Sciatic Nerve Block at the Knee

Indications and Clinical Considerations

Sciatic nerve block at the knee is useful in the evaluation and management of distal lower extremity pain thought to be subserved by the sciatic nerve. The technique also is useful for providing surgical anesthesia for the distal lower extremity when combined with lateral femoral cutaneous, femoral, and obturator nerve block, or lumbar plexus block. It is used for this indication primarily for patients who would not tolerate the sympathetic changes induced by spinal or epidural anesthesia and who need distal extremity amputations or debridement.

Sciatic nerve block with local anesthetic can be used diagnostically during differential neural blockade on an anatomic basis in the evaluation of distal lower extremity pain. If destruction of the sciatic nerve is being considered, this technique is useful as a prognostic indicator of the degree of motor and sensory impairment that the patient may experience. Sciatic nerve block with local anesthetic may be used to palliate acute pain emergencies, including distal lower extremity fractures and postoperative pain relief, while waiting for pharmacologic methods to become effective. Sciatic nerve block with local anesthetic and steroid is occasionally used in the treatment of persistent distal lower extremity pain when the pain is thought to be secondary to inflammation or when entrapment of the sciatic nerve by the piriformis muscle is suspected. Destruction of the sciatic nerve is occasionally indicated for the palliation of persistent distal lower extremity pain secondary to invasive tumor that is mediated by the sciatic nerve and has not responded to more conservative measures.

Clinically Relevant Anatomy

The sciatic nerve innervates the distal lower extremity and foot, with the exception of the medial aspects of the calf and foot, which are subserved by the saphenous nerve. The largest nerve in the body, the sciatic nerve is derived from the L4, L5, and S1-S3 nerve roots. The roots fuse together in front of the anterior surface of the lateral sacrum on the anterior surface of the piriformis muscle ( Fig. 170.1 ). The nerve travels inferiorly and leaves the pelvis just below the piriformis muscle via the sciatic notch. The sciatic nerve lies anterior to the gluteus maximus muscle and, at this muscle’s lower border, lies halfway between the greater trochanter and the ischial tuberosity. The sciatic nerve courses downward past the lesser trochanter to lie posterior and medial to the femur. In the midthigh, the nerve gives off branches to the hamstring muscles and the adductor magnus muscle. In most patients, the nerve divides to form the tibial and common peroneal nerves in the upper portion of the popliteal fossa, although these nerves sometimes remain separate through their entire course ( Fig. 170.2 ). The tibial nerve continues downward to provide innervation to the distal lower extremity, whereas the common peroneal nerve travels laterally to innervate a portion of the knee joint and, via its lateral cutaneous branch, to provide sensory innervation to the back and lateral side of the upper calf ( Fig. 170.3 ).