To understand the potential use of ultrasound imaging for lumbar plexus block, detailed anatomy and background must be reviewed.

The lumbar plexus consists of contributions from the anterior rami of the L1 through L4 roots (and sometimes T12 or L5). The lumbar plexus forms the subcostal, iliohypogastric, ilioinguinal, lateral femoral cutaneous, genitofemoral, femoral, and obturator nerves. Anatomic dissections have revealed that the lumbar plexus lies within the psoas muscle, with the lateral femoral cutaneous nerve and femoral nerve in the same fascial plane. However, the obturator nerve can be found within a distinct muscular fold in about half of anatomic specimens. The lumbosacral trunk is formed from the anterior rami of L4 and L5. It contributes to the sacral plexus and enters the pelvis apart from the lumbar plexus.

Patient Position: Lateral decubitus with the operative side up or in the sitting position.

Indications: Surgery on the hip, thigh, or knee.

Needle Size: 9-cm, 22-gauge needle.

Surface Landmarks: The posterior spinous process of the fourth lumbar vertebra.

Ultrasound Landmarks: The transverse processes of the third and fourth lumbar vertebrae and the psoas muscle.

Transducer Type: Linear or curved array, 3 to 7 MHz