11 Proximal Sciatic Nerve Block
11.1 Anatomical Overview
The sacral plexus can be divided into three parts:
The sacral plexus is not subdivided in all anatomy textbooks. However, as the division is useful from the clinical aspect, it is used as a basis here.
11.1.1 Sciatic Plexus
The sciatic plexus derives its roots from part of the anterior ramus of the fourth lumbar nerve and from the entire anterior ramus of the fifth lumbar nerve, which together form the lumbosacral trunk, along with the anterior rami of the first and second and part of the third sacral nerves. The anterior ramus of the first sacral nerve is not only the biggest branch of the lumbosacral plexus but the biggest anterior ramus overall.
All of the roots of the plexus converge from their exit sites toward the greater sciatic foramen so that the plexus forms a triangular sheet, the apex of which points toward the infrapiriform foramen where the sciatic nerve emerges (Fig. 11.1). The nerve plexus lies for the most part on the piriformis muscle and is covered toward the pelvis by the parietal peritoneum and the tissues lying below it that form the parietal pelvic fascia and branches of the internal iliac artery. The articular branches that supply parts of the hip joint capsule and the periosteal branches that innervate the periosteum of the sciatic tuber, greater trochanter, and lesser trochanter also come from the sciatic plexus.
11.1.2 Sciatic Nerve (L4–S3)
The sciatic nerve derives its fibers from all the roots of the sacral plexus. It is the biggest and longest nerve in the body, it supplies the widest area, and at the same time it has the greatest resistance among all the nerve cords, with a tear strength of 91.5 kg (!). Excessive stretching can even tear the nerve trunk from its roots in the spinal cord. The roots of the sciatic nerve unite into the trunk immediately before the greater sciatic foramen at the lower border of the piriformis (Fig. 11.2).
The sciatic nerve consists of two components, the common fibular nerve (synonym: common peroneal nerve) and the tibial nerve, which are surrounded in the lesser pelvis and thigh by a shared connective-tissue sheath and therefore have the appearance of a single nerve trunk. The division into the two branches can occur at varying levels (Fig. 11.3 and Fig. 11.4).
At dissection, the nerve can almost always be separated into its two divisions as far as the hip region, even if they run in a common sheath (Ericksen et al 2002).
The sciatic nerve usually leaves the true pelvis (lesser pelvis) through the infrapiriform foramen as a 1.4 cm (up to 3 cm) wide and 0.4 to 0.5 cm (up to 0.9 cm) thick nerve cord (Fig. 11.3 and Fig. 11.4) and enters the gluteal region. It divides into the tibial nerve and the common fibular nerve at the latest where it enters the popliteal fossa.
The sciatic nerve provides motor innervation through its tibial division to all of the flexor muscles of the thigh (with the exception of the short head of the biceps femoris) and the lower leg and with its fibular division it innervates the short head of the biceps femoris and the fibular muscles and all the extensors in the lower leg and foot. It provides sensory innervation through both divisions to the skin of the lower leg and foot.
If the trunk of the sciatic nerve is paralyzed, external rotation of the thigh and knee flexion are greatly impaired. Complete anesthesia leads to the corresponding impairment. Because of the unopposed extensor action of the quadriceps muscle, the leg behaves like a stilt. The foot is unstable at the ankle and can no longer be dorsiflexed (foot drop).
11.1.3 Posterior Cutaneous Nerve of the Thigh (S1–S3)
The posterior cutaneous nerve of the thigh, which is the sensory supply to the posterior aspect of the thigh, leaves the pelvis together with the sciatic nerve and the inferior gluteal nerve through the infrapiriform foramen. The nerve lies medial to the sciatic nerve and reaches the posterior surface of the thigh under the gluteus maximus muscle. There it lies subfascially, but close to the fascia lata, through which it passes at varying levels. The distribution of the posterior cutaneous nerve of the thigh is variable and extends from the distal third of the buttocks as far as the distal boundary of the popliteal fossa.
Overview of the Nerves of the Sciatic Plexus
The superior gluteal nerve and the inferior gluteal nerve along with the posterior cutaneous nerve of the thigh and the sciatic nerve belong to the sciatic plexus.
Only the sciatic nerve (tibial nerve, common fibular nerve) and the posterior cutaneous nerve of the thigh are important for anesthesia and analgesia of the leg.
11.1.4 Periosteal Innervation
Relevant Facts for Anesthesia and Pain Therapy
The periosteum of the femur is supplied posteriorly by the sciatic nerve in the upper third, by the obturator nerve in the middle third, and in the distal third by the sciatic nerve laterally and by the femoral and obturator nerves medially.
Innervation of the knee is provided anteriorly by branches of the femoral nerve and sciatic nerve and posteriorly by parts of the sciatic nerve, the obturator nerve, and the saphenous nerve.
The periosteum of the tibia and fibula apart from the lateral head of the tibia and the head of the fibula (common fibular nerve) is supplied by the tibial nerve (important for lower leg amputations, fractures).
The ankle receives its sensory supply from the tibial nerve and sural nerve.
The periosteum of the tarsal bones is innervated by the sural nerve and parts of the tibial nerve, the metatarsals, and the phalanges by the deep fibular nerve and the terminal branches of the tibial nerve (Wagner 1994).
11.2 Anterior Proximal Sciatic Nerve Block (with Patient in Supine Position)
11.2.1 Technique of Anterior Sciatic Nerve Block
The classical technique of anterior sciatic nerve block was described in 1963 by G.P. Beck (Beck 1963). The technique was modified by Meier (Meier 1998) to simplify the procedure and allow placement of a catheter.
Anterior Block of the Sciatic Nerve (according to Beck, Classical Technique)
Inguinal crease, gap between the rectus femoris and sartorius muscles. (Note: We have dispensed with the extensive description of orientation with various landmarks and lines, as the method described below has been proven for years.)
The patient lies supine with the leg to be anesthetized extended.
The puncture site is approximately 10 cm (hand breadth) below the inguinal crease (Fig. 11.5).
The puncture site is the muscle gap between the rectus femoris and the sartorius. In this muscle gap, vertical pressure is exerted on the femur with two fingers, and the bone is used as an abutment (“two-finger grasp”; Fig. 11.6, Fig. 11.7, Fig. 11.8). The blood vessels are pushed medially by this maneuver and the likelihood of accidental vascular puncture is reduced (Fig. 11.7, Fig. 11.8, Fig. 11.9, Fig. 11.10). A 15-cm needle is then advanced at an angle of 75 to 85° to the skin in cranial, posterior and slightly lateral direction (Fig. 11.8 and Fig. 11.10). When a cranial direction is maintained, the sciatic nerve is reached further proximal by a few centimeters, depending on the angle of insertion; if the nerve is reached more tangentially, it is easier to advance an indwelling catheter (Fig. 11.11). For ultrasound visualization, a curved array transducer is recommended owing to the depth (Fig. 11.12, Chapter 11.2.5). The sciatic nerve can be reached well using this technique.
Branches of the femoral nerve are often stimulated after the needle is advanced a few centimeters. The position of the needle tip is corrected (usually laterally) until no further response can be detected from the quadriceps femoris muscle and it is then advanced further. Stimulation is initially with a current of 0.8 to 1.0 mA. After 6 to 10 cm, the adductor fascia is reached, which is often signaled by an obvious loss of resistance. The needle is advanced further until a motor response is produced in one of the two divisions of the sciatic nerve (fibular nerve, dorsiflexion: tibial nerve, plantar flexion; Fig. 11.13).
The correct position of the needle is indicated by a response in the foot. If there is a response in the ischiocrural muscles, the needle must be withdrawn significantly and corrected laterally (“below the femur”) (Fig. 11.14, Fig. 11.15, Fig. 11.16). Then 30 mL of a medium-acting or long-acting local anesthetic is injected.
In the continuous technique, following injection of the local anesthetic, a 20G catheter is advanced 4 cm proximally by the needle (Fig. 11.17 and Fig. 11.18). Short, mild resistance can occur when the catheter tip reaches the end of the needle (Fig. 11.17), but this is normally easily overcome, and the catheter can then be advanced smoothly.
The technique should be performed with peripheral nerve stimulation.
If sciatic nerve block is combined with a femoral nerve block, anesthetizing the femoral nerve first is useful as this leads to anesthesia of the anterior thigh and helps to improve patient comfort (Fig. 11.17).
11.2.2 Indications and Contraindications (in Combination with Femoral Nerve Block)
Operations on the knee, lower leg, or foot (e.g., knee replacement, tibial head osteotomy, arthrodesis, lateral ligament suture, forefoot operation).
Reposition of fractures in the lower leg and foot area.
Amputations in the thigh (Fig. 11.21), lower leg, and foot.
Regional sympathetic block (e.g., perfusion disorders, wound healing disorders, CRPS 1).
Pain therapy (e.g., postoperative, achillodynia, oligoarthritis).
Postoperative pain therapy after total knee replacement, particularly when there is incomplete extension in the knee joint (Fig. 11.22).
Traumatology (e.g., pain-free positioning for diagnostic investigation).
General contraindications (see Chapter 20.2)
Special contraindications: none
11.2.3 Side Effects and Complications
There are no known special complications of sciatic nerve block. There have been few reports of side effects. Major complications including late sequelae are very rare. Dysesthesia for 1 to 3 days, which resolved spontaneously, has been described.
11.2.4 Remarks on the Technique
In Beck′s classical anterior technique (Beck 1963) the sciatic nerve is reached relatively distally. For this reason, anesthesia of the posterior cutaneous nerve of the thigh is often inadequate, which may cause problems in patients requiring a tourniquet at the thigh. In addition, with the Beck technique the sciatic nerve may be difficult to locate. With the modified technique, directing the needle at an angle of 75° to the skin causes needle contact with the sciatic nerve 3 to 4 cm more proximally; therefore both the sciatic nerve and the posterior cutaneous nerve of the thigh are reached (Meier 1999a).
The diameter of the sciatic nerve, the largest nerve in the body, is impressive. In anatomical studies only part of the nerve was stained after injection of 10 mL of methylene blue through the catheter (Meier 1999a; Fig. 11.23).
Tips and Tricks
The leg should be in neutral position.
Digital support (“two-finger grasp”) markedly reduces the risk of vascular puncture and the skin–nerve distance is shortened considerably (Meier 1999b).
In contrast to the Beck classical anterior technique, the Meier approach is about 1 to 1.5 cm further medial and distal so that contact with the periosteum of the femur is avoided (Meier 1999a).
The course of the deep blood vessels can also be established with a Doppler probe (Büttner and Meier 1999).
Using the Meier approach, the tip of the needle reaches the nerve 3 to 4 cm more proximal than with the technique described by Beck. The posterior cutaneous nerve of the thigh can therefore also be anesthetized (Meier 1999b).
If no stimulation response is produced, the needle should be withdrawn and corrected laterally (Meier 2001).
The catheter can be advanced more smoothly if the local anesthetic has been injected beforehand.
Advancing the catheter more than 4 cm beyond the tip of the needle has no advantages (Fig. 11.24).
No change in patient position is required when the technique is combined with a femoral nerve block.
The onset of the block (as indicated by corresponding sympathetic block) can be checked through the rise in plantar temperature using a skin thermometer (Büttner and Meier 1999).
To achieve complete anesthesia of the sciatic nerve, at least 20 mL of a local anesthetic should be injected. When positioning the patient, it should be ensured that the leg to be anesthetized is in neutral position. Bone contact is not necessary and the stimulating needle can be advanced past the femur (lesser trochanter) much more easily when the leg is in neutral position.
The distance from the skin of the anterior thigh to the sciatic nerve is around 6 to 10 cm in adults when the needle is directed vertically. With the needle directed cranially, the nerve is reached after 8 to 12 cm, and sometimes only after 13 to 15 cm in muscular or obese patients (Brown 1996, Chelly et al 1997, Bridenbaugh and Wedel 1998, Ericksen et al 2002). Paresthesia should be avoided.
During stimulation, note that contractions of gluteus maximus or tensor fasciae latae do not represent an adequate response. The vicinity of the sciatic nerve is indicated by a response in the area supplied by the nerve (hamstring muscles, triceps surae, tibialis anterior, fibular group; Wagner and Missler 1987, Kaiser et al 1990, Wagner 1994).
The motor response should be optimized so that plantar flexion (tibial nerve) or dorsiflexion (common fibular nerve) is produced in the foot.
In a study by Neuburger et al (2001), the stimulated muscle group (whether innervated by the tibial nerve or fibular nerve) had no effect on the block result. With correct stimulation (below 0.5 mA), a success rate over 95% can be achieved (Niesel 1994, Chelly et al 1999).
Performing the block with an immobile needle technique is beneficial as it enables aspiration to exclude vascular puncture and accidental intravascular injection of local anesthetic (Winnie 1975, Büttner and Meier 1999).
For anesthesia, 30 mL of a medium-acting or long-acting local anesthetic of adequate concentration should be injected (Wagner and Taeger 1988, Bridenbaugh and Wedel 1998, Chelly and Delaunay 1999; Fig. 11.25). In combination with a lumbar plexus block (psoas or so-called “3-in-1 block”), a further 30 to 40 mL of local anesthetic is required. When complete anesthesia of the leg is required, the combination of an anterior sciatic nerve block with femoral nerve block is a good alternative, as both blocks can be performed in the same sterile field without a change in position or repeated disinfection (Fig. 11.7 and Fig. 11.10).
The femoral nerve block should be performed before the sciatic nerve block. This allows a largely pain-free anterior proximal sciatic nerve block to be performed.
A catheter for continuous anesthesia or analgesia can be placed (Meier 1999a) because the sciatic nerve is surrounded by a fascial sheath from its emergence from the infrapiriform foramen until it enters the popliteal fossa (Clara 1959, Benninghoff and Goerttler 1975). Advancing the catheter more than 4 to 5 cm can lead to deviation of the catheter into the true pelvis (lesser pelvis) and should be avoided (see Fig. 11.24).
In a study by Meier (1999b) of 85 patients, the catheter was kept in situ for an average of 4 days (up to a maximum of 8 days). No infections were observed at the puncture site. No side effects, complications, or neurological deficits were found.
Continuous anterior sciatic nerve block.
Pain therapy through a continuous anterior sciatic nerve block can be provided with 6 mL/h of ropivacaine 0.33% (3.3 mg/mL) or bolus injections of 20 mL of ropivacaine 0.2 to 0.375% (2–3.75 mg/mL) every 6 to 8 hours (Meier 2001). The maximum dose of ropivacaine should not exceed 37.5 mg/h. Büttner and Meier (1999) reviewed continuous pain therapy with ropivacaine in clinical practice, which was performed without complications in over 6,000 peripheral catheters.
The anterior technique allows block of the sciatic nerve with the patient in supine position. No change of position is required. The anesthesia can therefore also be performed, for example, in the presence of vertebral fractures, fractures of the pelvis or long bones, and also in the case of obesity, chronic polyarthritis, and other positioning problems. If a femoral nerve block is performed before the anterior sciatic nerve block and periosteal contact is avoided, the technique can be performed with little pain. The modified technique according to Meier enables anesthesia that includes the posterior cutaneous nerve of the thigh. A catheter can be placed readily without the potential risks seen with transgluteal or parasacral sciatic nerve block (Chapter 11.5 and Chapter 11.6). As a continuous technique, the procedure can be used for postoperative pain therapy after major surgery on the knee, lower leg, and foot, and for the treatment of pain syndromes distal to the knee and for regional sympathetic block.
11.2.5 Anterior Proximal Sciatic Nerve Block Using Ultrasound
Curved array: 5 to 2 MHz, linear multifrequency broadband transducer with phased-array technique Penetration depth: 10 to 20 cm
Needle: 8 to 15 cm
The sciatic nerve is visualized on the anterior proximal technique in supine position at the inside of the thigh 8 to 10 cm distal to the inguinal fold at the level of the lesser trochanter or slightly distal to it. The leg is abducted and rotated externally.
The following structures are sought:
The femur as a convex hyperechoic line with an acoustic shadow behind it
The femoral vessels (femoral artery and deep femoral artery)
Medial to and below the femoral arteries is the adductor muscle group, the largest muscle of which is the adductor magnus. The adductors originate at the femur. The sciatic nerve is in the layer between the adductor magnus and the gluteus maximus (Fig. 11.26). The sciatic nerve is hyperechoic here and has a round to oval shape (1.66 ± 0.68 cm maximum diameter) and can be visualized at a depth of 6.21 ± 0.68 cm below the surface of the skin (Chan et al 2006).