Sciatic Nerve Block

Anatomy and Physiology

The sacral plexus originates from the L4-S3 nerve roots, and is located just lateral to the sacrum. It gives rise to five distal nerves: the sciatic nerve, posterior femoral nerve, superior gluteal nerve, inferior gluteal nerve, and pudendal nerve. The largest of these is the sciatic nerve, which courses beneath the gluteal muscles and then down the posterior leg.

 Just above the popliteal crease, the sciatic nerve divides into the tibial and common peroneal nerves. These two nerves innervate almost the entire lower leg.  While not completely accurate, it is useful to think of the tibial nerve as supplying the posterior lower leg and the plantar surface of the foot, while the common peroneal supplies the anterior lower leg and dorsal surface of the foot. The common peroneal nerve terminates as the deep and superficial peroneal nerves of the foot.  Likewise,  the tibial nerve becomes the sural and posterior tibial nerves.

Regional anesthetic blockade of the sciatic nerve is possible at several anatomic locations. From proximal to distal, these sites are sacral plexus, classic transgluteal approach, subgluteal approach, anterior approach, and the popliteal approach. As with most other regional anesthetic techniques, ultrasound visualization has become the standard of practice.[4][5][6][7]

The sacral plexus approach is achieved by drawing a line between the posterior superior iliac spine (PSIS) and the ischial tuberosity on the side to be anesthetized, with the patient in the prone or lateral position. The sacral plexus is normally located approximately 8 cm distal to the PSIS along this line, although individual variation exists from 6 cm to 12 cm. While sacral plexus block is uncommonly used in regular clinical practice, ultrasound approaches have been well described. Of note is that the sacral plexus is the only peripheral sacral nerve location which is proximal enough to provide anesthesia for surgery of the hip when combined with lumbar plexus blockade. Sensory articular branches to the hip arise from the superior gluteal nerve, originating from the sacral plexus.

The transgluteal approach (aka Labat’s technique)  was the originally described and long preferred approach to sciatic nerve block because of its relatively consistent anatomic location. With the patient in Sim’s position,  the midpoint of a line from the greater trochanter of the hip to the posterior superior iliac spine is determined. A perpendicular to this first line is drawn from this midpoint. A second line is marked from the greater trochanter to the sacral hiatus. The spot where the perpendicular to the first line intersects the second line becomes the location of needle placement. While relatively consistent, the introduction of ultrasound into regional anesthetic practice has led to a shift away from the classic sciatic nerve block in favor of subgluteal, anterior, and popliteal approaches.

The subgluteal approach to sciatic nerve blockade has become the dominant approach to sciatic nerve block for procedures requiring complete knee or distal upper leg anesthesia (e.g., tourniquet placement).  Visualizing along a line between the greater trochanter and ischial tuberosity utilizing ultrasound, the sciatic nerve is located in the subgluteal fascial plane above the quadratus femoris muscle. While this approach is usually performed in the lateral position, a supine variant with the hip flexed is also described.

The anterior sciatic approach occurs at the level of the lesser trochanter and has the advantage of being performed in the supine position. This is particularly useful when the patient cannot be positioned laterally. The thigh is externally rotated, and the knee flexed. Ultrasound visualization is used to locate the lesser trochanter of the femur. The sciatic nerve is then located both medial and deep to this bony landmark. Anesthetic results are similar to the subgluteal approach.

The popliteal approach is the most distal location for sciatic nerve blockade. The sciatic nerve is anesthetized just proximal to its division into the tibial nerve and the more lateral common peroneal nerve,  approximately 6 cm above the popliteal crease. Originally described as a landmark-based blind technique, it has become a mainstay of knee and lower leg anesthesia and analgesia since the introduction of ultrasound visualization.  The popliteal artery is located in the crease of the knee, as is the tibial nerve lying immediately posterior to it. The tibial nerve is continuously visualized as the probe is moved slowly in a proximal direction until the common peroneal nerve joins with it to form the sciatic nerve. A blockade utilizing the popliteal approach can be performed in the supine or lateral position.  It is important to note that this approach will not relieve the pain from a tourniquet placed on the thigh due to the distal block location.

 It is worth noting that selective tibial or common peroneal nerve blocks may be performed in the popliteal region. A selective tibial nerve block is one well-utilized technique for relief of post total knee arthroplasty pain in the posterior knee or calf region.  It also preserves the ability to dorsiflex the foot since the common peroneal nerve is spared. Dorsiflexion is crucial for postoperative ambulation, as it prevents falls from a dragging foot getting caught on the floor as it moves forward.

Any of the above approaches to the sciatic nerve will provide satisfactory anesthesia of the distal lower leg and foot (with the exception of the small strip of saphenous nerve sensory innervation previously mentioned).  Several factors determine the choice of approach. The site of the surgical procedure, presence and location of a tourniquet, accessibility of block location, and acceptability of postoperative motor weakness are among the most important to consider.[8][9][10]

Indications

Sciatic nerve blockade is indicated whenever analgesia or anesthesia of the lower extremity is appropriate. This can range from surgical or nonsurgical procedures to acute or chronic pain relief therapies. Depending on the exact location of the lower extremity coverage required, a sciatic nerve block may need to be combined with blockade of nerves originating from the lumbar plexus to achieve the desired result.

Contraindications

Sciatic nerve blockade is absolutely contraindicated in two situations: active area of infection where needle puncture will occur and the refusal of a competent patient to undergo the procedure. Other contraindications are relative and include alteration of normal coagulation mechanisms, systemic sepsis or isolated infection, and documented neurological disease or deficit. Before performing a regional anesthetic on these patients, careful consideration must be given to the expected benefit of the block, the potential of a complication occurring, and the expected degree of injury from such a complication.

Complications

Potential complications related to sciatic nerve blocks include infection at the injection site, bleeding, nerve injury, and local anesthetic toxicity. Fortunately, all of these are rare, but simple precautions can minimize these occurrences if followed regularly. All nerve blocks should be performed under sterile technique to minimize the chance of infection at the injection site. Direct ultrasound visualization of the needle, nerve, and any nearby vascular structures minimizes the chance of nerve injury, local anesthetic toxicity, and accidental puncture of a blood vessel. Frequent negative aspiration during injection of local anesthetic, even when ultrasound visualization is utilized, is recommended.

Clinical Significance

The sciatic nerve originates from the sacral plexus (L4-S3) and provides most of the motor and sensory innervation to the leg. The long course of the sciatic nerve, from the sciatic notch in the gluteal region to the popliteal fossa, allows for multiple possible sites for an anesthetic blockade. Positioning is an important consideration in sciatic nerve block, with sacral plexus and transgluteal approaches not available in the supine position. The only approach that routinely uses the supine position is the anterior sciatic approach, although both the popliteal and subgluteal approaches can be performed in the supine position also. Consideration must be given to factors such as tourniquet placement, site of surgery, availability of access, and postoperative motor impairment when choosing a sciatic nerve block approach.

Enhancing Healthcare Team Outcomes

Anesthesiologists and pain specialists often perform sciatic nerve blocks. However, the nurse must be dedicated to the monitoring of the patient during the procedure. Also, resuscitation equipment must be in the room before the procedure.

The sciatic nerve block can be associated with several complications, and it may be advisable to use ultrasound for guidance.