Sciatic blocks procedure guide

INTRODUCTION — The sciatic nerve is the largest peripheral nerve in the human body and provides sensory and motor innervation to portions of the leg, especially distal to the knee joint. Sciatic nerve block is used for anesthesia and analgesia for a wide variety of lower extremity surgical procedures.

This topic will discuss the anatomy, ultrasound imaging, nerve stimulator guidance, and injection techniques for sciatic nerve block at the level of the gluteus maximus muscle. Block of the sciatic nerve at the level of the popliteal fossa is discussed separately. (See "Popliteal block procedure guide".)

General considerations common to all peripheral nerve blocks, including patient preparation and monitoring, use of aseptic technique, localization techniques, drug choices, contraindications, and complications, are discussed separately. (See "Overview of peripheral nerve blocks" and "Ultrasound for peripheral nerve blocks".)

ANATOMY

Course of the sciatic nerve — The sciatic nerve is formed by the convergence of the ventral rami of spinal nerves L4-S3 along the posterior ischium (figure 1). Each ventral ramus splits into dorsal and ventral divisions. The dorsal divisions come together to form the peroneal component of the sciatic nerve, which lies on the lateral aspect of the nerve. The ventral divisions come together to form the tibial component of the sciatic nerve, which lies on the medial aspect of the nerve. The two components of the sciatic nerve course together as the common sciatic nerve until they again split just proximal to the popliteal crease. No fibers are shared between dorsal (peroneal) and ventral (tibial) divisions of the lumbosacral plexus [1]. This fiber arrangement has implications for nerve localization and needle positioning during stimulation-based nerve blockade (figure 2). (See 'Innervation' below.)

The sciatic nerve exits the pelvis through the sciatic foramen, ventral to the piriformis muscle. Here, the sciatic travels in close proximity to the posterior femoral cutaneous nerve (PFCN; also known as posterior cutaneous nerve of the thigh), which innervates the superficial posterior thigh and popliteal fossa.

Beyond the inferior border of the piriformis, the sciatic nerve is located in the subgluteal space, which is bordered dorsally by the gluteus maximus muscle. Ventrally, the sciatic nerve courses along a series of muscles that connect the pelvis and femur. Listed from proximal to distal, these are the superior and inferior gemelli, obturator internus, and quadratus femoris muscles, though at the level of the transgluteal sciatic block, the ventral muscle is the quadratus femoris [2]. More distally, at the level where an infragluteal sciatic block might be placed, the gluteus maximus muscle has tapered, and the biceps femoris muscle may be seen dorsal to the nerve, with the adductor magnus muscle ventral. The subgluteal space also contains the inferior gluteal artery and vein medially, the comitans vessels of the sciatic nerve, and ascending femoral circumflex artery laterally. Two important bony landmarks, the greater trochanter and the ischial tuberosity, help to identify the location of the sciatic nerve within the subgluteal space when either an ultrasound-guided or nerve stimulation-based approach is used (figure 3). At a point between these bony landmarks, the nerve is reliably found within the fascial plane between the gluteus maximus dorsally and quadratus femoris ventrally (figure 4).

Distal to the gluteal fold, the sciatic nerve enters what is commonly referred to as the infragluteal space. Within this space, the nerve lies dorsal to the adductor magnus muscle and is crossed by the long head of the biceps femoris obliquely as it enters the posterior compartment of the thigh (figure 5).

In the proximal half of the thigh, the sciatic nerve is significantly wider (roughly 2 cm, lateral to medial) than it is thick (roughly 0.5 cm anterior to posterior), and lies at a depth of approximately 3 cm from the skin of the posterior thigh in people with normal body mass index [3]. In the distal half of the thigh, the nerve becomes almost circular in cross-section and lies progressively more superficial as it approaches its bifurcation in the popliteal fossa.

The sciatic nerve is surrounded by a connective tissue sheath, referred to as the paraneural sheath, or paraneurium [4,5]. More distally beyond the bifurcation, the paraneurium divides and the common peroneal and tibial nerves are surrounded by their own, separate paraneural sheaths.

Innervation

Sensory

●Cutaneous – The sciatic nerve provides sensory innervation to the skin of the leg and foot below the level of the knee, with the exception of a medial strip innervated by the saphenous nerve (figure 6).

•The common peroneal nerve innervates the lateral skin of the calf (via the lateral sural cutaneous nerve), and the remaining skin of the calf and dorsum of the foot (via branches of the superficial peroneal nerve). The deep branch of the peroneal nerve innervates only the webspace between the first and second toes.

•Branches of the tibial nerve innervate the plantar surface of the foot.

The skin of the posterior thigh is innervated by the posterior femoral cutaneous nerve (PFCN) which derives from the sacral plexus and is separated from, but close to, the sciatic nerve along most of its course.

●Bony innervation – The sciatic nerve innervates a distal portion of the posterior femur and all bones below the knee, with the exception of the medial tibial plateau and medial malleolus, which are innervated by branches of the femoral nerve.

Motor — The sciatic nerve innervates the posterior thigh muscles, including the hamstrings (semitendinosus, semimembranosus, and short and long heads of the biceps femoris). Nerves to these muscles derive variably from the sciatic nerve at or above the ischial tuberosity [6]. After its division approximately 6 to 10 cm above the popliteal crease, the sciatic nerve continues as tibial and common peroneal nerves and innervates the muscles of the calf and foot.

●The common peroneal nerve and its terminal branches innervate extensor muscles of the anterior compartment of the calf (ie, tibialis anterior, extensor hallucis longus, and extensor digitorum longus). Their combined motor functions are eversion and dorsiflexion of the foot and ankle.

●The tibial nerve innervates muscles of the posterior compartment of the calf whose motor functions are inversion and plantar flexion of the foot.

CLINICAL ANATOMIC CORRELATIONS — The proximal sciatic nerve block is used to provide anesthesia and/or analgesia for surgery from the midthigh to the knee. The popliteal block (ie, block of the sciatic nerve in the popliteal fossa) can be used for surgery below the knee.

Multiple factors impact the clinical effects of sciatic nerve blockade. Speed of onset, density of motor or sensory block, and perhaps even block success may relate to whether injection is performed inside or outside the paraneural space. Subparaneural injection results in extensive cranial-caudal spread, which has been identified by ultrasound and magnetic resonance imaging. Subparaneural injection has been associated with increased block reliability and faster block onset [5]. In multiple studies, subparaneural injection has been shown to be safe, with minimal risk of nerve injury [4,5,7].

The posterior femoral cutaneous nerve (PFCN) of the thigh runs in the subgluteal space along with the sciatic nerve and is often blocked with transgluteal block [8,9]; this is important when a sciatic block is performed for procedures in the mid to distal thigh (eg, above the knee amputation). Block of the PFCN of the thigh is not required for pain related to upper or midthigh tourniquet placement [10]. (See 'Transgluteal versus infragluteal sciatic block' below.)

TERMINOLOGY — The sciatic nerve can be blocked anywhere along its course. Proximal blocks can be performed using an anterior (supine) or a posterior (lateral or prone) approach. The anterior approach will not be described here. Proximal posterior sciatic approaches are described at levels relative to the gluteus maximus muscle and gluteal fold, and terminology is not standardized.

●In this topic, we use the term transgluteal to refer to a block that is performed with needle insertion through the gluteus maximus muscle, targeting the sciatic nerve anterior (ventral) to the gluteus maximus muscle between bony landmarks of the ischial tuberosity and the greater trochanter. The term subgluteal will be avoided in this topic to prevent provoking confusion between transgluteal and infragluteal block approaches (figure 4).

●Infragluteal block will refer to a slightly more distal block, at or distal to the gluteal fold where the sciatic nerve is anterior (ventral) to either the gluteus maximus or biceps femoris muscle (figure 5).

PATIENT POSITIONING — For either a transgluteal or infragluteal block, position the patient in the lateral decubitus position but rolled slightly forward, with the limb to be blocked in the nondependent position, flexed 45 degrees at the hip and 90 degrees at the knee (Sim's position).

ULTRASOUND GUIDANCE VERSUS NERVE STIMULATION — We use ultrasound guidance to perform sciatic nerve blocks. When ultrasound is not available, nerve stimulation is a reasonable alternative. Nerve stimulation is useful for nerve confirmation during ultrasound-guided block, and should be used if the nerve cannot be visualized (eg, due to altered anatomy, obesity, edema). The benefit of nerve stimulation confirmation was demonstrated in a study of 40 patients who underwent ultrasound-guided sciatic nerve block performed at the level of the mid-femur [11]. Nerve stimulation was required to confirm the nerve in 37.5 percent of patients due to inadequate ultrasound visualization.

Ultrasound guidance has been widely adopted for sciatic nerve block due to a favorable safety profile in addition to increased rates of block success. In a 2015 meta-analysis of 10 small randomized trials that compared ultrasound-guided with nerve stimulator-guided sciatic block, ultrasound guidance resulted in an increased rate of successful block (96 versus 78 percent, risk ratio [RR] 1.2, 95% CI 1.03-1.39) and reduced rate of vascular puncture (2 studies, 0 versus 11 percent, RR 0.13, 95% CI 0.02-0.97), with similar time to perform the block. There was marked heterogeneity among studies [12]. Benefits of ultrasound guided peripheral nerve blocks are discussed separately. (See "Overview of peripheral nerve blocks", section on 'Ultrasound guidance'.)

TRANSGLUTEAL VERSUS INFRAGLUTEAL SCIATIC BLOCK — The approach to sciatic nerve block may be based on optimal ultrasound visualization, depending on patient body habitus and anatomy.

The transgluteal and infragluteal sciatic approaches provide a similar distribution of block, except that the transgluteal block is more likely to include the posterior femoral cutaneous nerve (PFCN). Transgluteal block anesthetizes the PFCN in >90 percent of blocks [10,13]. It can be missed in up to one third of infragluteal blocks [14], and if necessary can be blocked separately [15]. PFCN blockade is clinically relevant for surgery that involves a posterior thigh incision (eg, above knee amputation). In cases without such an incision, PFCN coverage should not influence the sciatic approach chosen.

The infragluteal approach may be less uncomfortable for the patient, as it avoids the need to insert the needle through the gluteus maximus. The infragluteal approach may be preferred in patients with obesity, as the distance from the skin to the nerve is less at the needle insertion site, and for the parabiceps infragluteal variant, does not require palpation of bony landmarks.

TRANSGLUTEAL BLOCK

Landmarks for the block — The location for ultrasound placement and/or needle insertion (for nerve stimulator-guided block) is determined by palpating bony landmarks. The Labat approach and Winnie's modification of it are named after the individuals who described them. We use Winnie's modification for transgluteal block. For nerve stimulator-guided block we mark the skin with reference lines as described here to guide needle placement. For ultrasound guidance, we do not mark the skin, though the bony landmarks can provide useful reference for ultrasound transducer placement (picture 1).

●Labat approach – Draw a line between the posterior superior iliac spine (PSIS) and the greater trochanter. Draw a perpendicular line starting at the midpoint of the first line, extending 4 cm inferomedially. The distal end of this line marks the site of transducer placement or needle insertion (figure 3 and picture 2) [16].

●Winnie's modification – Mark as for the Labat approach. Add a third line between the sacral hiatus and the greater trochanter. Extend the perpendicular line as necessary to meet this third line; place the transducer or insert the needle at this intersection. This modification should be more accurate than the Labat approach, because it serves as a way to "cross-reference," the nerve site from the Labat approach against the length of the sacrum, which presumably varies with the patient's height [17].

Ultrasound-guided block

Ultrasound imaging

●Select a low frequency 2-5 mHz, curvilinear ultrasound transducer, with a low frequency preset. Set the ultrasound depth at approximately 6 to 8 cm.

●Place the transducer midway between greater trochanter and the PSIS in an orientation transverse to the course of the nerve (perpendicular to the axis of the femur) (picture 1).

●Identify the ischial tuberosity medially and the greater trochanter laterally. Visualize the gluteus maximus muscle spanning the ultrasound view, superficial (dorsal) to the quadratus femoris muscle and separated by a hyperechoic fascial plane (figure 4).

●Identify the sciatic nerve (hyperechoic, flat, and wide) in the plane between the two muscles, often in close proximity to the ischial tuberosity.

Performing the block

●Insert an echogenic B-bevel 20 to 22 gauge, 10 cm needle using an in-plane approach, in a lateral to medial trajectory through the gluteus maximus muscle (picture 3). An out of plane approach can also be used.

●The needle typically becomes difficult to see due to the steep needle angle relative to the ultrasound beam during this procedure. As the needle approaches the lateral aspect of the nerve, inject small volumes of saline through the needle to help identify the tip (movie 1).

●When the needle tip is in close proximity to the sciatic nerve, inject 1 to 2 mL saline, visualizing spread around the sciatic nerve. Spread immediately above or below the nerve is acceptable.

●Following negative aspiration, inject approximately 20 mL of local anesthetic (LA) in 5 mL increments, with gentle aspiration between injections. Visualize spread of LA around the nerve. (See 'Local anesthetic choice and dose' below.)

Nerve stimulator-guided block — Nerve stimulation can be used on its own to perform the block, or it can be used for confirmation of the nerve during ultrasound-guided block. For nerve stimulator-guided block, we use a single injection, preferably after eliciting a tibial nerve stimulation motor response, as described here. Some authors advocate a multiple stimulation approach, which involves injecting one-half of the LA after eliciting a tibial response and the other one-half after a common peroneal response. The two injection technique has been associated with faster block onset time and increased block success [18]. However, we do not favor this approach due to added complexity, higher rate of paresthesia, and occasional difficulty eliciting the second stimulation pattern.

●Nerve stimulation equipment – The authors set the nerve stimulator to a pulse width of 100 ms at 1-2 Hz and an initial current of 1-1.2 mA. We use a shielded, B-bevel, 20 to 22 gauge 10 cm stimulating needle. Connect the stimulator to the needle and to an electrode on the patient's skin. (See "Overview of peripheral nerve blocks", section on 'Equipment for nerve stimulator guidance'.)

●Performing the block

•Determine needle insertion site, as described above. (See 'Landmarks for the block' above.)

•Infiltrate the skin and soft tissue at the injection site with 1% lidocaine, 1 to 3 mL, using a 25 gauge needle.

•Insert the needle perpendicular to the skin, and advance, watching for stimulation responses. The first stimulation response typically occurs in the gluteus muscles, which indicates that the needle is superficial to the target.

•Advance the needle until a motor response occurs in the calf, ankle, or foot. Two patterns of evoked response may be seen.

-Tibial nerve stimulation – Inversion and plantar flexion of the foot

-Common peroneal stimulation – Eversion and dorsiflexion of the foot

The mnemonics "TIP" (tibial = inversion, plantarflexion) and "PED" (peroneal = eversion, dorsiflexion) can help one remember these stimulation patterns.

•For a single injection technique, stimulation of the tibial nerve (plantar flexion) is the preferred stimulation response, and has been associated with increased block success and faster onset, compared with common peroneal stimulation (dorsiflexion) [19].

-If common peroneal stimulation occurs (dorsiflexion), withdraw the needle slightly and redirect medially, looking for a tibial response.

-If no motor response occurs, withdraw the needle and redirect cephalad 10 to 15 degrees and make a second pass. If no twitch is found on this pass, redirect caudad 10 to 15 degrees and make a third pass.

-If no stimulation is observed after two redirections, reassess landmarks and use a new needle insertion site. Consider one of the alternative strategies for localizing the nerve described above, or ultrasound guidance if available. (See 'Landmarks for the block' above.)

•Once plantar flexion of the foot occurs, decrease the stimulus current until the motor response disappears, then turn back up to the minimal current that results in a response. The goal is a minimal stimulation response between 0.2 and 0.5 mA. The presence of a motor response at <0.2 mA strongly suggests intraneural needle placement and the needle should be repositioned without injecting.

•After negative aspiration, inject 30 mL of LA containing epinephrine (as a vascular marker) in 5 mL increments, with gentle aspiration between injections. Stop injection and reposition the needle tip if blood appears in the syringe, the patient complains of pain or paresthesia, or if there is resistance to injection.

INFRAGLUTEAL BLOCK

Landmarks for the block — Several methods may be used to identify the site for ultrasound transducer placement and/or needle insertion (for nerve stimulator guided block) for infragluteal sciatic block. For nerve stimulator-guided block we mark the skin as described here to guide needle placement. For ultrasound guidance, we do not mark the skin, though the bony landmarks can provide useful reference for ultrasound transducer placement.

●Di Benedetto approach – Draw a line between the greater trochanter and the ischial tuberosity. Draw a second line perpendicular to the first, starting at the midpoint of the first line and extending 4 cm distally. Place the ultrasound transducer or insert the block needle at the end of this second line (picture 4 and figure 7) [2].

●Infragluteal parabiceps approach – In patients with obesity in whom bony landmarks cannot be identified, the initial ultrasound transducer placement may be determined by identifying the lateral edge of the biceps femoris [20]. With the patient actively flexing the knee, palpate the taut tendon emerging at the gluteal crease, and place the transducer just lateral to the tendon, at the level of the gluteal crease. If there are multiple gluteal folds, use the most proximal one.

●Wadwha approach – Another, simplified approach is useful for nerve stimulator-guided block in cases where nerve localization is difficult or requires multiple passes. For this approach, position the patient prone, and insert the needle 3 cm lateral to the ischial tuberosity.

In one study of fresh cadavers positioned prone, the sciatic nerve was found on dissection to lie an average of 2.8 cm lateral to the midpoint of the ischial tuberosity; needles placed using the Di Benedetto approach were an average of 2.27 cm lateral to the sciatic nerve [21]. In a follow-up randomized crossover trial in 40 patients who had sciatic nerve blocks for surgery, needle insertion 3 cm lateral to the ischial tuberosity resulted in fewer needle passes, and increased likelihood of localizing the nerve, compared with localization with the Di Benedetto approach.

Ultrasound-guided block

Ultrasound imaging

●Select a low frequency 2-5 mHz, curvilinear ultrasound transducer, using a low frequency setting designed for ultrasound penetration to deep structures. Set the depth on the ultrasound machine at approximately 6 to 8 cm.

●Place the transducer at the location described above (figure 7) in an orientation transverse to the course of the nerve (ie, perpendicular to the axis of the femur).

●Identify the biceps femoris tendon [22], the gluteus maximus muscle, and the adductor magnus muscle. The sciatic nerve will appear as a hyperechoic flat and wide structure, just lateral to the biceps femoris tendon, deep to the gluteus maximus muscle and superficial to the adductor magnus muscle (figure 8).

Performing the block

●Insert an echogenic B-bevel 20 to 22 gauge, 10 cm needle using an in-plane approach, in a lateral to medial trajectory with an insertion site 1 cm away from the ultrasound transducer (picture 3). An out of plane approach can also be used.

●Gently advance the needle tip until it approaches the paraneural sheath, if appreciable, on the lateral side of the sciatic nerve. Direct the needle immediately deep (ventral) to the sciatic nerve, positioning the needle tip at the lateral edge of the nerve. Following negative aspiration, inject a small amount of local anesthetic (LA; approximately 3 mL). Stop injection and reposition the needle if the patient has pain or paresthesia, or if the nerve swells on ultrasound, any of which may indicate intraneural injection.

●Continue to inject approximately 20 mL of LA in 5 mL increments with gentle aspiration between injections. Visualize spread within the paraneural sheath. The nerve may appear to float in a ring of LA (called the "donut sign" [23]) but this is not required for successful block. Simple displacement of the nerve and formation of a fluid pocket within the paraneural sheath is acceptable [24,25]. (See 'Local anesthetic choice and dose' below.)

Nerve stimulator-guided block — Nerve stimulation can be used on its own to perform the block, or it can be used for confirmation of needle/nerve proximity during ultrasound-guided block. For nerve stimulator-guided block, we use a single injection, preferably after eliciting a tibial nerve stimulation motor response, as described here. Some authors advocate a multiple stimulation approach, which involves injecting one-half of the LA after eliciting a tibial response and the other one-half after a peroneal response. The two injection technique has been associated with faster block onset time and increased block success [18]. However, we do not favor this approach due to added complexity, higher rate of paresthesia, and occasional difficulty eliciting the second stimulation pattern.

●Nerve stimulation equipment – The authors set the nerve stimulator to a pulse width of 100 ms at 1-2 Hz and an initial current of 1.0 mA. We use a shielded, B-bevel, 20 to 22 gauge 10 cm stimulating needle. Connect the stimulator to the needle and to an electrode on the patient's skin. (See "Overview of peripheral nerve blocks", section on 'Equipment for nerve stimulator guidance'.)

●Performing the block

•Determine needle insertion site, as described above. (See 'Landmarks for the block' above.)

•Infiltrate the skin and soft tissue at the injection site with 1% lidocaine, 1 to 3 mL, using a 25 gauge needle.

•Insert the needle perpendicular to the skin, and advance, watching for stimulation responses. If bone is contacted, withdraw the needle and redirect medially.

•Advance the needle until a motor response occurs in the calf, ankle, or foot. Two patterns of evoked response may be seen.

-Common peroneal stimulation – Eversion and dorsiflexion of the foot

-Tibial nerve stimulation – Inversion and plantar flexion of the foot

The mnemonics "TIP" (tibial = inversion, plantarflexion) and "PED" (peroneal = eversion, dorsiflexion) can help one remember these stimulation patterns.

•For a single injection technique, stimulation of the tibial nerve (plantar flexion) is the preferred stimulation response and has been associated with increased block success and faster onset, compared with common peroneal stimulation (dorsiflexion) [19] in transgluteal sciatic blockade.

-If common peroneal stimulation occurs (dorsiflexion), withdraw the needle and redirect medially.

-If biceps femoris stimulation (knee flexion) is elicited, withdraw the needle and redirect laterally.

-If no stimulation is observed after two redirections, landmarks should be reassessed and a new needle insertion site should be considered. Alternative strategies for localizing the nerve (eg, Wadwha's technique as above) or ultrasound should be considered.

•Once plantar flexion of the foot occurs, decrease the stimulus current until the motor response disappears, then turn back up to the minimal current that results in a response. The goal is a minimal stimulation response between 0.2 and 0.5 mA. The presence of a motor response at <0.2 mA strongly suggests intraneural needle placement and the needle should be repositioned without injecting.

•After negative aspiration, inject 30 mL of LA containing epinephrine (as a vascular marker) in 5 mL increments with gentle aspiration between injections. Stop injection and reposition the needle tip if blood appears in the syringe, the patient complains of pain or paresthesia, or if there is resistance to injection.

LOCAL ANESTHETIC CHOICE AND DOSE — Local anesthetics (LAs) are chosen according to the goal of the block (surgical anesthesia or analgesia) and the desired duration of the effect of the block (table 1). LAs for peripheral nerve blocks and the use of adjuvant drugs are discussed in more detail separately. (See "Overview of peripheral nerve blocks", section on 'Drugs'.).

●Choice of LA – Usual options for sciatic block are as follows:

•Surgical anesthesia only – 2% lidocaine or 1.5% mepivacaine.

•Postoperative analgesia – 0.25 to 0.5% bupivacaine or 0.5% ropivacaine.

•Surgical anesthesia and postoperative analgesia, either:

-For rapid onset – Equal volumes of a short-acting LA (2% lidocaine or 1.5% mepivacaine) plus a long-acting LA (0.5% bupivacaine or 1% ropivacaine). Note that mixing LAs results in onset and duration that are both intermediate between the two agents [26].

-When rapid onset is not required (ie, 30 minutes for onset is acceptable) – 0.25 to 0.5% bupivacaine or 0.5% ropivacaine.

●Volume of LA – LA volumes used for sciatic blocks vary widely.

•Ultrasound-guided block – LA volumes of 20 mL are typically used, extrapolating from literature of nerve stimulator-guided sciatic blockade suggesting adequacy of 17 mL in 95 percent of subjects [27]. In our practice we reduce the volume only when multiple nerve blocks will be used for surgical anesthesia (eg, sciatic, femoral, lateral femoral cutaneous, and obturator blocks for surgical anesthesia of above knee amputation or total knee arthroplasty). In such cases we reduce the dose to avoid LA toxicity.

A commonly cited end-point with regard to LA spread is the "donut," or "halo sign," which refers to circumferential spread of LA around the nerve. However, circumferential spread is not necessarily required or predictive of a successful block [24]. Furthermore, care should be taken to avoid inadvertent nerve puncture when performing multiple needle passes in order to achieve circumferential spread.

•Nerve stimulator-guided block – Higher LA volume, up to 30 mL, may be required for nerve stimulator-guided block, as injection may occur further from the target nerve, and potentially separated from the nerve by connective tissue barriers. In a dose finding study in patients who underwent nerve stimulator guided sciatic nerve block, the effective dose required to produce sensory and motor block in 95 percent of subjects was 17 mL for sciatic block [27].

If multiple procedures are planned (eg, additional femoral block, or local infiltration), the total dose of LA should be calculated to avoid excessive dosing and to minimize the risk of local anesthetic systemic toxicity (LAST) (table 1). (See "Local anesthetic systemic toxicity".)

CONTINUOUS SCIATIC NERVE BLOCK — Continuous sciatic block catheters may be used to provide prolonged postoperative analgesia beyond the duration provided by a single-injection block.

●Catheter placement technique – The technique for placing the catheter is similar to the technique used for single injection block.

•Use a 19 or 20 gauge Tuohy needle for this technique. Insulated, stimulating Touhy needles are available.

•Place the needle tip and catheter near the nerve, given the approach chosen. If nerve stimulation is used, place the needle tip such that tibial nerve stimulation occurs at 0.2-0.5 mA. (See 'Nerve stimulator-guided block' above.)

•After negative aspiration for blood, inject a small volume of saline or local anesthetic (LA) to distend the space into which the catheter will be threaded.

•While stabilizing the needle, insert a catheter through the needle and advance far enough to allow the catheter perforations to bathe the nerve. The authors insert the catheter 4 to 5 cm beyond the tip of the needle. The optimal distance for catheter insertion has not been well studied, and practice varies. The catheter should be inserted far enough to prevent dislodgement, but inserting it too far may increase the chance of piercing the paraneurium and placing the catheter tip too far from the nerve.

•If using ultrasound, carefully visualize catheter insertion to identify and correct catheter passage away from the nerve.

•Remove the needle without withdrawing the catheter.

•After negative aspiration, inject 15 to 30 mL of LA through the catheter. If using ultrasound, confirmation of continued satisfactory perineural spread through the catheter is crucial to successful catheter function.

•Secure the catheter well. The authors use skin adhesive and a locking device to prevent catheter migration. Others tunnel the catheter away from the insertion site.

●Positioning, draping, and securing the catheter – Important technical aspects to consider when placing a catheter for continuous sciatic block include:

•Placing a perineural catheter takes longer than a single-injection block and sufficient time must be allocated prior to surgery (budget 15 to 30 minutes).

•Create a large sterile field to avoid contamination of the catheter. Sterile towels and/or a clear plastic drape can be used.

•Wear a mask, cap, and sterile gloves during catheter placement. Use a sterile ultrasound sheath to avoid contamination of the surgical field.

•Secure the catheter well to prevent leakage and/or dislodgement.

-We apply surgical glue at the catheter insertion site, primarily to prevent leakage, which can disrupt dressing integrity lead to catheter dislodgement. The glue also helps fix the catheter in place.

-We apply liquid adhesive (eg, tincture of benzoin or similar) around the insertion site.

-We use a catheter fixation device. Avoid placing such devices within the surgical field or where a tourniquet will be placed.

•Coil the excess catheter and fix it to the skin with a transparent adhesive dressing. Ensure the insertion site, fixating device, and catheter are well covered and visible.

●Infusion drug dose – After injecting a bolus of LA as described above, we start an infusion of 0.125% bupivacaine or 0.2% ropivacaine at 5 to 8 mL/hour postoperatively, with a continuous infusion and a patient-controlled bolus of 5 mL per 30 minutes. Ambulatory catheters, which can be managed and removed at home by the patient, are well-described and have an excellent safety record [28]. Ambulatory patients receive comprehensive instructions on the signs and symptoms of LA toxicity and on pump maintenance, and the acute pain service phone number prior to discharge.

SIDE EFFECTS AND COMPLICATIONS — Major complications of sciatic block are very rare. Complications common to all peripheral nerve blocks (eg, nerve injury, bleeding, local anesthetic systemic toxicity [LAST], infection) are discussed in detail separately. (See "Overview of peripheral nerve blocks", section on 'Complications'.)

Motor weakness associated with sciatic block and other lower extremity blocks can cause patient falls and poor rehabilitation after surgery. The most important motor deficit associated with sciatic block is the inability to dorsiflex the ankle (ie, foot drop), which causes the foot to drag and catch on the ground. The resulting falls can lead to significant morbidity and mortality. (See "Falls in older persons: Risk factors and patient evaluation", section on 'Morbidity and mortality'.)

Sciatic block may result in motor block of the hamstrings, which can also cause falls. Motor block is expected but not universal following sciatic nerve blocks [29,30]. It may be reduced but not eliminated by using dilute local anesthetic (LA) solutions and possibly by using ropivacaine rather than bupivacaine [31]. Patients and families should be alerted to the risk of motor block after popliteal block, particularly in ambulatory settings.

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Local and regional anesthesia".)

SUMMARY AND RECOMMENDATIONS

●Anatomy

•The sciatic nerve is derived from spinal nerves L4-S3 (figure 2). After exiting the pelvis, the sciatic nerve travels through the subgluteal space, followed by the infragluteal space, the sites at which transgluteal and infragluteal sciatic blocks, respectively, are performed (figure 1). (See 'Anatomy' above.)

•The sciatic nerve provides sensory innervation to the skin of the lower leg, except for a medial strip that is innervated by the saphenous nerve. It innervates the hamstrings and muscles of the calf and foot (figure 6). (See 'Sensory' above and 'Motor' above.)

•Sciatic nerve block is used for anesthesia or analgesia of the posterior knee or for surgery below the knee. (See 'Clinical anatomic correlations' above.)

●Block technique – We use ultrasound guidance, with or without nerve stimulator confirmation. If ultrasound is not available, nerve stimulator guidance is a reasonable alternative (see 'Ultrasound guidance versus nerve stimulation' above). Block technique is described briefly here, and in more detail above.

Position the patient in the lateral decubitus position but rolled slightly forward, with the limb to be blocked in the nondependent position, flexed 45 degrees at the hip and 90 degrees at the knee (figure 3).

•Transgluteal block

-Place the ultrasound transducer or mark the patient and insert the needle according to the graphic (figure 3 and picture 1). (See 'Landmarks for the block' above.)

-For ultrasound-guided block, identify the sciatic nerve in the plane between the gluteus maximus and quadratus femoris muscles (figure 4). Insert the block needle in plane, positioning the tip near the lateral edge of the nerve. After negative aspiration, inject approximately 20 mL of local anesthetic (LA) in 5 mL increments, with gentle aspiration between injections, visualizing spread around the nerve (movie 1). (See 'Ultrasound-guided block' above.)

-For nerve stimulator-guided block, insert the needle perpendicular to the skin and advance, watching for motor response in the foot. After appropriate tibial nerve stimulation (ie, foot inversion and plantar flexion) and after negative aspiration, inject 30 mL of LA with epinephrine (as a vascular marker) in 5 mL increments, with gentle aspiration between injections. (See 'Nerve stimulator-guided block' above.)

•Infragluteal block

-Place the transducer or mark the patient and insert the needle according to the graphic (figure 7).

-For ultrasound-guided block, identify the sciatic nerve lateral to the biceps femoris tendon, deep to the gluteus maximus muscle and superficial to the adductor magnus muscle (figure 8). Insert the needle in or out of plane, positioning the tip near the lateral edge of the nerve. After negative aspiration, inject approximately 20 mL of LA in 5 mL increments, with gentle aspiration between injections, visualizing spread around the nerve. (See 'Ultrasound-guided block' above.)

-For nerve stimulator-guided block, insert the needle perpendicular to the skin and advance, watching for motor response in the foot. If bone is contacted, redirect the needle medially. After appropriate tibial nerve stimulation (ie, foot inversion and plantar flexion), and after negative aspiration, inject 30 mL of LA with epinephrine (as a vascular marker) in 5 mL increments, with gentle aspiration between injections. (See 'Nerve stimulator-guided block' above.)

●Continuous block technique – Continuous block is performed as described for single injection block, using a Touhy needle with a 19 or 20 gauge catheter inserted through it and 4 to 6 cm beyond the needle tip. (See 'Continuous sciatic nerve block' above.)

●LA choice (see 'Local anesthetic choice and dose' above)

•For single injection block:

-For surgical anesthesia – 2% lidocaine or 1.5% mepivacaine.

-For postoperative analgesia – 0.25 to 0.5% bupivacaine or 0.5% ropivacaine

•For continuous block– Bolus injection as for single-injection block, followed by continuous infusion of 0.125% bupivacaine or 0.2% ropivacaine at 5 to 8 mL per hour

●Side effects and complications – Major side effects of sciatic block are very rare. Patients should be alerted to the possibility of motor block, which can cause foot drop or hamstring weakness. (See 'Side effects and complications' above.)