• Admir Hadzic, MD

        INTRODUCTION

Distal sciatic nerve block (popliteal fossa block) is a relatively simple technique that results in reliable surgical anesthesia of the calf, tibia, fibula, ankle, and foot.^1,2 Consequently, this technique is used primarily for anesthesia or analgesia for foot, ankle, and lower-leg surgery.³ The sciatic nerve can be approached from either the posterior approach described by Rorie,³ or the lateral approach, which eliminates the need to reposition patients in the supine position.¹ With the lateral popliteal fossa block, patients remain supine for the block and catheter placement, rather than being prone and then turning supine after the block is placed. Both approaches provide equivalent surgical anesthesia after nerve blockade.¹ With both approaches, catheters can be inserted to provide prolonged postoperative analgesia; catheters, however, are more easily secured in the lateral position. Because of the slower resolution of neural blockade in the lower extremity, popliteal fossa block performed with long-acting local anesthetics such as ropivacaine can provide 12–24 hours of analgesia after foot surgery. The remarkable safety of the block has been demonstrated in numerous studies.^3,4

        Analgesia with lower-extremity blocks typically lasts longer than analgesia with ankle block. For instance, McLeod found that lateral popliteal fossa block with 0.5% bupivacaine lasted 18 hours when compared with ankle block, which lasted only 6.2 hours.⁵ Popliteal fossa block has also been used as an effective analgesic technique in children.⁶ In a study of the efficacy of the popliteal sciatic nerve blockade (0.75 mL/kg of ropivacaine 0.2%) after foot and ankle surgery, 19 of 20 children required no analgesic agents during the first 8–12 hours postoperatively. Blocking the sciatic nerve in the popliteal fossa is an excellent choice for foot and ankle surgery.¹ When used as a sole technique in outpatients, popliteal fossa block provides excellent anesthesia and postoperative analgesia, allows use of a calf tourniquet, and is devoid of disadvantages of neuraxial blockade.⁷

Indications & Contraindications

The popliteal block is one of the most commonly used regional anesthesia techniques in regional anesthesia practice. Common indications include corrective foot surgery, foot debridement, short saphenous vein stripping, repair of the Achilles tendon, and others.⁸ As opposed to the more proximal block of the sciatic nerve, popliteal fossa block anesthetizes the leg distal to the hamstring muscles, allowing patients to retain knee flexion.^9,10

Functional Anatomy

The sciatic nerve is a nerve bundle consisting of two separate nerve trunks, the tibial and common peroneal nerves. A common epineural sheath envelops these two nerves at their outset in the pelvis.¹¹ As the sciatic nerve descends toward the knee, the two components eventually diverge in the popliteal fossa, giving rise to tibial and common peroneal nerves (Figure 38–1). This division of the sciatic nerve occurs usually between 50 and 120 mm proximal to the popliteal fossa crease.^12,13 From its divergence from the sciatic nerve, the common peroneal nerve continues its path downward and descends along the head and neck of the fibula. Its major branches in this region are branches to the knee joint and cutaneous branches that form the sural nerve. Its terminal branches are superficial and deep peroneal nerves. The tibial nerve is the larger of the two divisions of the sciatic nerve and continues its path vertically through the popliteal fossa. Its terminal branches are the medial and lateral plantar nerves. Its collateral branches give rise to the cutaneous sural nerves, muscular branches to the muscles to the calf, and articular branches to the ankle joint. The tibial nerve is enveloped by a well-defined epineural sheath; consequently, single injection of a large volume of local anesthetic into the sheath of the tibial nerve may carry a higher success rate than injection into the sheath of the common peroneal nerve.¹¹ Note that in contrast to the common assumption, the sciatic nerve and popliteal vessels are not enveloped by the same tissue sheath; consequently, the concepts of the neurovascular sheath are not applicable to this block.¹¹ Instead, in the popliteal fossa the sciatic nerve components are lateral and superficial to the popliteal artery and vein. This anatomic characteristic is important in understanding why vascular punctures and systemic toxicity are so rare after popliteal blockade.

Figure 38–1. Anatomy of the distal sciatic nerve. The sciatic nerve descends between the hamstring muscles and diverges as tibial and common peroneal nerves approximately at or below 7–8 cm above the popliteal fossa crease. A common epineural sheath that serves as a conduit for injected local anesthetic is shown dissected.

Distribution of Anesthesia

Popliteal blockade results in anesthesia of the entire distal two thirds of the lower extremity, with the exception of the medial aspect of the leg)¹⁴ (Figure 38–2). Cutaneous innervation of the medial leg below the knee, however, is provided by the saphenous nerve, a superficial terminal extension of the femoral nerve. Depending on the level of surgery, the addition of saphenous nerve block may be required for surgery Popliteal block alone is typically sufficient as anesthesia for the tourniquet pain, because this pain is the result of the pressure and ischemia of the deep muscle beds.

Choice of Local Anesthetic

Popliteal blockade requires a larger volume of local anesthetic (35–45 mL) to achieve anesthesia of both divisions of the nerve.⁷ The choice of type, volume, and concentration of local anesthetic should be based on the patient’s size and general condition and whether the block is planned for surgical anesthesia or pain management. The type and concentration of local anesthetics and the choice of additives to local anesthetic influence the onset and, particularly, the duration of the blockade (Table 38–1).

        TECHNIQUES

Intertendinous (Posterior) Approach

The patient is in the prone position.¹⁵ The foot on the side to be blocked should be positioned so that even the slightest movement of the foot or toes can be easily observed. This is best achieved by allowing the foot to protrude off the bed.

Figure 38–2. Sensory distribution of anesthesia after popliteal blockade.

Equipment

A standard regional anesthesia tray is prepared with the following equipment:

    Sterile towels and 4-in. × 4-in. gauze packs Three 20-mL syringes with local anesthetic Sterile gloves, marking pen, and surface electrode One 1½-in., 25-gauge needle for skin infiltration A 5-cm long, short-bevel, insulated stimulating needle Peripheral nerve stimulator

Table 38–1.

Local Anesthetics Choice for Popliteal Block

Anatomic Landmarks

Landmarks for the intertendinous approach to popliteal block are easily recognizable even in obese patients (Figure 38–3). The landmarks should be routinely outlined by a marking pen: (1) popliteal fossa crease, (2) tendon of biceps femoris (laterally), and (3) tendons of semitendinosus and semimembranosus (medially).

        The needle insertion point is marked at 7 above the popliteal fossa crease at the midpoint between the tendons. This point is just above the sciatic nerve in the popliteal fossa in nearly two thirds of patients (Figure 38–4).

Clinical Pearls

Technique

After application of an antiseptic solution, local anesthetic is infiltrated subcutaneously at the site of the block needle entry. The practitioner is best positioned on the side of the patient with the palpating hand on the biceps femoris muscle while observing the motor response of the foot and toes (Figure 38–6). The needle is introduced at the midpoint between the tendons. The nerve stimulator should be initially set to deliver 1.5 mA current (2 Hz, 100 psec). When the needle is inserted in a correct plane, advancement of the needle should not result in local muscular twitches; the first response to nerve stimulation is typically that of the sciatic nerve (foot twitch). After the initial stimulation of the sciatic nerve is obtained, the stimulating current is gradually decreased until twitches are still seen or felt at 0.2–0.5 mA. This typically occurs at a depth of 3–5 cm from the skin. After negative aspiration for blood, 35–45 mL of local anesthetic is slowly injected.

Figure 38–3. Popliteal block. Landmarks for the intertendinous approach. Seven centimeters above the popliteal fossa crease (1) the sciatic nerve is shown positioned between the tendons of the biceps femoris (2) and semitendinosus (3) muscles.

Figure 38–4. MRI of the popliteal fossa 7 cm above the popliteal fossa crease. 1, Tendon of the biceps femoris muscle; 2, tendon of the semitendinosus muscle; 3, sciatic nerve in the popliteal fossa (shown are both components: the tibial nerve is positioned more anteriorly and medially, whereas the common peroneal nerve is more posterior and lateral); 4, popliteal artery and vein; 5, femur.

Figure 38–5. The landmarks for the popliteal block can be accentuated by asking the patient to flex the leg.

Figure 38–6. Popliteal block, intertendinous approach. The needle is inserted between tendons of the biceps femoris and semitendinosus muscles.

Clinical Pearls

Related posts:

Regional Anesthesia in the Patient with Preexisting Neurologic Disease. The History of Local Anesthesia. Stimulating Catheters. Ultrasound-Assisted Nerve Blocks in Adults. Cutaneous Blocks for the Upper Extremity. Cervical Plexus Block.