Muscles and fascia
Four muscles form the posterior abdominal wall on each side of the lumbar spine: psoas major, iliacus, quadratus lumborum and the posterior part of the diaphragm.
| Muscle | Origin | Insertion | Nerve | Main action |
|---|---|---|---|---|
| Psoas major | Bodies and discs of T12 to L5, transverse processes of lumbar vertebrae | Lesser trochanter of femur (with iliacus) | Anterior rami L1 to L3 | Flexes hip; flexes trunk laterally |
| Iliacus | Iliac fossa | Lesser trochanter, via the psoas tendon | Femoral nerve (L2, L3) | Flexes hip |
| Quadratus lumborum | Iliolumbar ligament and posterior iliac crest | 12th rib and transverse processes of L1 to L4 | Anterior rami T12 to L4 | Fixes the 12th rib in inspiration; lateral flexion |
Psoas minor lies on the front of psoas major and is absent in a large minority of people. The diaphragm contributes its crura: the right crus arises from L1 to L3 and the left from L1 to L2. The median arcuate ligament joins the crura in front of the aorta at T12; the medial arcuate ligament arches over psoas, and the lateral arcuate ligament over quadratus lumborum.
Each muscle is covered by its own fascia. Psoas fascia is a closed sheath running to the thigh, so infection inside it tracks down below the inguinal ligament.
Vessels and lymphatics
The abdominal aorta and inferior vena cava run vertically in front of the lumbar spine, with the aorta slightly left of the midline and the IVC to the right.
Abdominal aorta
The aorta passes through the aortic hiatus behind the median arcuate ligament at T12 and divides into the common iliac arteries at L4, at about the level of the iliac crests.
- Anterior unpaired branches: coeliac trunk (T12), superior mesenteric artery (L1), inferior mesenteric artery (L3)
- Lateral paired branches: inferior phrenic, middle suprarenal, renal (L1 to L2), gonadal (L2)
- Posterior branches: four pairs of lumbar arteries and the unpaired median sacral artery
Inferior vena cava
The IVC forms at L5, behind the right common iliac artery, from the two common iliac veins. It ascends on the right psoas and right crus and leaves through the caval opening in the central tendon at T8. It receives the lumbar, right gonadal, renal, right suprarenal, inferior phrenic and hepatic veins. The left gonadal and left suprarenal veins drain into the left renal vein, which crosses in front of the aorta just below the origin of the superior mesenteric artery.
Lymphatics
Para-aortic (lumbar) nodes lie along the aorta and drain the posterior wall, kidneys, adrenals and gonads. Their efferents form the lumbar trunks, which join the intestinal trunk at the cisterna chyli, a thin sac behind the right crus at about L1 to L2, the start of the thoracic duct.
Nerves
The lumbar plexus forms inside psoas major from the anterior rami of L1 to L4, and its branches are identified by where they leave the muscle.
| Where it emerges from psoas | Nerve | Roots |
|---|---|---|
| Lateral border | Iliohypogastric, ilioinguinal | L1 |
| Lateral border | Lateral cutaneous nerve of thigh | L2, L3 |
| Lateral border, low | Femoral nerve | L2 to L4 |
| Anterior surface | Genitofemoral nerve | L1, L2 |
| Medial border | Obturator nerve | L2 to L4 |
| Medial border | Lumbosacral trunk | L4, L5 |
The subcostal nerve (T12) runs along the lower border of the 12th rib, in front of quadratus lumborum. The lumbar sympathetic trunks lie on the vertebral bodies along the medial border of psoas; the right trunk is hidden behind the IVC and the left lies beside the aorta. Around the aortic branches lie the coeliac, superior mesenteric and inferior mesenteric plexuses, continuing down as the superior hypogastric plexus below the bifurcation.
Relations
The kidneys, adrenal glands and ureters lie directly on the posterior abdominal wall, within their fascia, separated from the peritoneal cavity by the parietal peritoneum.
Each kidney rests on the diaphragm above, and on psoas, quadratus lumborum and transversus abdominis below. The subcostal, iliohypogastric and ilioinguinal nerves pass behind the lower pole. The ureter descends on psoas major, crossed anteriorly by the gonadal vessels, and enters the pelvis by crossing the bifurcation of the common iliac artery. The duodenum, pancreas and ascending and descending colon lie on the wall as secondary retroperitoneal organs.
Clinical relevance
Disease on the posterior abdominal wall often presents through its effect on psoas, the great vessels or the nerves.
- Psoas abscess: classically from tuberculosis of the lumbar spine, it tracks within the psoas sheath and can point as a swelling in the groin below the inguinal ligament.
- Psoas sign: pain on hip extension when an inflamed retrocaecal appendix lies on the right psoas.
- Abdominal aortic aneurysm: most are infrarenal; a rupture usually bleeds into the retroperitoneum first, and back pain is common.
- Nutcracker phenomenon: compression of the left renal vein between the aorta and superior mesenteric artery.
- Lumbar sympathectomy: the chain lies close to the lumbar veins, the IVC on the right and the aorta on the left.