Chapter 38. Block of the Sciatic Nerve in the Popliteal Fossa

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.

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).

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.

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

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).

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 μsec). 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.

There are two basic types of motor responses that can be elicited with sciatic nerve stimulation at the level of the popliteal fossa. Stimulation of the common peroneal nerve results in dorsiflexion and eversion of the foot, whereas stimulation of the tibial nerve results in plantar flexion and inversion (Figure 38–7). As the stimulating current is being decreased, the twitch of the great toe often remains the only motor response seen with currents of <0.5 mA. Either response is adequate when the response is still present with current intensity of 0.2–0.4 mA (0.1 msec) as long as a large volume of local anesthetic is used. However, when the stimulation can not be accomplished with current <0.5 mA, stimulation of the tibial nerve may result in a higher success rate. Some common responses to nerve stimulation and the course of action to obtain the proper response are shown in Table 38–2.16

Block Dynamics and Perioperative Management

The intertendinous approach to popliteal block is associated with relatively minor patient discomfort because the needle passes only through the adipose tissue of the popliteal fossa. Regardless, adequate sedation and analgesia are always important to ensure a still and tranquil patient. Midazolam 1–2 mg after the patient is positioned and alfentanil 250–500 mcg just before block placement suffices for most patients. A typical onset time for this block is 10–25 minutes, depending on the type, concentration, and volume of local anesthetic used. The first signs of the onset of blockade are usually spontaneously reported by the patient who reports that the foot “feels different” or that he or she is unable to wiggle the toes. Sensory anesthesia of the skin with this block is often the last to develop. Inadequate skin anesthesia despite the apparent timely onset of the blockade is common because it may take up to 30 minutes for full blockade to develop. However, local infiltration at the site of the incision by the surgeon is often all that is needed to allow the surgery to proceed until the block fully sets in.

Continuous Popliteal Block

Continuous popliteal block is an advanced regional anesthesia technique, and adequate experience with the single-shot technique is necessary to ensure its efficacy. The technique is similar to the single-shot injection; however, slight angulation of the needle cephalad is necessary to facilitate insertion of the catheter. Securing and maintaining of the catheter are easy and convenient. This technique can be used for surgery and postoperative pain management in patients undergoing a wide variety of lower leg, foot, and ankle surgeries.

Catheters may be inserted through a variety of approaches and equipment/infusion-specific techniques. There are also a variety of infusion systems, such as simple elastomeric pumps, which are easy for the patient to use. These infusion systems are disposable, deliver a fixed rate, and are generally inexpensive. Mechanical, battery-operated pumps offer more flexibility of programming and bolus dosing, but tend to be more costly.10 Souron et al.17 reported the use of the continuous catheter technique to provide high-quality postoperative analgesia after oncologic orthopedic surgery of the leg.

Ilfeld9 reported excellent postoperative analgesia using a continuous catheter in the popliteal fossa and a portable infusion pump for outpatients having moderately painful, lower extremity orthopedic surgery. Eighty percent of patients receiving ropivacaine infusion did not require oral opioid therapy and reported an average resting pain score of less than 1 (on a verbal pain score scale of zero to 10). Seven percent of patients who received placebo delayed their first dose of oral opioid until after infusion discontinuation, with an average resting pain score of 3–4 (out of 10). Breakthrough pain resulted in the worst resting pain score, with the difference between treatment and control groups being even more pronounced. The patients who received ropivacaine experienced a significant decrease in sleep disturbances, oral opioid use, and opioid-related adverse effects. These benefits were attained for ambulatory patients with the use of a portable, programmable, patient-controlled infusion pump. The degree of analgesia and the relative simplicity of the catheter pump system led to a very high rate of satisfaction for all subjects receiving ropivacaine.

Klein et al.18 examined the efficacy and complications of long-acting popliteal nerve block after discharge home. This prospective study included 1791 patients who had received an upper- or lower-extremity nerve block with 0.5% ropivacaine and were discharged the day of surgery. In all, 2382 blocks were placed: 1119 upper-extremity blocks and 1263 lower-extremity blocks. There were 733 interscalene, 193 supraclavicular, 193 axillary, 338 lumbar plexus, 263 femoral, and 662 sciatic blocks. Block efficacy was demonstrated by a low rate of conversion to general anesthesia (1–6%) and a low percentage of opioid use in the postanesthesia care unit (8–11%). The results showed that long-acting popliteal nerve block may be used in the ambulatory setting with a high degree of efficacy, safety, and satisfaction. Chelly et al.19 documented the benefits of a continuous lateral popliteal sciatic nerve block infusion technique for postoperative analgesia in patients who had undergone open reduction and internal fixation of the ankle. Continuous infusion of ropivacaine 0.2% was associated with a significant reduction of morphine consumption by 29% and 62% during postoperative days 1 and 2, respectively.

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, insulated stimulating needle
• Catheter
• Peripheral nerve stimulator

The needle insertion site is marked at 7 cm proximal to the popliteal fossa crease and between the tendons of the biceps femoris and semitendinosus muscles.

Technique

The continuous popliteal block technique is similar to the single-shot technique. With the patient in the prone position, the skin is infiltrated with local anesthetic using a 25-gauge needle at the injection site 7 cm above the popliteal fossa crease and between the tendons of biceps femoris and semitendinosus muscles. A 5- to 10-cm needle connected to the nerve stimulator (1.5 mA current) is inserted at the midpoint between the tendons of the biceps femoris and semitendinosus muscles (Figure 38–8). The block needle is advanced slowly with a slight cranial direction while seeking a plantar or dorsiflexion of the foot or toes. After obtaining appropriate motor response, the needle is manipulated until the desired response is seen or felt using a current of approximately 0.5 mA. The catheter should be advanced some 5 cm beyond the needle tip (see Figure 38–8). The needle is then withdrawn back to the skin while simultaneously advancing the catheter to prevent its inadvertent removal. Before activating, the catheter is checked for inadvertent intravascular placement by negative aspiration test for blood.

Continuous Infusion

Continuous infusion is initiated after an initial bolus of local anesthetic through the catheter. For this purpose, we routinely use ropivacaine 0.2% (15–20 mL). Diluted bupivacaine or l-bupivacaine is also suitable, but may result in more motor blockade. The infusion is maintained at 10 mL/h or 5 mL/h when a patient-controlled analgesia (PCA) dose is planned (5 mL). The dosing interval for the PCA dose is 20–60 minutes.

Popliteal (Lateral) Approach

The main advantage of the lateral approach to the popliteal block is that the patient does not need to be positioned in the prone position as with all posterior approaches.5,16,20

Regional Anesthesia Anatomy

The sciatic nerve is positioned between the biceps and semitendinosus muscles (see Figure 38–4). During block performance, stimulation of the common peroneal nerve is usually obtained first (65%) because this nerve is positioned lateral and more superficial than the tibial nerve.

Patient Positioning

The patient is in the supine position. 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 placing the foot on a foot rest. Attention should be paid so that the Achilles tendon is also protruding off the foot rest. This positioning allows easy visualization of any foot movement during nerve stimulation.

Equipment

The equipment is identical with that for the posterior intertendinous approach except that a 10-cm stimulating needle is used.

Anatomic Landmarks

Landmarks for the lateral approach to popliteal block include popliteal fossa crease, vastus lateralis muscle, and biceps femoris muscle (Figure 38–9). The needle insertion site is marked in the groove between the vastus lateralis and biceps femoris muscles (Figure 38–10).

Technique

The operator should be seated, facing the side to be blocked. The height of the bed with the patient is adjusted to allow for an ergonomic position and a greater precision during block placement. This position also allows the performer to simultaneously monitor both the patient and the responses to nerve stimulation. The site of needle insertion is cleaned with an antiseptic solution and infiltrated with local anesthetic at the site of estimated needle insertion using a 1½-in., 25-gauge needle.

A 10-cm, 22-gauge needle is connected to a nerve stimulator, inserted in a horizontal plane between the vastus lateralis and biceps femoris muscles, and advanced to contact the femur (see Figure 38–10). The contact with the femur is important because it provides information on the depth of the nerve (typically 1–2 cm beyond the skin–femur distance) as well as on the angle at which the needle will need to be redirected posterior to stimulate the nerve. The current intensity is initially set at 1.5 mA. Keeping the fingers of the palpating hands firmly pressed and immobile in the groove, the needle is then withdrawn to the skin, redirected 30 degrees posterior to the angle at which the femur was contacted, and advanced toward the nerve (Figure 38–11).

When the sciatic nerve is not localized on the first needle pass, the needle is withdrawn to the skin level and the following procedure is followed:

1. 1. Make sure that the nerve stimulator is functional and properly connected to the patient and to the needle and that it is set to deliver current of desired intensity.

2. 2. Make sure that the leg is not externally rotated in the hip joint and that the foot forms a 90-degree angle to the horizontal plane of the table. Any deviation from this angle changes the relationship of the sciatic nerve to the femur and biceps femoris muscle.

3. 3. Mentally visualize the plane of the initial needle insertion and redirect the needle in a slightly posterior direction (5–10 degrees posterior angulation).

4. 4. If the above maneuver fails, withdraw the needle and reinsert with an additional 5–10 degrees posterior redirection.

5. 5. If the above maneuvers fail, withdraw the needle to the skin and reinsert 1 cm inferior to the initial insertion site; then repeat the above steps.

6. 6. Failure to obtain foot response to nerve stimulation should prompt reassessment of the landmarks and arm position. In addition, the stimulating current should be increased to 2 mA.

After the initial stimulation of the sciatic nerve is obtained, the stimulating current is gradually decreased until motor response of the foot or toes is still seen or felt at 0.2–0.5 mA. This typically occurs at a depth of 5–7 cm. At this point, the needle should be stabilized, and after negative aspiration for blood, 35–45 mL of local anesthetic are slowly injected. The hands should be kept as immobile as possible to prevent injection outside the sheath of the sciatic nerve.

Some common responses during block placement using a nerve stimulator and the course of proper action to obtain twitches of the foot (Table 38–3).

Continuous Popliteal Block Through the Lateral Approach

The continuous popliteal block technique through the lateral approach is similar to the single-injection technique. With the patient in the prone position, the skin is infiltrated with local anesthetic at the injection site 8 cm above the popliteal fossa crease and in the groove between the biceps femoris and vastus lateralis muscles using a 25-gauge needle. A 10-cm Tuohy-style tip needle for continuous nerve block is connected to the nerve stimulator (at 1.5 mA current intensity) and inserted to contact the femur. Once the femur is contacted, the needle is withdrawn to the skin and redirected in a slight cranial and posterior direction relative to the plane in which the femur was contacted (usually 30 degrees to the horizontal plane) (Figure 38–13).

The needle is advanced slowly while seeking a plantar flexion or dorsiflexion of the foot or toes. After obtaining appropriate twitches, the needle is manipulated until the desired response is seen or felt using a current of 0.5 mA. The catheter should be advanced some 5–7 cm beyond the tip of the needle. The management of the catheter is similar to that of the intertendinous technique.

Complications after a popliteal block are uncommon. Table 38–4 provides specific instructions on possible complications and how to avoid them.

Popliteal sciatic block is an extremely useful technique to accomplish anesthesia or analgesia after ankle and foot surgery. Posterior and lateral approaches are both highly effective and applicable in numerous clinical scenarios.