LOWER EXTREMITY CONDUCTION BLOCKS

Anatomic Considerations

The lower extremity has a more complex nerve supply than the upper extremity because it depends on two

Figure 45-10 Metacarpal (or transthecal) nerve block, showing the head of the metacarpal located by palpation (1).

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Lumbar Plexus and Lumbar Plexus Nerves

The lumbar plexus supplies the anterior aspect of the limb. It lies within the substance of the psoas muscle and in a fascial plane called the psoas compartment, where it can be approached by a posterior route through the quadratus lumborum muscle. The lumbar plexus contributes four nerves to the lower extremity: femoral, lateral cutaneous, obturator, and genitofemoral. The psoas and iliacus muscles share the same aponeuroses, the fascia iliaca. At their emergence from the psoas muscle, all lumbar plexus nerves run a variable part of their course just below this fascia. A local anesthetic injected at the inner surface of this fascia and spreading along it contacts virtually all lumbar plexus nerves if the injected volume is sufficient. This is the concept sustaining the technique of the fascia iliaca compartment block.

The femoral nerve is the largest branch of the lumbar plexus. It emerges from the psoas major muscle in the groove separating the psoas and the iliacus muscles, passes behind the inguinal ligament, and enters the groin in the femoral triangle (i.e., Scarpa's triangle). It runs lateral to the femoral vessels from which it is separated by the lower part of the psoas major muscle. At this level, it splits into anterior and posterior terminal divisions. Contrary to the brachial plexus, which is enclosed in the axillary sheath, the femoral nerve remains outside the sheath surrounding the femoral vessels. Its main division branch, the saphenous nerve, which is a purely sensory nerve, continues the general direction of the femoral nerve, lateral to the femoral artery in the adductor canal (i.e., Hunter's canal) together with the motor branch supplying the vastus medialis muscle. At the distal extremity of the adductor magnus muscle, it crosses the femoral vessels anteriorly, passes behind the sartorius muscle, descends along the medial border of the tibia, and ends its course on the medial side of the ankle, where it gives off two terminal branches. In the adductor canal, it gives off twigs to the medial cutaneous nerve of the thigh and the obturator nerve to form a subsartorial plexus. It also gives off patellar branches that contribute to the patellar plexus and the medial cutaneous branches. It supplies the medial part of the leg and the foot.

Sacral Plexus and Sciatic Nerve

The sacral plexus supplies the posterior aspect of the limb. It lies on the anterior aspect of the piriformis muscle, behind the posterior wall of the pelvic cavity, and cannot be directly approached by a block needle. It contributes two nerves to the lower extremity: posterior femoral cutaneous and sciatic. The sciatic nerve is the largest mixed nerve of the body, and it is formed by two distinct nerves, the common peroneal nerve and the tibial nerve, which are enclosed in the same perineural sheath. The sciatic nerve leaves the pelvis through the greater sciatic foramen and then passes between the greater trochanter of the femur and the ischial tuberosity. Below the piriformis muscle, it runs in the subgluteal space within a true aponeurotic canal, in which the pressure is negative and can be detected by an LOR technique. It is separated from the hip joint by the obturator externus muscle. In the thigh, the sciatic nerve runs on the adductor magnus, along the lower border of the femur and toward the popliteal fossa. At the upper part of the popliteal fossa, the tibial and common peroneal nerves separate. The common peroneal nerve supplies the lateral aspect of the leg and the dorsum of the foot. The tibial nerve supplies the dorsal aspect of the leg and the plantar surface of the foot.