The posterior femoral cutaneous nerve accompanies the sciatic nerve medially along with the inferior gluteal artery, and is reliably blocked using this approach.

This block is commonly performed in combination with a lumbar plexus block for hip procedures. The depth at which the sciatic nerve is often located is often the same as the one of the femoral nerve. Consequently, we first perform the lumbar plexus block.

The sciatic nerve usually innervates no muscles in the gluteal region. Direct gluteal stimulation on advancement of the needle indicates that the sciatic nerve is still deep to the needle tip.

Pelvic splanchnic and distal sympathetic block are possible given their close proximity to the block site leading to possible urinary retention.

In morbidly obese patients it is sometime necessary to use a 15-cm needle. However, we recommend to first start with a 10-cm needle, unless there is clear evidence that a 15-cm is required.

Close pudendal nerve proximity may lead to genital tingling during needle positioning, and anesthesia following the block. If genital tingling is elicited during needle placement, the needle should be repositioned more laterally and superficially.

This approach is useful for hip procedures as it allows for the concurrent block of additional sacral branches involved in hip joint innervation. Two of these nerves that exit through the greater sciatic foramen are the superior gluteal nerve (L4-S1, coursing superior to the piriformis muscle) and the nerve to the quadratus femoris muscle (L4-S1, coursing anterior to the sciatic nerve).

Weakness in leg adduction with this block is due to block of the sciatic branch to the hamstring portion of the adductor magnus rather than obturator block.

Limit advance of the needle to 2 cm beyond the border of the sacrum to minimize the chance of pelvic organ damage.

A pillow may be placed between the legs at the level of the knee.

Appropriate positioning is critical to establish the proper site for the introduction of the needle.

Already at this level, the sciatic nerve is separated into the common peroneal and the tibial nerves, and the posterior femoral cutaneous nerve of the thigh has branched.

The stimulation of the sciatic nerve is almost always preceded by the stimulation of the gluteus maximus.

A bone contact usually indicates that the needle is too lateral.

Stimulation of the piriformis muscle indicates that the needle is too cephalad.

A motor response at the level of the toes increases the likelihood of success.

When patients complain of pelvic discomfort, it suggests that the needle is too anterior and is going through the greater sciatic notch.

Because the sciatic nerve is found at a depth of 8 to 13 cm, no redirection of the needle should be attempted after it passes the skin to avoid bending the needle.

This approach can be uncomfortable for the patient and therefore requires appropriate local anesthesia with a 38-mm needle and an appropriate sedation.

This approach is not recommended in anticoagulated patients.

A new posterior approach has been described in adults: The patient is positioned either prone or in the lateral position. The site of introduction of the needle is 10 cm lateral from the midpoint of the intergluteal sulcus.

Positioning of the patient is important, as with any regional anesthesia technique. The patient is placed in true lateral position (not Sim's). Both hips and knees remain slightly flexed and the buttocks are at a right angle with the bed. This position aligns the midline of the patient with the horizontal plane of the bed making it easier to judge the angle of insertion of the needle.

The prone position can be used but it is usually unnecessary and more time consuming.

The sciatic nerve enters the upper gluteal area describing a downward curve from medial to lateral before running down parallel to the midline. Thus, it is recommended to attempt the block in the lower three-fourths of the buttocks because in the upper fourth the nerve is located less than 10 cm from the midline.

The differences between male and female pelvises lie in the shape and diameters of the inner pelvis and not in the total width (bicrestal diameter), which is virtually the same in both sexes. The female inner pelvis is wider while the male bones are thicker. In fact, different hormone-dependent patterns of fat deposition in both sexes explain the perceived differences in pelvis width.

Deposition of fat in the buttocks does not affect the position of the sciatic nerve with respect to the midline but it does affect its depth and its relative position with respect to the lateral side of the body. In other words, the nerve's relative position in the buttocks changes, but not its absolute position with respect to the midline, which remains fixed at 10 cm.

The gluteus maximus muscle is the only muscle that covers the sciatic nerve superficially in most of the gluteal region. In the uppermost area of the buttocks the small piriformis muscle is also superficial to the nerve.

In our experience any response that can be elicited from stimulation of the sciatic nerve (e.g., dorsal or plantar flexion, eversion and inversion) provides a similar success rate provided the response is obtained at low output.

When reposition of the needle is necessary it is important to realize that small changes in the angle of insertion of the needle have a profound impact in the position of the tip of the needle when dealing with deep structures like the sciatic nerve. A modest 10° correction angle for example has been calculated to move the tip of the needle approximately 1.6 cm at a depth of 9 cm.

The gluteal approach as compared with other sciatic approaches (classic, high lateral) reduces the risk of misplacements or dislocation of the catheter after surgery and reduces the risk of vascular puncture.

This block is usually performed in combination with a single or continuous lumbar plexus or femoral block.

A variant of the gluteal approach is the subgluteal approach (SG). A line is drawn perpendicularly and extending caudally for 4 cm in the middle of the ischial tuberosity–greater trochanter line. The end of this line represents the site of introduction of the needle (SG) (Fig. 12-7).

Gluteal or subgluteal approach is preferred in obese patients, because at this level the sciatic nerve is the most superficial.

Compared with the classic posterior approach of Labat, this approach, as well as subgluteal, is less painful because the needle is introduced in the groove between the biceps femoris and lateral border of quadriceps femoris muscles.

If the femur is contacted, the needle needs to be withdrawn and redirected medially (toward sacrum).

If direct stimulation of gluteus maximus muscle is noticed (too shallow), needle should be advanced a few millimeters until distal response with foot movement is recorded.

Hamstring contraction represents either a direct muscle contraction or the stimulation of tibial nerve, except the short head of biceps femoris, which is innervated by common peroneal nerve (and not considered part of the hamstring muscles). This response is not reliable for the block below the knee. The needle should be readjusted to elicit a distal (toes) response.

If patient reports painful paresthesia along the leg, the needle should be withdrawn and redirected to achieve painless stimulation with movement of the toes with a current of or below 0.5 mA.

In this approach, the nerve stimulator has a dual function. First, it helps prevent damage to the femoral nerve during the first 3 to 5 cm of needle introduction, and second, it helps to localize the sciatic nerve. If, during the introduction of the needle, it is found that the needle is directly on the femoral nerve (presence of movement of the patella with a current of 0.5 mA), the needle is directed slightly medially before continuing with any further advancement (Fig. 12-11).

Except for morbidly obese patients (>100 kg), it is rarely necessary to introduce the 150-mm needle. In most cases, the nerve is found at an approximate depth of 10 to 12 cm.

Introduction of an insulated needle through the quadriceps is easy. However, if at a depth of 10 to 12 cm, the introduction of the needle becomes more difficult, this is a sign that the needle is very close to the femur. In such cases, it should be withdrawn to the skin and redirected slightly more medially.

The onset time for a sciatic nerve block is usually longer than other blocks. It usually requires 20 to 30 minutes or even longer.

Using the anterior sciatic approach also allows one to block the femoral nerve through the same skin introduction point. However, to avoid any risk of injury of the femoral nerve by the insulated needle on its way to the sciatic nerve, it is recommended to first block the sciatic nerve and then proceed to block the femoral nerve while retrieving the needle. This technique offers the advantage of only one stick. However, with such a distal approach to the femoral nerve, it is not possible to also block the femoral cutaneous nerve or the obturator (three-in-one block).

Beck first described an anterior approach. The Chelly and Delaunay approach and the Beck approach use different anatomic landmarks, but both lead to the same site (Fig. 12-12) and both approach the sciatic nerve above the lesser trochanter.

Van Elstraete et al. described new landmarks (2.5 cm medial from the femoral artery and 2.5 cm distal from the inguinal crease). In this case the 150-mm insulated needle connected to a nerve stimulator is introduced at a 10° to 15° angle relative to the vertical plane. This latter approach offers the advantage of being based only on a landmark
found below the femoral crease. Van Elstraete appears to approach the sciatic nerve below the lesser trochanter.

Anterior approach to the sciatic nerve is especially interesting in morbidly obese and trauma patients who cannot be moved. However, in patients weighing over 100 kg, the 150-mm needle may be too short. In these cases, it is possible to approach the sciatic nerve using the lithotomy approach described by Raj, with the patient supine and the hip and the knee flexed. This gluteal approach is based on the identification of the ischial tuberosity and greater trochanter. The middle of the ischial tuberosity–greater trochanter line represents the site of introduction of the needle. The needle is introduced perpendicular to the skin, and stimulation of the sciatic nerve is produced within 4 to 12 cm (depending on the size of the thigh) (Fig. 12-13).

This is one of the easier blocks to perform.

This block can also be performed with a 50-mm needle with the patient placed in the lateral position.

The anatomic landmarks can be easily identified by asking the patient to perform an active flexion of the leg.

The best positioning of the leg and foot can be achieved by either positioning the patient with the foot outside of the stretcher or placing a pillow under the leg.

Minimal discomfort should be experienced by the patient because only interstitial fat lies between the skin and the nerve. If the patient complains of pain, this suggests that the needle is going through the semi-tendinous/semi-membranous tendons rather than being medial to them.

It is possible to perform the same block using the lithotomy approach.

For surgery performed at the level of the knee, especially with the use of a tourniquet placed at the thigh, a femoral or a lumbar plexus block is required to complete the anesthesia. For surgery below the knee, especially when a tourniquet is placed at the level of the calf or below, a saphenous nerve block is sufficient.

There are at least three techniques that can be used to perform a saphenous nerve block at the level of the knee and with the patient in the supine position. First, a 22-gauge, 38-mm blunt needle mounted on a 10-mL syringe (containing 10 mL of local anesthetic solution) is introduced perpendicular to the skin and through the sartorius muscle, 3 to 4 cm cephalad from the medial femoral condyle. Within 3 cm, a loss of resistance is felt. After negative aspiration for blood, 10 mL of local anesthetic is injected slowly, with aspiration for blood after 5 mL. Second, a 50-mm insulated needle connected to a nerve stimulator (2 mA, 2 Hz, 0.1 ms) is introduced perpendicular to the skin, 3 cm distally to the medial femoral epicondyle. Within 2 to 3 cm, the patient should report an electrical paresthesia in the medial aspect of the leg. The needle position is adjusted to maintain the paresthesia with a current of less than 0.4 mA. After negative aspiration for blood, 6 to 8 mL of local anesthetic is injected slowly. Third, a 25-gauge, 38-mm needle is introduced 3 cm below the tibial plateau and directed from the middle to the medial border of the leg. Ten milliliters of local anesthetic solution is then injected subcutaneously.

This block can be performed at any level of the popliteal fossa. However, the more cephalad from the popliteal crease, the closer are the common peroneal and tibial nerves. Conversely, the more distal, the more separated the nerves become. Indeed, the tibial nerve runs in the middle of the popliteal fossa while the common peroneal nerve runs in a lateral direction toward the head of the fibula.

Variants of the proposed technique include using the perpendicular line in the middle of the popliteal crease as a starting point. If no motor response is elicited, the needle is directed more laterally.

The deep femoral vessels become the popliteal vessels after passing through the adductor hiatus. The popliteal vessels, which are initially medial and anterior and separated from the sciatic nerve, run medial and anterior to the tibial nerve at the lower part of the popliteal fossa. Consequently, the risk of vessel puncture is minimal if the needle is kept in the lateral upper quadrant of the popliteal fossa or if the needle is introduced above the adductor hiatus (above the popliteal fossa). However, this latter approach requires a 100-mm needle to be introduced deeper and through the hamstring muscles, which is more painful to patients.

If the medial aspects of the lower leg and ankle are involved in the surgery, the addition of a saphenous nerve block will be necessary to provide complete anesthesia. This can be achieved by blocking the saphenous nerve selectively above or below the knee or by performing a femoral nerve block. The latter is especially recommended if thigh coverage will be used during surgery.

In most individuals the division of the sciatic nerve into the common peroneal nerve and the tibial nerve occurs below the suggested level of needle insertion. However, if this block is performed closer to the knee, a double stimulation and injection technique accounting separately for both branches may be necessary to achieve a complete block in the sciatic nerve territory of the lower leg.

If the local biceps femoris muscle twitch cannot be elicited, the needle can be inserted in a perpendicular direction until the femur is contacted. Then the needle is withdrawn and reinserted in 10° to 15° increments posterior to the horizontal plane until a sciatic nerve response is obtained.

Plantar flexion (tibial nerve response) as the elicited motor response may yield a more complete block and a faster onset time than dorsiflexion (peroneal nerve response).

This approach is a great alternative to the posterior popliteal and the classic approaches to the sciatic nerve if the patient cannot be positioned in prone or lateral decubitus position.

Slightly supporting the knee by placing a small towel under the popliteal fossa allows for easier needle operation.