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The lower extremity is supplied by nerves that are widely separated from each other as they enter the thigh. For this reason, lower extremity blocks are technically more difficult to perform than those of the upper extremity. Major nerves to the lower extremity include the sciatic, posterior femoral cutaneous, femoral, lateral femoral cutaneous, and obturator nerves (Figure 16-1). Lower extremity blocks provide for superior postoperative pain relief and are ideal for patients with comorbidities that make an epidural or spinal anesthetic otherwise unsafe.
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The nerve supply to the lower extremity is derived from the lumbar and sacral plexuses (Figure 16-2). The lumbar plexus is formed by the L1 to L4 anterior rami and it innervates the anterior aspect of the thigh. The plexus lies between the psoas major and quadratus lumborum muscles within the psoas compartment. The lower components of the plexus, L2 to L4, innervate the anterior and medial aspects of the thigh. The anterior divisions of L2 to L4 form the obturator nerve, and the posterior divisions of L2 to L4 form the femoral nerve. The lateral femoral cutaneous nerve is formed from posterior divisions of L2 and L3.
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The sacral plexus gives rise to the posterior femoral cutaneous and the sciatic nerves. It innervates the rest of the lower extremity aside from the anterior aspect of the thigh and medial aspect of the leg. The posterior cutaneous nerve is derived from S1 to S3, and the sciatic nerve is derived from the anterior rami of L4 to L5. Both nerves pass through the pelvis and greater sciatic foramen, and are blocked via the same technique. At or above the popliteal fossa, the sciatic nerve separates into the tibial nerve, which passes medially, and the common peroneal nerve, which passes laterally down the leg.
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The lumbar plexus block provides anesthesia to the hip and anterolateral and medial aspects of the thigh, and the saphenous nerve below the knee. The block is useful for hip procedures such as arthroplasty or open reduction internal fixation (ORIF). In conjunction with a proximal sciatic block, this block can provide total anesthesia to the lower limb.
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The patient is placed in the sitting position, or in the lateral position with the ...