Formation
The lumbar plexus forms inside the substance of psoas major, in front of the transverse processes of the lumbar vertebrae, from the ventral rami of L1, L2, L3 and part of L4.
- L1, often joined by a branch from T12, splits into an upper part that becomes the iliohypogastric and ilioinguinal nerves and a lower part that joins L2 to form the genitofemoral nerve.
- L2, L3 and L4 each split into anterior and posterior divisions. The anterior divisions unite as the obturator nerve. The posterior divisions form the femoral nerve (L2–L4) and the lateral cutaneous nerve of thigh (L2, L3).
- L4 also sends a branch down to join L5 as the lumbosacral trunk, which crosses the ala of the sacrum to join the sacral plexus.
The ventral rami receive grey rami communicantes from the sympathetic trunk; L1 and L2 also send white rami to it. Muscular twigs go directly from the rami to psoas major, psoas minor and quadratus lumborum.
Branches
The lumbar plexus has six named branches, best learned by where each one leaves psoas major.
| Branch | Roots | Leaves psoas | Supply |
|---|---|---|---|
| Iliohypogastric nerve | L1 (T12) | Lateral border | Internal oblique and transversus abdominis; skin above the pubis and over the upper lateral buttock |
| Ilioinguinal nerve | L1 | Lateral border | Lowest fibres of internal oblique and transversus; skin of the upper medial thigh, root of penis and anterior scrotum or mons pubis and labium majus |
| Genitofemoral nerve | L1, L2 | Anterior surface | Genital branch: cremaster and anterior scrotal or labial skin. Femoral branch: skin over the femoral triangle |
| Lateral cutaneous nerve of thigh | L2, L3 | Lateral border | Skin of the anterolateral thigh |
| Femoral nerve | L2–L4, posterior divisions | Lateral border | Iliacus, pectineus, sartorius, quadriceps; anterior thigh and medial leg |
| Obturator nerve | L2–L4, anterior divisions | Medial border | Hip adductors, gracilis, obturator externus; medial thigh |
Two inconstant or connecting branches complete the picture: the accessory obturator nerve (L3, L4), present in a minority of people, which leaves the medial border and supplies pectineus; and the lumbosacral trunk (L4, L5), which also appears at the medial border.
Course of the branches and relations
After leaving psoas the branches fan out across the posterior abdominal wall and pelvis, each with a predictable path.
- Iliohypogastric and ilioinguinal nerves run laterally in front of quadratus lumborum, behind the kidney, pierce transversus abdominis near the iliac crest and travel forward between transversus and internal oblique. The ilioinguinal nerve then enters the inguinal canal, lying on the spermatic cord or round ligament, and leaves through the superficial inguinal ring. It does not pass through the deep ring.
- Genitofemoral nerve pierces the front of psoas and descends on its surface behind the ureter. Its genital branch enters the deep inguinal ring; its femoral branch passes under the inguinal ligament inside the femoral sheath, lateral to the femoral artery.
- Lateral cutaneous nerve of thigh crosses iliacus towards the anterior superior iliac spine and enters the thigh under or through the inguinal ligament just medial to the spine.
- Femoral nerve runs in the groove between psoas and iliacus and passes under the inguinal ligament lateral to the femoral artery.
- Obturator nerve descends along the lateral pelvic wall to the obturator canal.
Anteriorly the plexus is covered by psoas, with the kidney, ureter, gonadal vessels and colon in front of that. Posteriorly lie the transverse processes and quadratus lumborum.
Clinical relevance
Lumbar plexus lesions produce groin pain and weakness of hip flexion, knee extension and hip adduction together, a combination no single peripheral nerve lesion can explain.
- Meralgia paraesthetica: compression of the lateral cutaneous nerve of thigh near the anterior superior iliac spine, from obesity, pregnancy, tight belts or iliac crest bone graft harvest, gives burning numbness of the lateral thigh with no weakness.
- Hernia repair and lower abdominal incisions: the ilioinguinal, iliohypogastric and genital branch of the genitofemoral nerve can be trapped by mesh or sutures, a recognised cause of chronic groin pain.
- Retroperitoneal haematoma and psoas abscess: both expand within the psoas sheath and compress the plexus, typically the femoral nerve first.
- Lateral transpsoas spinal surgery: lateral lumbar interbody fusion passes through psoas and can injure the plexus, most often causing thigh numbness or weakness of hip flexion.
- Cremasteric reflex: stroking the upper medial thigh contracts cremaster and raises the testis. It tests L1 and L2 through the genitofemoral and ilioinguinal nerves.
- Regional blocks: the lumbar plexus (psoas compartment) block and the fascia iliaca block anaesthetise the femoral and lateral cutaneous nerves for hip surgery.