where the connective tissues decrease and the nerves are superficial. No exact mathematical data is available looking into onset times for nerves with varying diameter, as no previous studies are done in this area.