BLOCK AT A GLANCE
Block of the sciatic nerve at the popliteal fossa.
Indications: Foot and ankle surgery; analgesia after major knee surgery
Goal: Local anesthetic (LA) spread within the sciatic nerve sheath (Vloka’s sheath) between tibial and common peroneal nerves
Local anesthetic volume: 15 to 20 mL
The popliteal block is a commonly used anesthesia technique for foot and ankle procedures, particularly in the setting of ambulatory surgery. It provides a sensory-motor block of the lower extremity below the knee. The anesthetic effect is similar to that of the proximal sciatic nerve block but spares the hamstring muscles.
The introduction of ultrasound (US) and research on the functional anesthesia anatomy of the popliteal space resulted in substantial refinement and standardization of popliteal block techniques. In particular, US allows monitoring of the LA spread proximally/distally, and determination of the level where the sciatic nerve divides, as the optimal injection site. Monitoring of the spread of the injectate also allows for a reduction of the LA volume and dose required for a successful block. The popliteal block can be performed in different patient positions and with different needle insertion techniques to obtain consistent results.
The popliteal block results in complete motor block below the knee limiting the ability to ambulate without assistive devices.
The sciatic nerve descends through the posterior compartment of the thigh into the popliteal fossa where its main components, the tibial nerve (TN) and the common peroneal nerve (CPN), diverge. The level at which the TN and CPN divergence occurs varies being approximately 2 to 4 cm proximal to the popliteal crease. From their origin in the pelvis, the TN and CPN are enveloped by a common connective tissue sheath that continues along the nerves individually after their separation (Figure 29-1). The nomenclature, consistency, and function of the connective tissue sheath around the sciatic nerve have been the subject of considerable debate. In this text, we opt to call it the Vloka sheath in recognition of Dr. Jerry Vloka’s contribution to the understanding of the sciatic sheath and its role in sciatic popliteal block.
Anatomy of the sciatic nerve in the popliteal fossa.
In the popliteal fossa, the nerve becomes relatively superficial as the biceps femoris tapers into a tendon. The TN continues its course along the posterior aspect of the popliteal vein and artery while the CPN courses laterally deep to the tendon of the biceps femoris muscle and descends around the head and neck of the fibula. The semimembranosus muscle and the tendon of the semitendinosus muscle are located at the medial side of the popliteal fossa.
CROSS-SECTIONAL ANATOMY AND ULTRASOUND VIEW