Sciatic Nerve Block – Landmarks and Nerve Stimulator Technique

INTRODUCTION

Victor Pauchet, a French surgeon, first described the sciatic nerve block in L’Anesthesie Regionale in 1920: “the site of needle insertion for blocking the sciatic nerve at the level of hip: 3 cm along the perpendicular that bisects a line drawn between the greater trochanter and the posterior superior iliac spine.”1 This technique has since been referred to as “The classic approach of Labat,” possibly because it was first described in the English language literature in 1923 by Gaston Labat, a student of Pauchet, in his book Regional Anesthesia: Its Technic and Clinical Application.2 Labat’s book went through several reprints of the first edition of one of the first English-language textbooks of regional anesthesia. Curiously, this book was very similar to L’Anesthesie Regionale.3 In the same year, Labat founded the American Society of Regional Anesthesia (ASRA). Anecdotally, Labat intended to name the new group “The Labat Society” in his own honor, but the name ASRA remains today as we know it.Alon Winnie eventually modified the Labat approach in 1975.4 Alternatives, such as the anterior approach described by George Beck in 1963 and the lithotomy approach described by Prithvi Raj in 1975, were devised to allow the sciatic nerve to be blocked in the supine patient.5.6 A number of other approaches have been proposed, most of which include minor modifications. The most useful of these newer techniques are likely the subglu-teal and parasacral approach introduced by Pia di Benedetto and Philippe Cuvillon, respectively.7.8 In this chapter, we focus on the classic approach to sciatic nerve block, parasacral and subgluteal modifications, and the anterior approach.

Indications and Contraindications

Indications for sciatic nerve block include lower-limb surgery, combined with a femoral or psoas compartment block.9 For distal surgery of the lower extremity, however, more distal approaches such as ankle block or popliteal sciatic nerve block are preferable whenever feasible. Note that the sciatic nerve block often needs to be combined with additional blocks, such as lumbar plexus (femoral or saphenous nerve) when anesthesia of the entire lower extremity is desired.

Contraindications to sciatic nerve block may include include local infection and bed sores at the site of insertion, coagulopa-thy, preexisting central or peripheral nervous systems disorders, and allergy to local anesthesia.

Functional Anatomy

The union of the lumbosacral trunk with the first three sacral nerves forms the sacral plexus (Figure 82D–1). The lumbosa-cral trunk originates from the anastomosis of the last two lum-bar nerves with the anterior branch of the first sacral nerve. This structure receives the anterior branches of the second and third sacral nerves, forming the sacral plexus. The sacral plexus is shaped like a triangle pointing toward the sciatic notch, with its base spanning across the anterior sacral foramina. It rests on the anterior aspect of the piriformis muscle and is covered by the pelvic fascia, which separates it from the hypogastric vessels and pelvic organs. Seven nerves stem from the sacral plexus: six collateral branches and one terminal branch—the sciatic nerve, the largest nerve of the plexus (Figure 82D–2).

FIGURE 82D–1. Formation of the sacral plexus.

FIGURE 82D–3. Parasacral area. Transversal section at the S3 level. 1, Iliac bone; 2, sacrum; 3, gluteal muscle; 4, piriformis muscle; 5, pelvic aponeurosis; 6, inferior gluteal plexus; 7, lumbosacral trunk; 8, first sacral root; 9, second sacral root

The sciatic nerve is the largest peripheral nerve in the body and measures more than 1 cm in width at its origin. It exits the pelvis through the greater sciatic notch below the piriformis muscle, then descends between the greater trochanter of the femur and the ischial tuberosity. The nerve then runs along the posterior thigh to the lower third of the femur, where it diverges into two large branches, the tibial and common peroneal nerves. This division may occur at any level proximal to the lower third of the femur.10,11 The common peroneal and tibial nerves are separated from their onset at the sacral plexus (15%); in this case, the common peroneal nerve typically pierces the piriformis muscle. The course of the sciatic nerve can be estimated by drawing a line on the back of the thigh beginning from the apex of the popliteal fossa to the midpoint of the line joining the ischial tuberosity to the apex of the greater trochan-ter. From its onset, the sciatic nerve also gives off numerous articular (hip, knee) and muscular branches.

FIGURE 82D–2. Course of the sciatic nerve at the exit from the pelvis.

FIGURE 82D–4. Computed tomographic (CT) scan of the parasacral area, at the S3 level. 1, Iliac bone; 2, sacrum; 3, gluteal muscle; 4, piriformis muscle; 5, pelvic aponeurosis; 6, inferior gluteal plexus; 7, lumbosacral trunk; 8, first sacral root; 9, bladder.

In the upper part of its course, the sciatic nerve lies deep in the gluteus maximus muscle and rests on the posterior surface of the ischium (Figures 82D–3 and 82D–4). The sciatic nerve crosses the external rotators, obturator internus, and gemelli muscles, then passes on to the quadratus femoris. The quadratus femoris separates the sciatic nerve from the obtura-tor externus and the hip joint. Medially, the posterior cutane-ous nerve of the thigh and the inferior gluteal plexus accompany the sciatic nerve, whereas more distally the sciatic nerve lies on the adductor magnus. The long head of the biceps femoris crosses the sciatic nerve obliquely. The articular branches of the sciatic nerve arise from the upper part of the nerve and supply the hip joint by perforating the posterior part of its capsule. However, the articular branches are some-times derived directly from the sacral plexus. The muscular branches of the sciatic nerve innervate the gluteus, the biceps femoris, the ischial head of the adductor magnus, the semiten-dinosus, and the semimembranosus muscles (Figure 82D–5; Table 82D–1). The branches for the ischial head of the adductor magnus and semimembranosus muscles arise from a common trunk. The nerve to the short head of the biceps femoris comes from the common peroneal division, whereas the other muscular branches arise from the tibial division of the sciatic nerve.

FIGURE 82D–5. Sciatic nerve. Downward course and motor branches to the hamstrings muscles.

TABLE 82D–1.Branches, source, and motor innervation of the sacral plexus.

NerveSourceMuscular Innervation
Nerve to obturator internus muscleLumbosacral trunk and S1Obturator internus
Superior gluteal nerveLumbosacral trunk and S1Gluteus medius
Gluteus minimus
Tensor fasciae lata
Nerve to piriformis muscleS2Piriformis
Nerve to biceps femoris superiorAnterior portion of plexusBiceps femoris superior
Nerve to biceps femoris inferior and quadratus femorisAnterior portion of plexusBiceps femoris inferior
Quadratus femoris
Branch to coxofemoral articulation
Posterior femoral cutaneous nerve (lesser sciatic nerve)Lumbosacral trunk, S1, S2Inferior gluteal n. to gluteus maximus muscle
Sensory branch to buttock, thigh, popliteal fossa, and lateral aspect of knee

The parasacral area is delineated by the ventral aponeurosis of the piriformis muscle dorsally, by the pelvic aponeurosis medially, and by the aponeurosis of the obturator internis muscle laterally.8 The common peroneal component passes through the piriformis muscle or above it, and only the tibial component passes below the muscle.

The tibial and common peroneal elements of the sciatic nerve each have their own outer layer of epineurium. Both components are further enclosed by a dense layer of connective tissue, which runs from the origin of the sciatic nerve to its bifurcation. This layer has been given several names over the years, but lately it is commonly referred to as the “paraneural sheath” of the sciatic nerve. Injection of local anesthetic deep to this sheath (but outside the epineurium of the tibial or com-mon peroneal nerves) has been shown to spread a considerable distance proximally and distally, and result in a rapid onset, dense block. This is not considered an “intraneural” injection as the injection occurs outside of the epineurium.12–14

Choice of Local Anesthetic

Despite its large size, sciatic block requires a relatively low vol-ume of local anesthetic to achieve anesthesia of the entire trunk of the nerve.15 Generally, 20–25 mL of local anesthetic is suf-ficient. The choice of the type and concentration of local anes-thetic should be based on whether the block is planned for surgical anesthesia or pain management16 (Table 82D–2). When seeking prolonged pain relief, longer-acting local anes-thetic may be more appropriate.17,18 Addition of epinephrine may be justified in patients undergoing above the knee amputa-tion, in whom prolonged analgesia is desired.

TABLE 82D–2.Local anesthetic choices for sciatic nerve block: duration of anesthesia and analgesia.

 Onset (min)Anesthesia (h)Analgesia (h)
3% 2-Chloroprocaine10–1522.5
1.5% Mepivacaine10–154–55–8
2% Lidocaine10–205–65–8
0.5% Ropivacaine15–206–126–24
0.75% Ropivacaine10–158–128–24
0.5% Bupivacaine15–308–1610–48

Equipment

As with all regional anesthesia techniques, the heart rate, blood pressure, and pulse oximetry are routinely monitored before performing the block. Resuscitation equipment and emergency medications must be immediately available and ready to use. Supplemental oxygen via face mask is routinely used before giving sedation. A standard regional anesthesia tray is prepared with the following equipment:

  • Sterile towels and 4-in. × 4-in. gauze packs
  • 20-mL syringe with local anesthetic
  • Sterile gloves, marking pen, and surface electrode
  • One 1.5 -in., 25-gauge needle for skin infiltration
  • A 10-cm long, short-bevel, insulated stimulating needle (15 cm for anterior approach)
  • Peripheral nerve stimulator
  • Injection pressure monitor

Interpreting Responses to Nerve Stimulation

Twitches of the hamstrings, calf, foot, or toes at 0.3–0.5 mA current all can be used as signs of successful localization of the sciatic plexus (nerve). Table 82D–3 presents common responses to nerve stimulation and the course of action to take to obtain the proper response.19

TABLE 82D–3.Common responses to nerve stimulation and action to take.

Response ObtainedInterpretationProblemAction
Local twitch of the gluteus muscleDirect stimulation of the gluteus muscleToo shallow (superficial) placement of the needleContinue advancing the needle
Needle contacts bone but local twitch of the gluteus muscle not elicitedNeedle inserted close to the caudal aspect of the iliac bone or the lateral aspect of the sacrumToo superior or too medial needle insertionSlightly laterally and caudally redirect the needle
Needle encounters bone and sciatic twitches elicitedNeedle missed the plane of the sciatic nerve and is stopped by the hip joint or ischial boneNeedle inserted too laterally (hip joint) or medially (ischial bone)Withdraw the needle and redirect slightly medially or laterally (5–10 degrees)
Hamstring twitchStimulation of the main trunk of the sciatic nerveNone. These branches are within the sciatic nerve sheath at this levelAccept and inject local anesthetic
The needle placed deep (10 cm) but no twitches elicited and no bone contactNeedle has passed through the sciatic notchToo inferior needle placementWithdraw and redirect the needle slightly laterally, or cephalad
Paresthesia of the genital organsNeedle is stimulating the inferior roots of the sacral plexus (pudendal nerve)Too inferior and too medial needle placementWithdraw and redirect the needle slightly cephalad and laterally

BLOCK DYNAMICS AND PERIOPERATIVE MANAGEMENT

Sciatic nerve block may cause patient discomfort because the needle passes through the gluteus muscles. Adequate sedation and analgesia are important to ensure patient comfort. Midazolam 2–4 mg can be given for patient positioning, and alfentanil 500–750 mcg is given just before needle insertion. A typical onset time for this block is 10–25 minutes, depend-ing on the type, concentration, and volume of local anesthetic used. The first signs of block onset are usually reported by the patient as a feeling that the foot is “different” or that they cannot wiggle their toes.

Clinical Pearls

  • Inadequate skin anesthesia despite an apparent timely onset of the blockade can occur. Local infiltration at the site of the incision by the surgeon is often all that is needed to allow the surgery to proceed.

POSTERIOR APPROACHES TO SCIATIC NERVE BLOCK

General Considerations

The posterior approach to sciatic block has wide clinical applicability for surgery and pain management of the lower extremity. The block require expertise with more basic nerve blocks for successful and safe practice.20,21 It is particularly well suited for surgery on the knee, calf, Achilles tendon, ankle, and foot. It provides complete anesthesia of the leg below the knee with the exception of the medial strip of skin, which is innervated by the saphenous nerve (Figure 82D–6). When combined with a femoral nerve or lumbar plexus block, anesthesia of almost the entire leg can be achieved.

FIGURE 82D–6. Sciatic nerve. Cutaneous innervation.

Distribution of Anesthesia

Sciatic nerve block results in anesthesia of the skin of the pos-terior aspect of the thigh, hamstrings, and biceps muscles, part of the hip and knee joints, and the entire leg below the knee, with the exception of the skin of the medial aspect of the lower leg (see Figure 82D–6). Depending on the level of surgery, the addition of a saphenous or femoral nerve block may be required.

Classic Posterior Approach

Anatomic Landmarks

Landmarks for the posterior approach to sciatic blockade are easily identified in most patients (Figure 82D–7A, B). Proper palpation technique is of utmost importance because the adipose tissue over the gluteal area may obscure these bony prominences. The landmarks are outlined by a marking pen:

  1. Greater trochanter
  2. Posterior superior iliac spine
  3. Needle insertion site 4 cm distal to the midpoint between the two landmarks
FIGURE 82D–7. Sciatic nerve block, posterior approach.

Technique

The patient is in the lateral decubitus position with a slight forward tilt–this prevents the “sag” of the soft tissues in the gluteal area and significantly facilitates block placement. The foot on the side to be blocked should be positioned over the dependent leg so that twitches of the foot or toes can be easily noted. After cleaning with an antiseptic solution, local anesthetic is infiltrated subcutaneously at the determined needle insertion site.

Clinical Pearls

  • Raise the height of the bed enough and assume an ergo-nomic position to allow a comfortable and stable posi-tion for the patient during block placement and for observation of the motor responses to nerve stimulation.

The fingers of the palpating hand should be firmly pressed on the gluteus muscle to decrease the skin–nerve distance (Figure 82D–8). The palpating hand should not be moved during block placement; even small movements of the palpat-ing hand can substantially change the position of the needle insertion site because the skin and soft tissues in the gluteal region are highly mobile. The needle is introduced at an angle perpendicular to the spherical skin plane (Figure 82D–8A and B). The nerve stimulator should be initially set to deliver 1.0–1.5 mA current (2 Hz, 100 μsec) to allow detection of twitches of the gluteal muscles and stimulation of the sciatic nerve.

FIGURE 82D–8. A and B. Sciatic nerve block, posterior approach. Needle insertion is in the perpendicular plane; the palpating hand is firmly pressed to decrease the skin–nerve distance and stabilize the anatomy.

As the needle is advanced, the first twitches observed are from the gluteal muscles. These twitches merely indicate that the needle position is still too shallow. The goal is to achieve visible or palpable twitches of the hamstrings, calf muscles, foot, or toes at 0.3–0.5 mA current. Twitches of the hamstrings are equally acceptable because this approach blocks the nerve proximal to the separation of the neuronal branches to the hamstrings muscle. Once the gluteal twitches disappear, brisk response of the sciatic nerve to stimulation is observed (ham-strings, calf, foot, or toe twitches). After the initial stimulation of the sciatic nerve is obtained, the stimulating current is gradu-ally decreased until twitches are still seen or felt at 0.3–0.5 mA current. This typically occurs at a depth of 5–8 cm.After negative aspiration for blood, 15–25 mL of local anes-thetic is injected (Figure 82D–9). Any resistance to the injec-tion of local anesthetic should prompt needle withdrawal by 1 mm. The injection is then reattempted. Persistent resistance to injections should prompt complete needle withdrawal and ensuring needle patency before reintroduction.

FIGURE 82D–9. Sciatic nerve block, posterior approach. Dispersion of the local anesthetic after injection.

Clinical Pearls

  • Since the level of the blockade with this approach is above the departure of the branches for hamstring muscles, twitch of any of the hamstring muscles can be accepted as a reliable sign of localization of the sciatic nerve without deliberately seeking foot response.
  • When the first needle pass does not result in nerve local-ization, do not regard it as a failure. Instead, use a sys-tematic approach to troubleshooting:
  • Ascertain the nerve stimulator is functional, properly connected, and set to deliver the desired current.
  • Mentally visualize the plane of the initial needle inser-tion, and redirect the needle in a slightly caudal direction (5–10 degrees) to the initial insertion plane.
  • If the above maneuver fails, withdraw the needle to the skin and redirect it slightly cephalad (5–10 degrees) to the initial insertion plane.
  • Failure to obtain hamstrings or foot response to nerve stimulation should prompt a reassessment of the land-marks and patient position.

Continuous Block

The continuous sciatic nerve block is an advanced regional anes-thesia technique, and experience with the single-shot technique is recommended to ensure its efficacy and safety. Continuous sciatic nerve block was described by Gross in 1956.22 The current technique used is similar to the single-shot injection; however, slight angulation of the needle in the caudal direction is necessary to facilitate threading of the catheter. Securing and maintenance 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. Perhaps the single most important indication for use of this block is for amputation of the lower extremity.

Technique

Patient positioning, marking of landmarks, skin preparation and local anesthetic infiltration are performed as described above. An 8–10 cm long, insulated stimulating needle (prefer-ably Tuohy-style tip) is inserted in the same manner as for the single-injection technique. The opening of the needle should face distally (pointing toward the patient’s foot) to facilitate catheter insertion.

Clinical Pearls

  • When insertion of the catheter proves difficult, lowering the angle of the needle can be helpful.
  • It is useful to inject some local anesthetic intramuscu-larly to prevent pain on advancement of larger gauge and blunt-tipped needles typically used for this block.

After obtaining the motor response at a current of 0.3–0.5 mA, a 20-mL bolus of local anesthetic is injected. This is followed by insertion of the catheter 5 cm beyond the needle tip (Figure 82D–10). The catheter is then aspirated to check for inadvertent intravascular placement.

A number of techniques to secure the catheter to the skin have been proposed. A benzoin skin preparation followed by application of a clear dressing and a cloth tape is one such simple and efficacious method. The infusion port should be clearly marked as “continuous sciatic block.”

FIGURE 82D–10. Continuous sciatic nerve block, posterior approach. Shown is the course of the catheter (1) and the fusiform-shaped contrast area indicating the spread of the local anesthetic in the sheath of the sciatic nerve (2). In this example, a mere 2 mL of the local anesthetic is injected.

Continuous Infusion

Continuous infusion is always initiated after an initial bolus of dilute local anesthetic through the catheter. Ropivacaine 0.2% is commonly used for this purpose (15–20 mL). Diluted solu-tions of bupivacaine or L-bupivacaine are also suitable, but can result in undesirably greater motor blockade. The infusion is initiated at 10 mL/h or 5 mL/h when a patient-controlled anal-gesia (PCA) dose is planned (5 mL).23,24

Parasacral Approach

Described by Mansour in 1993, the parasacral sciatic nerve block is well suited for continuous infusion of local anes-thetic.9,25–30 In addition, this block has characteristics of a plexus block and yields anesthesia of the entire sacral plexus, and obturator nerve.26,27,31 Ripart25 reported a 94% success rate in his series of 400 parasacral sciatic nerve block cases.The parasacral approach to sciatic blockade has a wide clini-cal applicability for surgery and pain management of the lower extremity, particularly when combined with a femoral or psoas compartment block.26,31 This technique is associated with a high success rate and is particularly well suited for surgery on the popliteal fossa and the knee.32

Distribution of Anesthesia

Parasacral sciatic nerve blockade results in anesthesia of the skin of the posterior thigh, hamstrings, and biceps femoris muscles; part of the hip and knee joint; and the entire leg below the knee except the medial cutaneous skin of the lower leg (see Figure 82D–6). Morris26 demonstrated extension of anesthesia to the obturator nerve after sciatic nerve block, as tested by the presence of adduc-tor muscle weakness on a numeric scale. Jochum however,32 sug-gested that the obturator nerve is sporadically affected by the parasacral sciatic nerve block.

Anatomic Landmarks

Landmarks for the parasacral approach to sciatic blockade are easily identified in most patients (Figure 82D–11). Careful palpation technique is important because the adipose tissue over the gluteal area may obscure these bony prominences (Figures 82D–12 and 82D–13). The following landmarks are outlined by a marking pen:

  • Posterior–superior iliac spine (PSIS)
  • Ischial tuberosity (IT)
  • A line between the PSIS and the IT is drawn. The needle insertion point lies 6 cm caudad to the PSIS on this line. The insulated needle is inserted at this point and advanced in a sagittal plane.

Technique

The patient is positioned in a lateral decubitus position, similar to the position required for the classic posterior approach to sciatic block (Figure 82D–13). The dependent limb is kept straight while the limb to be blocked is flexed at both the hip and knee. Appropriate sedation and analgesia are mandatory to ensure the patient’s comfort throughout the procedure. After cleaning with an antiseptic solution, local anesthetic is infiltrated subcutaneously at the deter-mined needle insertion site.

  • Contact with the bone usually indicates the needle contact with the wings of the sacrum or the iliac bone, superior to and near the greater sciatic notch.
  • In this case, the needle is withdrawn and redirected slightly caudally and laterally.
  • Contact with the bone can be used as a depth test. The needle depth is noted; the needle should not be advanced more than 2 cm beyond this depth. At this site, the sciatic nerve is approached at the top of the greater sciatic foramen while leaving the pelvis. Advancing the needle deeper may expose pelvic viscera and vessels to risk of injury.
FIGURE 82D–11. Parasacral approach to sciatic nerve block. Shown are the posterior superior iliac spine (PSIS) and the ischial tuberosity (IT). The needle insertion site is marked as 6 cm caudad to the PSIS on the line connecting PSIS with IT

FIGURE 82D–12. Parasacral sciatic nerve block. Palpation technique to identify posterior superior iliac spine (PSIS).

FIGURE 82D–13. Parasacral sciatic nerve block. Proper palpation technique to identify the ischial tuberosity. (IT, ischial tuberosity; PSIS, posterior superior iliac spine.)

FIGURE 82D–14. Parasacral sciatic nerve block. Needle insertion is perpendicular to the horizontal plane.

The needle is inserted perpendicular to the skin and advanced slowly (Figure 82D–14). The motor response of the sciatic plexus is usually obtained at a depth between 6 and 8 cm. The goal is to achieve visible or palpable twitches of the hamstrings, calf muscles, foot, or toes at the current intensity of 0.3–0.5 mA. The distal motor response may be either a tibial or a peroneal response—it is not necessary to stimulate both components (Figure 82D–15).33,34 Twitches of the hamstrings are equally acceptable because this approach blocks the sciatic nerve proximal to the separation of the neuronal branches to the hamstring muscles.

FIGURE 82D–15. Sciatic nerve stimulation: motor response of the common peroneal and tibial nerves indicate proper localization of the sciatic nerve.

Once an appropriate response is obtained, 20-25 mL of local anesthetic is injected slowly with intermittent aspiration (Figure 82D–16).

Cuvillon et al.8 compared the parasacral sciatic nerve block with Winnie’s approach with one or two stimulations. Winnie’s approach using the double-injection technique required more time to perform the block compared with Winnie’s single-injection technique and the parasacral method. Although the onset of sensory and motor blocks were signifi-cantly faster with the double- injection method, the additional time needed to perform the double-injection block eliminated the advantage of the faster onset.

FIGURE 82D–16. Parasacral nerve block: dispersion of the contrast after injection, the “negative” contrast sign, and a typical fusiform distribution of the injectate.

Continuous Parasacral Sciatic Nerve Block

The continuous parasacral sciatic nerve block is similar to the single-shot injection; however, slight caudal angulation of the needle is necessary to facilitate threading of the catheter. Secur-ing and maintenance of the catheter are easy and convenient.36 This technique can be used for surgery and postoperative pain management in patients undergoing a wide variety of knee, lower leg, foot, and ankle surgeries.

FIGURE 82D–17. Equipment for continuous sciatic block

The technique is identical to the single injection technique, except a continuous-block needle is used (Figure 82D–17). The opening of the needle should face distally to facilitate cath-eter insertion.37 The initial intensity of the stimulating current should be 1.0–1.5 mA.

Clinical Pearls

  • It is useful to inject some local anesthetic intramuscularly to decrease pain during placement of the continuous nerve block needle.

After obtaining the motor response at 0.3–0.5 mA, a 20 mL bolus of local anesthetic is injected and the catheter inserted 3-5 cm beyond the needle tip. If confirmation of catheter place-ment is desired, contrast media can be injected through the catheter and radiographic images can be studied.31 The pres-ence of a spindle 2–3 cm in length with an oblique orientation crossing the sciatic notch on the anteroposterior radiograph and/or shadowing of the sacral roots are considered to indicate injection into the correct plane and adequate placement of the catheter (see Figure 82D–18). After securing the catheter, an infusion (e.g., ropivacaine 0.2% at 5 mL/hr with 5 mL/q60min patient-controlled bolus) is initiated.

FIGURE 82D–18. Parasacral sciatic block: shown is the course of the catheter and visualization of the injectate around the sciatic nerve.

ALTERNATIVE POSTERIOR APPROACHES

Di Benedetto38 described a subgluteal approach to the sciatic nerve block in 2002. This technique is a good alternative to the more proximal approaches to the sciatic nerve, with the poten-tial for reducing the discomfort experienced by the patient dur-ing block placement. The landmarks with the subgluteal approach are the greater trochanter of the femur, the ischial tuberosity, and a line between the two with the midpoint marked. From the midpoint, another line is drawn perpendicu-larly and extended 4 cm in the caudal direction to identify the needle insertion point. A 20-gauge, 10-cm needle and nerve stimulator are used for this approach, and 20 mL of local anes-thetic is injected when twitches of the sciatic nerve are obtained at ≤0.5 mA current.

Of note, ultrasound-guided subgluteal approach to sciatic block has become one of the most common sciatic nerve block techniques in modern regional anesthesia. Traditional approaches to the sciatic nerve at the pelvic level require identification of pelvic bone structures. Although the size of the buttocks is vari-able among different individuals and in the same individual over time, the relation of the sciatic nerve to the pelvis is constant throughout life. Using this premise, Franco has suggested a more simplified approach to the sciatic nerve block that does not require palpation of deep bony structures. The landmarks with this approach are the midline of the intergluteal sulcus, and a point 10 cm lateral to the midline of the intergluteal sulcus where the block needle will be inserted. The curvature of the buttocks is disregarded when locating the needle insertion point. Care must be taken not to stretch the soft tissues when marking the needle insertion site as subsequent recoil of the tissues will occur, causing the distance to the nerve to be underestimated. The patient is either in the prone or lateral position and the needle is inserted parallel to the midline.39

Complications in Posterior Approaches and How to Avoid Them

Table 82D–4 lists instructions on possible complications of sciatic nerve blockade and methods to decrease the risk.

TABLE 82D–4.Complications and how to avoid them.

InfectionUse strict aseptic technique
HematomaAvoid multiple needle insertions, particularly in anticoagulated patients
Vascular punctureAvoid deep needle insertion (pelvic vessels)
Local anesthetic toxicityAvoid using large volumes and doses of local anesthetic owing to the proximity of the large vessels and the potential for rapid absorption
Injection of local anesthetic should be carried out slowly and with frequent aspiration to rule out intravascular injection
Nerve injurySciatic nerve has a unique predisposition for mechanical and pressure injury
Use nerve stimulation and slow needle advancement
Never inject local anesthesia when the patient complains of pain or when abnormally high pressure on injection is noted
Never assume that the needle is obstructed with tissue debris when resistance to injection is met
When stimulation is obtained with a current intensity of <0.2 mA, withdraw the needle slightly to obtain the same response with a current intensity of >0.2 mA before injecting local anesthetic
Nerve injuryAdvance the needle slowly when twitches of the gluteus muscle cease to avoid impaling the sciatic nerve on a rapidly advancing needle
OtherInstruct the patient and nursing staff on the care of the insensate extremity
Explain the need for frequent body repositioning to avoid stretching and prolonged ischemia (sitting) on the anesthetized sciatic nerve
Advise heel padding during prolonged bed rest or sleep to prevent pressure sores from developing
Perforation of pelvic organsDirecting the needle medially should be exercised with this complication in mind
Anesthesia of the pudendal nerveThe pudendal nerve, a branch of the sacral plexus, may be anesthetized by the parasacral nerve block owing to diffusion of the injected local anesthesia
Inform patients that this problem is transient
TourniquetInjection of the local anesthetic within the sciatic nerve sheath, epinephrine, and a tourniquet over the site of injection all can combine to cause ischemia of the sciatic nerve

ANTERIOR APPROACH

General Considerations

The anterior approach to a sciatic block is an advanced nerve block technique. The block is suited for surgery on the leg below the knee, particularly on the ankle and foot. It provides complete anesthesia of the leg below the knee with the excep-tion of the medial strip of skin innervated by the saphenous nerve (Figure 82D–19). When combined with a femoral nerve block, anesthesia of the entire knee and lower leg is achieved.40 The anterior approach is much less clinically applicable than posterior approaches because the distribution of anesthesia is more limited and a higher level of skill is required. In addition, this technique is not suitable for catheter insertion because of the deep location and perpendicular angle of needle insertion required to reach the sciatic nerve. Consequently, this block is best reserved for patients who cannot easily be moved to the lateral position needed for the posterior approach; e.g., patients with spinal injuries or under general anesthesia.41,42

FIGURE 82D–19. Distribution of anesthesia with anterior approach to sciatic nerve block

Since the original description by Beck5 several clinicians have attempted to devise more reliable landmarks and tech-nique for this block. However, all described approaches derive the needle insertion site at nearly identical points regardless of whether they use bony prominences, soft tissue, or femoral artery as landmarks.40,43–47 In addition, even if these different approaches varied slightly in the site of needle insertion, the long path by the needle required to reach the sciatic nerve (8–12 cm) and the tendency of long, blunt-tipped needles to bend on insertion through the tissues make any such differ-ences meaningless when it comes to increasing the precision of the technique.

Equipment

A standard regional anesthesia tray is prepared with the follow-ing equipment:

  • Sterile towels and 4-in.× 4-in. gauze packs
  • 20-mL syringes with local anesthetic
  • Sterile gloves, marking pen, and surface electrode
  • One 1.5-in., 25-gauge needle for skin infiltration
  • A 15-cm long, short-bevel, insulated stimulating needle
  • Peripheral nerve stimulator

Anatomic Landmarks

The following landmarks should routinely be outlined using a marking pen (Figure 82D–20):

  1. Femoral crease
  2. Femoral artery pulse
  3. Needle insertion point marked 4–5 cm distally on the line passing through the pulse of the femoral artery and perpendicular to the femoral crease

Technique

The leg is fully extended on the table with the patient in the supine position. After cleaning the area with an antiseptic solu-tion, local anesthetic is infiltrated subcutaneously at the needle insertion site. The fingers of the palpating hand should be firmly pressed against the quadriceps muscle to decrease the skin–nerve distance. The block needle (connected to a nerve stimulator set to deliver a current of 1.5 mA) is introduced at a perpendicular angle to the skin plane (Figure 82D–21). Motor response of the sciatic nerve is typically obtained at a depth of 8–12 cm. Accept-able responses are visible or palpable twitches of the calf muscles, foot, or toes at a current of 0.3–0.5 mA. After negative aspira-tion for blood, 20 mL of local anesthetic is slowly injected. If high injection pressure is detected, the needle should be with-drawn by 1 mm and injection attempted again. If high injection pressure persists, the needle should be withdrawn and flushed prior to further attempts.

Clinical Pearls

  • Local twitches of the quadriceps muscle are often elic-ited during needle advancement. The needle should be advanced past these twitches.
  • Although there is a concern of femoral nerve injury with further needle advancement, at this level, the femoral nerve is divided into smaller terminal branches that are movable and unlikely to be penetrated by a slowly advancing, blunt-tipped needle.
  • Resting the patient’s heel on the bed surface may prevent the foot from twitching even when the sciatic nerve is stimulated. This can be prevented by placing the ankle on a footrest or by having an assistant continuously palpate the calf or Achilles tendon.
  • Because branches to the hamstring muscle may depart the main trunk of the sciatic nerve at the level of needle insertion, twitches of the hamstrings should not be accepted as a reliable sign of sciatic nerve localization.

FIGURE 82D–20. Sciatic nerve block through the anterior approach.

FIGURE 82D–21. Sciatic nerve block through the anterior approach. Needle insertion

Bone contact is frequently encountered during needle advancement. This indicates that the needle has contacted the femur (usually lesser trochanter). In this case, the foot is first rotated laterally, which should swing the lesser trochanter out of the path of the needle and allow deeper advancement of the needle and nerve localization. If this maneuver fails, the needle is redirected slightly medially or reinserted more medially. Table 82D–5 lists some common responses to nerve stimulation and the course of action to take to obtain the proper response.

TABLE 82D–5.Interpreting responses to nerve stimulation.

Response ObtainedInterpretationProblemAction
Twitch of the quadriceps muscle (patella twitch)Common; stimulation of the branches of the femoral nerveToo shallow (superficial) placement of needleContinue advancing the needle
Local twitch at the femoral crease areaDirect stimulation of the iliopsoas or pectineus musclesToo superior insertion of needleStop the procedure and reassess the landmarks
Hamstring twitchNeedle may be stimulating branch(es) of the sciatic nerve to the hamstring muscle; direct stimulation of the hamstrings with higher current is also possibleUnreliable-difficult to determine whether the needle is in the proximity of the sciatic nerveWithdraw the needle and redirect slightly medially or laterally (5–10 degrees)
The needle is placed deep (12–15 cm) but twitches were not elicited and bone is not contactedThe needle is likely too medialWithdraw and redirect slightly laterally
Twitches of the calf, foot, or toesStimulation of the sciatic nerveNoneAccept and inject local anesthetic

SUMMARY

Although the sciatic nerve block was described in 1920, many practitioners avoided this block because of its perceived com-plexity. Sciatic nerve block is an important technique for the regional anesthesiologist to master because the combination of this block and a femoral nerve block or lumbar plexus block can anesthetize almost the entire leg. Although the posterior sciatic nerve block has an intermediate level of difficulty, with practice and knowledge of anatomy, high success rates can be achieved. Many approaches have been proposed; the most rel-evant have been presented in this chapter. Finally, nearly all described approaches are similar in clinical efficacy; therefore, learning one single approach well is recommended as this would suffice for most clinical indications.

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Elizabeth Gartner, Elisabeth Fouché, Olivier Choquet, Admir Hadzic, and Jerry D. Vloka



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