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  • Indications: surgery on the hip, anterior thigh, and knee
  • Landmarks: iliac crest, spinous processes (midline)
  • Nerve stimulation: quadriceps muscle at 0.5–1.0 mA
  • Local anesthetic: 25–35 mL

Figure 18-1.

To accomplish the lumbar plexus block, a needle is inserted perpendicular to the skin plane, 4 cm lateral to the midline at the level of L3-L5.

The lumbar plexus block (psoas compartment block) is an advanced nerve block technique. Because the placement of the needle is in the deep muscles, the potential for systemic toxicity is greater than it is with more superficial techniques. The proximity of the lumbar nerve roots to the epidural space also carries a risk of epidural spread of the local anesthetic. For these reasons, care should be taken when selecting the type, volume, and concentration of local anesthetic, particularly in elderly, frail, or obese patients. The lumbar plexus block provides anesthesia or analgesia to the entire distribution of the lumbar plexus, including the anterolateral and medial thigh, the knee, and the saphenous nerve below the knee. When combined with a sciatic nerve block, anesthesia of the entire leg can be achieved. Because of the complexity of the technique, potential for complications, and existence of simpler alternatives (e.g. fascia iliaca or femoral blocks), the benefits of lumbar plexus block should always be weighed against the risks.

The lumbar plexus is composed of five to six peripheral nerves that have their origins in the spinal roots of L1 to L4, with a contribution from T12 (Figures 18-2 and 18-3). After the roots emerge from the intervertebral foramina, they divide into anterior and posterior branches. The small posterior branches supply innervation to the skin of the lower back and paravertebral muscles. The anterior branches form the lumbar plexus within the substance of the psoas muscle and emerge from the muscle as individual nerves in the pelvis.

Figure 18-2.

Anatomy of the lumbar plexus. Roots of the lumbar plexus (arrows) are seen within the substance of the psoas major muscle (PsMM); the lumbar plexus is exposed through the abdominal cavity.

Figure 18-3.

Organization of the Lumbar Plexus into peripheral nerves.

The major branches of the lumbar plexus are the iliohypogastric (L1), ilioinguinal (L1), genitofemoral nerve (L1/L2), lateral femoral cutaneous nerve (L2/L3), and the femoral and obturator nerves (L2,3,4). Although not a lumbar nerve root, the T12 spinal nerve contributes to the iliohypogastric nerve in about 50% of cases.

The femoral nerve supplies the quadriceps muscle (knee extension), the skin of the anteromedial thigh, and the medial aspect of the leg below the knee and foot (Figure 18-4A and B). The obturator nerve sends motor branches to the adductors of the hip ...

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