Sciatic Nerve Blocks
A. Parasacral Approach
Carl Rest
Patient Position: Lateral with surgical side up.
Indications: Surgical anesthesia and postoperative analgesia of the posterior thigh and leg, as well as the knee, ankle, and foot.
Needle Size: 21-gauge, 100-mm insulated stimulating needle.
Volume: 15 to 20 mL of ropivacaine 0.5% or 0.2% depending on the indication
Anatomic Landmarks: The ischial tuberosity and the posterior superior iliac spine (PSIS) are major surface landmarks for this block. The sciatic nerve is comprised of L4 through S3 ventral rami and exits the pelvis through the inferior portion of the greater sciatic foramen, deep (anterior) to the piriformis muscle. It lies deep to the gluteus maximus muscle and medial to both the gluteus medius and gluteus minimus. It is the most lateral of all structures passing inferior to the piriformis muscle. Medial to it lies the internal pudendal and inferior gluteal vessels and nerves. Occasionally the sciatic nerve divides prior to entering the gluteal region, with the common fibular nerve passing superior to, or through the piriformis muscle.
Approach and Technique: The PSIS and the inferior aspect of the ischial tuberosity are palpated and marked, and a line drawn between the two points (Fig. 12-1). The needle insertion lies along this line, slightly caudal to the superior aspect of the gluteal cleft, or approximately 6–8 cm caudal to the PSIS. The needle is connected to a nerve stimulator (2 Hz, 1.5 mA, 0.1 ms) and advanced in a parasagittal plane. If the sacrum is contacted, the needle is withdrawn and redirected slightly laterally, walking off the border of the sacrum into the greater sciatic foramen. The endpoint of needle advancement is a sustained sciatic stimulation eliciting foot plantar flexion/inversion (tibial nerve) or dorsiflexion/eversion (common fibular nerve) motor response with a current less than 0.5 mA (Fig. 12-2). Following negative aspiration for blood, local anesthetic is slowly injected in 5-mL increments, with intermittent aspiration.
 Figure 12-1. The PSIS and the inferior aspect of the ischial tuberosity are palpated and marked, and a line drawn between the two points. |
Tips
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.
 Figure 12-2. The endpoint of needle advancement is a sustained sciatic stimulation eliciting foot plantar flexion/inversion (tibial nerve) or dorsiflexion/eversion (common fibular nerve) motor response with a current less than 0.5 mA. |
Suggested Readings
Birnbaum K, Prescher A, Hessler S, et al. The sensory innervation of the hip joint—an anatomical study. Surg Radiol Anat 1997;19:371–375.
Bruell P. Sciatic nerve block: parasacral approach. Reg Anesth Pain Med 1998;23:78.
Jochum D, Iohom G, Choquet O, et al. Adding a selective obturator nerve block to the parasacral sciatic block: an evaluation. Anesth Analg 2004;99:1544–1549.
Mansour NY, Bennetts FE. An observational study of combined continuous lumbar plexus and single shot sciatic nerve blocks for post–knee surgery analgesia. Reg Anesth 1996;21:287–291.
Morris GF, Lang SA, Dust WN, et al. The parasacral sciatic nerve block. Reg Anesth 1997;22:223–228.
Ripart J, Cuvillon P, Nouvellon E, et al. Parasacral approach to block the sciatic nerve: a 400-case survey. J Reg Anesth Pain Med 2005;30:193–197.
B. Posterior Approach
Daneshvari R. Solariki
Patient Position: Lateral, with the operative site up and the knee flexed (Sims position).
Indications: Anesthesia and immediate postoperative analgesia for surgery at and below the knee or requiring the use of a thigh tourniquet for more than 30 minutes.
Needle Size: 21-gauge, 150-mm insulated needle.
Volume: 15 to 20 mL.
Anatomic Landmarks: The greater trochanter, the posterosuperior iliac spine, and the sacral hiatus.
Approach and Technique: The center of the greater trochanter and the posterior iliac spine are identified and marked, and a line is drawn between these two points. Next, the sacral hiatus is identified and marked. Another line is drawn from the greater trochanter to the sacral hiatus. A perpendicular line is drawn to the midpoint of the greater trochanter–posterior iliac spine line. The intersection between this line and the greater trochanter–sacral hiatus line represents the point of insertion of the needle. The insulated needle connected to a nerve stimulator (1.5 mA, 2 Hz, 0.1 ms) is introduced perpendicular to the skin (Fig. 12-3). The stimulation of the sciatic nerve produces a flexion of the foot and toes or an inversion of the foot (tibial nerve) or a dorsiflexion of the foot and extension of the toes or an eversion of the foot (common peroneal nerve). The needle is positioned to maintain the same motor response with a current of less than 0.5 mA. After negative aspiration for blood, the local anesthetic is injected slowly, with repeated aspiration for blood every 5 mL.
 Figure 12-3. The insulated needle connected to a nerve stimulator is introduced perpendicular to the skin. |
Tips
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.
Suggested Readings
Carlo D, Franco MD. Posterior approach to the sciatic nerve in adults: is Euclidean geometry still necessary? Anesthesiology 2003;98:723–728.
Hahn M, McQuillan PM, Sheplock GJ. Regional anesthesia. St. Louis, Mosby-Year Book, 1996:131.
Labat G. Regional anesthesia: its technique and clinical applications. Philadelphia: WB Saunders, 2nd ed., 1930:330.
Winnie AP. Regional anesthesia. Surg Clin North Am 1975;54:861–892.
C. 10-cm Midgluteal Approach
Carlo D. Franco
Patient Position: The patient is placed in the lateral decubitus position with the side to be blocked up. Both lower extremities are flexed slightly at the hips and knees with the buttocks forming a 90° angle with the bed.
Indications: Anesthesia and postoperative analgesia for any surgical procedure in the lower extremity involving the posterior thigh and any area distal to the knee excluding the medial side of the leg, which is innervated by the saphenous nerve, a branch of the femoral nerve.
Needle Size: Usually a 21-gauge, 100-mm insulated needle suffices. In some cases a 20-gauge, 150-mm insulated needle is necessary.
Anesthetic Volume: 25 to 35 mL.
Anatomic Landmarks: The intergluteal sulcus between the buttocks is the only landmark for this approach.
Approach and Technique: This simple approach is based on the fact that the sciatic nerve runs parallel to and about 10 cm from the midline (intergluteal sulcus) in all adults regardless of gender and body habitus. Thus, the block can be performed at 10 cm from the midline at about any point in the gluteal area including the subgluteal fold. In the gluteal area it is usually performed lateral to the midpoint of the intergluteal sulcus only because this point is easy to visualize and teach. The 10-cm measurement must be linear as shown in Figure 12-4, disregarding any individual contour on the patient’s buttocks. This linear distance reflects the distance between the midline and the area immediately lateral to the ischial tuberosity where the nerve runs.
 Figure 12-4. With the patient in true lateral position the needle insertion point is easily found by measuring 10 linear cm from the midline (intergluteal sulcus). No other landmarks are identified. |
 Figure 12-5. The needle is advanced parallel to the patient’s midline at 10 cm from it without the need to find any additional landmarks. |
A local anesthetic wheal is raised at this point and an insulated needle connected to a nerve stimulator (around 1.5 mA, 1 Hz, 0.1 ms) is then slowly advanced parallel to the midline (parallel to the bed) as shown in Figure 12-5. Usually a motor twitch of the gluteus maximus can be easily seen as the needle passes through this muscle and continues to be visible until the needle reaches the deep surface of the gluteus maximus. The needle then needs to traverse the small amount of connective tissue deep to this muscle before reaching the sciatic nerve. The tip of the needle is then carefully manipulated until a response is still visible at 0.5 mA. The injection of local anesthetic is given slowly with frequent aspirations.
If the needle fails to elicit a sciatic nerve response, the reposition is easy since the nerve could only be either lateral or medial to the needle. The needle is withdrawn completely and a very small (10°) correction is made to the angle of insertion, first lateral and then if necessary medial.
Tips
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.Related posts:
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