I. Anatomy. The sacral plexus is located on the surface of the posterior pelvic wall, anterior to the piriformis muscle. It is formed from the ventral rami of the sacral spinal nerves (S1-S4). It also receives contributions from the lumbar spinal nerves (L4 and L5) via the lumbosacral trunk (Fig. 10.1). The ventral rami enter the pelvis through the anterior sacral foramina and converge to form the sacral plexus. The sacral plexus has a flattened triangular shape with its base oriented lateral to the sacral foramina and its vertex toward the greater sciatic foramen (Figs. 10.1 and 11.1). It lies anterior to the piriformis muscle and posterior to the presacral fascia, which separates it from the intrapelvic viscera. The sacral plexus provides sensory and motor innervation to portions of the lower extremity, including the hip, knee, and ankle joints. The most important branch for lower extremity surgery is the sciatic nerve.

A. The lumbosacral trunk (L4 and L5) and the anterior divisions of the S1-S3 ventral rami give rise to the tibial nerve (TN), whereas the posterior divisions of S1-S3 ventral rami give rise to the common peroneal nerve (CPN). These two distinct nerves travel together to form the sciatic nerve (SN). They are independent nerves that do not mix nerve fibers, but they share a common trajectory and common extraneural connective tissue sheath until they physically diverge from each other within the popliteal fossa.

B. As the sciatic nerve exits the pelvis through the infrapiriform foramen at the inferior border of the piriformis, the larger TN is medial and slightly anterior to the CPN. From the point where the sacral plexus first enters the pelvis until the sciatic nerve leaves the gluteal compartment just distal to the ischial tuberosity (IT) of the ischium and greater trochanter (GT) of the femur, it is covered by the mass of the gluteus maximus (Figs. 11.1 and 11.2). Within the gluteal region, the sciatic nerve is located just lateral to the posterior femoral cutaneous nerve and the inferior gluteal artery. Between the IT (medially) and the GT (laterally), the sciatic nerve is in a welldefined intermuscular compartment (subgluteal compartment) superficial to the quadratus femoris and deep to gluteus maximus.

C. The sciatic nerve descends caudally within the posterior thigh, by passing between the lateral border of the IT and the medial border of the proximal femur (Figs. 11.1 and 11.2). As the sciatic nerve courses into the posterior compartment of the proximal thigh, it lies posterior to the lesser trochanter of the femur. Within the proximal posterior compartment of the thigh, just distal to the inferior border of the gluteus maximus, the sciatic nerve lies on the posterior surface of the adductor magnus muscle immediately lateral to the tendon of the biceps femoris muscle (Figs. 11.2 and 11.3). At this location, the sciatic nerve is relatively superficial and covered only by skin and subcutaneous tissue.

D. Within the midthigh (approximately halfway between the lateral aspect of the GT and the popliteal crease), the sciatic nerve is located posterior and medial to the shaft of the femur in a myofascial plane: dorsal (superficial) to the adductor magnus and ventral (deep) to the belly of the long head of the biceps femoris.

E. As the sciatic nerve continues further distally within the middle third of the posterior compartment of the thigh toward the popliteal fossa, it lies in an intermuscular plane between the adductor magnus and the long head of the biceps femoris muscles (Fig. 11.4).

F. Within the apex of the popliteal fossa, the sciatic nerve is bordered laterally by the long head of the biceps femoris muscle tendon and medially by the semimembranosus-semitendinosus (SM-ST) tendons (Figs. 11.3 and 11.4). The divergence of the sciatic nerve into the physically separate TN and CPN components (Figs. 11.3 and 11.4) occurs within the proximal aspect of the popliteal fossa (6 to 9 cm above the popliteal crease, but with a range of 0 to 14 cm), but it may also occur at any point between the sacral plexus and the popliteal crease. Within the popliteal fossa, the sciatic nerve lies posterolateral to the popliteal artery and vein.

II. Drugs

A. The choice of local anesthetic for sciatic nerve block is dependent on the requirements for onset of anesthesia and duration of analgesia for single-injection techniques, as well as the anatomic

location where the block is performed. The sciatic nerve block is different from the individual nerve blocks of the lumbar plexus because the approach (anatomic location) for sciatic nerve block may have an impact on the onset and total local anesthetic mass requirements.

B. Specifically, the proximal approaches to sciatic nerve block have a shorter latency to complete anesthesia and lower total anesthetic requirements compared with the distal popliteal approaches (2). This is likely owing to the difference in the quantitative architecture (ratio of neural tissue and nonneural tissue within and external to the epineurium) between the proximal and distal approaches (3,4).

C. With the advent of continuous perineural catheter techniques, the anesthesiologist can extend the anesthetic duration (Tables 11.1 and 11.2) of surgical anesthesia by simply redosing through the indwelling perineural catheter. More commonly, a postoperative infusion of a dilute local anesthetic (e.g., ropivacaine 0.2% or bupivacaine 0.125%) may be used to provide the optimal balance of postoperative analgesia with minimal motor block to facilitate postoperative rehabilitation and recovery. Alternatively, if a neuraxial technique is chosen as the primary anesthetic, a loading dose (10 to 15 mL) of the analgesic infusion of ropivacaine 0.2% may be started intraoperatively. The typical postoperative regimen consists of running the analgesic infusion at 4 to 10 mL/h with or without a patient-controlled bolus of 2 to 5 mL every 20 to 60 minutes based on institutional protocols.

III. Approaches and techniques

USG has become the predominant technique for peripheral nerve localization. USG has not only been shown to increase the onset of complete sensory block but also decreases block performance time, block onset time, and local anesthetic requirements compared to PNS (5). There is no evidence to support the routine use of concurrent PNS to supplement a primary USG (dual-guidance) technique, especially when the target nerve is well visualized. However, a dual-guidance technique may be useful in two specific circumstances: (1) when target nerve visualization is difficult in specific circumstances, such as increased body habitus, deep nerve location (subgluteal or anterior approach), or anatomic variation and (2) evoked motor responses at current output ≤0.2 mA is highly suggestive of intraneural needle placement, and should prompt slight withdrawal or repositioning of the needle tip prior to local anesthetic injection; however, stimulatory thresholds >0.2 mA do not offer a fail-safe guarantee that the needle tip is extraneural. The sciatic nerve is the longest nerve in the body and may be blocked using the approaches such as subgluteal, anterior, midfemoral, or popliteal levels.

A. Ultrasound-guided subgluteal approach

1. Indications. In combination with a lumbar plexus block (or a combination of femoral, lateral femoral cutaneous, and obturator nerve blocks), complete anesthesia or analgesia for the entire lower extremity from the proximal thigh (distal to the hip joint) to the foot. In combination with a femoral nerve block (or saphenous nerve block above the knee), complete anesthesia or analgesia for the entire lower extremity from below the knee to the foot.

2. Contraindications. Usual contraindications to peripheral nerve blocks (i.e., lack of consent, infection at the injection site, allergy to local anesthetics). Preexisting neural compromise is considered a relative contraindication; a careful discussion regarding the potential risks and benefits of performing peripheral nerve blocks in patients with preexisting neural compromise is strongly recommended (6).

3. Single-injection technique

a. Patient position. The patient is typically placed in the lateral decubitus position with a slight forward tilt, hips flexed with the operative side to be blocked uppermost. The dependent limb should be straight, and the operative limb should be slightly flexed at both the hip and the knee. Alternatively, the patient may be placed in the prone position.

b. Ultrasound transducer selection-scanning technique. A low-frequency curved array transducer (2 to 5 MHz) is typically used for this block. The US transducer is initially placed at a 90-degree angle to the skin, with the long axis (LAX) of the probe directly over and parallel to a line connecting the IT and GT to obtain a short-axis (SAX) US image of the sciatic nerve (Fig. 11.5A and B).

c. Ultrasound anatomy (Fig. 11.6). The sciatic nerve appears as a hyperechoic, oval to lip-shaped polyfascicular structure (7,8). Slight adjustments in the transducer position will enhance the anisotropy of the sciatic nerve and optimize its sonographic appearance. The subgluteal compartment is bordered laterally by the GT and medially by the IT, with the sciatic nerve typically located within 3.0 cm lateral to the IT. Both IT and GT appear as intensely hyperechoic curved structures with posterior acoustic dropout. The sciatic nerve is sandwiched deep to the epimysium of the gluteus maximus muscle and superficial to the epimysium of the quadratus femoris muscle. The epimysium of the two muscles forms the subgluteal compartment and appears to suspend the sciatic nerve in between the GT and IT.