12 Sciatic Block
The sciatic nerve is one of the largest nerve trunks in the body, yet few surgical procedures can be performed with sciatic block alone. It is most often combined with femoral, lateral femoral cutaneous, or obturator nerve blocks. The block is also effective for analgesia of the lower leg and may provide pain relief from ankle fractures or tibial fractures before operative intervention.
This block may be indicated for patients needing analgesia before transport for definitive orthopedic surgical repair of lower leg or ankle fractures. For patients in whom it may be desirable to avoid the sympathectomy accompanying neuraxial block, sciatic block combined with femoral nerve block often allows ankle and foot procedures to be carried out. One group of patients in whom this block is often useful are those undergoing distal amputations of the lower extremity who have vascular compromise based on diabetes or peripheral vascular disease.
Sciatic nerve block requires from 20 to 25 mL of local anesthetic solution. When this volume is added to that required for other lower extremity peripheral blocks, the total may reach the upper end of an acceptable local anesthetic dose range. Conversely, uptake of local anesthetic from these lower extremity sites is not as rapid as with epidural or intercostal block; thus, a larger mass of local anesthetic may be appropriate in this region. If motor blockade is desired with this block, 1.5% mepivacaine or lidocaine may be necessary, whereas 0.5% bupivacaine or 0.5% to 0.75% ropivacaine will be effective.
The sciatic nerve is formed from L4 through S3 roots. These roots of the sacral plexus form on the anterior surface of the lateral sacrum and are assembled into the sciatic nerve on the anterior surface of the piriformis muscle. The sciatic nerve results from the fusion of two major nerve trunks. The “medial” sciatic nerve is functionally the tibial nerve, which forms from the ventral branches of the ventral rami of L4-L5 and S1-S3; the posterior branches of the ventral rami of these same nerves form the “lateral” sciatic nerve, which is functionally the peroneal nerve. As the sciatic nerve exits the pelvis, it is anterior to the piriformis muscle and is joined by another nerve, the posterior cutaneous nerve of the thigh. At the inferior border of the piriformis, the sciatic and posterior cutaneous nerves of the thigh lie posterior to the obturator internus, the gemelli, and the quadratus femoris. At this point, these nerves are anterior to the gluteus maximus. Here, the nerve is approximately equidistant from the ischial tuberosity and the greater trochanter (Figs. 12-1 to 12-3). The nerve continues downward through the thigh to lie along the posteromedial aspect of the femur. At the cephalad portion of the popliteal fossa, the sciatic nerve usually divides to form the tibial and common peroneal nerves. Occasionally, this division occurs much higher, and sometimes the tibial and peroneal nerves are separate through their entire course. In the popliteal fossa, the tibial nerve continues downward into the lower leg, whereas the common peroneal nerve travels laterally along the medial aspect of the short head of the biceps femoris muscle.