The lumbar plexus is formed from five lumbar nerve roots on each side, emerging from the L1-2 interspace through the L5-S1 interspace. The L2-4 nerve roots exit the intervertebral foramina and become embedded in the psoas major muscle, which is attached to the lateral surfaces and transverse processes of the lumbar vertebrae. Within the body of the muscle, these nerve roots split into anterior and posterior divisions, which then reunite to form individual peripheral nerves of the lumbar plexus. The most important of these are the femoral nerve, the lateral femoral cutaneous nerve, and the obturator nerve. In addition, the lumbar plexus gives rise to the genitofemoral nerve. The femoral and lateral femoral cutaneous nerves run in a fascial sleeve that divides the psoas into an anterior part, which includes two thirds of the muscle mass, and a posterior part consisting of the remaining third of the muscle mass. The obturator nerve can lie within this plane but is actually outside the fascial sleeve and contained in the substance of the psoas muscle itself in about 40%-50% of cases.
The femoral nerve supplies motor fibers to the quadriceps muscle, skin of the anteromedial thigh, and medial aspect of the leg below the knee and foot. See Femoral Nerve Block for more details. The lateral femoral cutaneous nerve is a purely cutaneous nerve, supplying the lateral aspect of the hip and thigh, down to the knee. The obturator nerve supplies motor branches to the adductors of the hip and supplies cutaneous innervation in a highly variable distribution on the medial thigh and around the knee joint. The genitofemoral nerve supplies the cremaster muscle and cutaneous innervation of the scrotal skin in males and supplies cutaneous innervation to the mons pubis and labia majora in females. A small femoral branch of the genitofemoral nerve also supplies innervation to the skin anterior to the upper part of the femoral triangle.
Other important structures include the kidney, which lies just anterior and lateral to the psoas muscle, the renal artery and vein; the inferior vena cava, which lies anterior and slightly medial on the right side; and the abdominal aorta, which is anterior and medial on the left.
Surface landmarks for psoas compartment include the iliac crest and spinous processes in the midline of the back. Ultrasound visualization of the psoas muscle in adults requires a low-frequency transducer (5-8 MHz) to reach a depth of 5-8 cm. Higher frequency transducers may be used in children.