Introduction and indications: The lumbar plexus consists of roots L1 through L4. In some patients, roots T12 and L5 contribute. The roots join together to form the subcostal, iliohypogastric, ilioinguinal, lateral femoral cuTaneous, genital-femoral and obturator nerves. The lumbar plexus usually lies within the posterior subsTance of the psoas muscle. The lateral femoral cuTaneous and femoral nerves usually lie within the same fascial plane, but the obturator nerve often lies within a separate fold of the muscle. Lumbar plexus block is used to provide intraoperative and postoperative analgesia for lower extremity surgery, sometimes in combination with sciatic nerve block. It may also be used as a series of injections for patients with chronic lower extremity pain.

Anatomy: The spinous process, the articular process, and the transverse process are imporTant landmarks. These bony structures are hyperechoic and create a shadow. When the transducer is moved slightly cephalad or caudad, a clear image of the psoas and quadratus lumborum muscles is obtained. The lumbar plexus generally is expected to lie within the posterior third of the psoas major muscle and 2 to 3 cm posterior to the anterior surface of the transverse process. The lumbar plexus is not always visualized under ultrasound but may be observed as a hyperechoic structure in a young population (Fig. 22.1).

Patient position: Lateral decubitus position with the side to be blocked uppermost.

Transducer: 40 to 60 mm curved array oscillating at 2 to 5 MHz.

Transducer position: The transducer is positioned transversely in the midline of the back at the L4 level to capture the spinous process. The transducer is then moved laterally (approximately 3 cm) while scanning the paravertebral region until a clear image of the articular and transverse processes is obtained.

Needle: A 100 to 150 mm, short-bevel, 21G insulated needle.

Local anesthetic: 20 to 35 mL of ropivacaine 0.5% to 0.75%. For pain therapy, 10 mL of ropivacaine 0.25%.

Approach and technique: The skin is washed with sterile solution, and the transducer is covered with a sterile sheath. Ultrasound-guided lumbar plexus block requires advanced skill because of the depth of needle placement. After the optimal transducer position is found, the transducer is angled laterally or medially in the transverse plane, depending on the needle insertion site, while the target is kept in the middle of the ultrasound image. This allows the practitioner to view the needle more easily.