Posterior Abdominal Wall

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The posterior abdominal wall is a musculoskeletal structure closely related to a number of vital retroperitoneal organs and neurovascular bundles, the relationship of which is of valuable clinical significance. Broadly speaking, the wall is formed by the lumbar vertebral spine (T12-L5) in the midline, surrounded to either side by muscle and fascia; this confers significant structural support and also creates the paravertebral gutters, home to the kidneys and their perinephric fat.

The scope of this section is to look at the posterior abdominal wall muscles, the abdominal aorta and the IVC in more depth, and to appreciate the general structure of the lumbar plexus and the network of lymphatic vessels.


Components of Posterior Abdominal Wall

1)     T12-L5 vertebrae

2)     Muscles:

psoas (major, minor)


Quadratus lumborum

(transversus abdominis and obliques, laterally)


3)     Diaphragm (superiorly)

4)     Fascia

5)     Nervous structures:

Lumbar plexus

6)     Vascular:

Abdominal aorta and IVC

Gross Anatomy


The three main paired muscles of the posterior abdominal wall are:

-        Psoas major (psoas minor absent in about 33% of population)

-        Iliacus

-        Quadratus lumborum






Psoas Major

Transverse process lumbar vertebrae,

Sides of T12-L5 bodies

Lesser trochanter of femur

Lumbar nerves L1, L2, L3 (anterior rami)

Flexes vertebral column

Acts with iliacus to flex trunk and thigh


Iliac fossa,

ala of sacrum,

anterior sacroiliac ligaments

Lesser trochanter of femur

Psoas major tendon

Femoral Nerve


Acts with psoas major

Flexes thigh and stabilises hip

Quadratus Lumborum

12th ribs

Lumbar transverse processes

Iliolumbar ligament

Iliac crest

T12, L1-L4

(anterior branches)

Extends and laterally flexes vertebral column




The fascial layers can be simplistically broken down into

1)     a continuation of the TA fascia

2)     psoas fascia

3)     thoracolumbar fascia (anterior, middle and posterior layers)

The transversalis fascia extends posteriorly to provide a layer of fascia between the posterior abdominal wall muscles and the parietal peritoneum.

The psoas fascia covers the homonymous muscle. It is attached medially to the lumbar vertebrae. Superiorly, a thicker layer forms the medial arcuate ligament. Laterally, it merges with the thoracolumbar fascia, and posteriorly it joins the iliac fascia.

The anterior layer of the thoracolumbar fascia encloses the quadratus lumborum muscles. It is attached to the transverse processes of the lumbar vertebrae, 12th rib, and iliac crest and continues laterally to join the aponeurosis of the TA muscle. Superiorly, it forms the lateral arcuate ligament.

The middle and posterior layers enclose the erector spinae. The posterior layer spans from the 12th rib to the iliac crest posteriorly; laterally it joins the IO and TA muscles and covers the lat dorsi.


The network of nerves of the posterior abdominal wall is quite complex, with both autonomic and somatic components to it.

Most of the nerves of the posterior abdominal wall arise from the Lumbar Spinal Nerves L1-L5, (except for the subcostal nerves, formed from the anterior rami T12), which give off posterior and anterior rami. The latter form the somatic lumbar plexus, whose branches are described below:

Nerve and values



Femoral Nerve (L2-L4)

Lateral border psoas major

Deep to inguinal ligament

Anterior thigh


Hip flexors/knee extensors

Obturator Nerve (L2-L4)

Medial border psoas major

Inferior to superior pubic ramus

Though obturator foramen

Medial thigh

Adductor muscles of thigh

Lumbosacral Trunk (L4, L5)

Passes over ala of sacrum

Formation of sacral plexus

With anterior rami S1-S4

Ilioinguinal and Iliohpogastric nerves (ant ramus L1, with contributions from T12)

Posterior to medial arcuate ligament

Pass anterior to Quadratus lumborum

Superior and parallel to ilac crest (ilioinguinal medial to iliohypogastric)

Pierce Transversus abdominis

Pass through internal and external oblique

Cutaneous innervation of inguinal and pubic regions

Motor supply to inguinal sections or IO and TA


Genitofemoral Nerve (L1, L2)

Pierces psoas major

Runs posteriorly deep to psoas fascia

Divides in

-        Femoral

-        Genital branches

Lateral to common/external iliac

Genital branch- innervation to tunica vaginalis, external and internal spermatic fasciae


Femoral branch- cutaneous innervation above inguinal ligament


Lateral cutaneous nerve of thigh (L2, L3)

Inferolateral to iliacus

Enters thigh deep to inguinal ligament

Medial to ASIS

Cutaneous innervation anterolateral thigh

Accessory Obturator Nerve (L3, L4). Present in 10%

Parallel to medial border of psoas

Anterior to obturator nerve

Superior to superior pubic ramus





The abdominal aorta contributes most of the arteries supplying the posterior abdominal wall, with the exception of the subcostal arteries which arise from the thoracic aorta.

The abdominal aorta is about 13cm in length and 2.5cm in width. It begins at the aortic hiatus in the diaphragm (T12 level) and ends with the bifurcation of R and L common iliacs at L4.

The common iliacs run along the medial border of the psoas and split into INTERNAL and EXTERNAL common iliacs at the pelvic brim. The internal iliac continues into the pelvis.

The external iliac follows iliopsoas and gives off inferior epigastric and deep circumflex iliac arteries (anterolateral abdominal wall supply)


The branches of the Abdominal aorta can be classified into what vascular plane they are found and as unpaired/paired and visceral/parietal, as summarised below:

Vascular plane





Anterior midline

Unpaired visceral

GI tract





Paired visceral

Urogenital and endocrine







Paired parietal



Inferior phrenic





Median Sacral Artery- unpaired parietal branch




The IVC, the largest vein in the body and a largely valveless, begins at L5 where the common iliac veins join (inferior to aortic bifurcation and posterior to R Common iliac. The IVC continues superiorly along the right side of L3-L5 and ultimately leaves the abdomen by exiting at the caval opening in the diaphragm at T8.

All of the Posterior abdominal wall veins drain into the Inferior Vena Cava, except for the LEFT testicular/ovarian vein, which drains into the Left renal vein.

The drainage into the IVC is analogous to the branches of the abdominal aorta, except for in the case of the unpaired visceral branches, where the veins are actually tributaries of the hepatic portal vein (which do ultimately drain into the IVC via the hepatic veins).



The lymphatic system surrounds the aorto-venous structures of the abdomen, and thus follow a somewhat similar path.

The external and internal iliac lymph nodes drain into the common iliac lymph nodes which consequently drain in to the right and left (or caval and aortic) lumbar lymph nodes. These nodes accept lymph from a variety of organs including the posterior abdominal wall, kidneys and ureters, testes or ovaries, uterus and Fallopian tubes. Lymph from these nodes then drains into the lumbar lymphatic trunks.

Lymph from the GI tract, spleen, pancreas and liver drains into the preaortic lymph nodes, which then drain into the intestinal lymphatic trunks.

These trunks converge with the descending thoracic lymph ducts to form the start of the thoracic duct: the cisterna chyli (there is often very wide anatomical variation in size and shapes of this structure). The thoracic duct leaves the abdomen through the aortic hiatus in the diaphragm; ultimately, it enters the venous system at the left venous angle (junction of L subclavian and internal jugular)

Clinical Anatomy

1)     Psoas Abscess

A psoas abscess is a relatively uncommon condition which can be quite hard to diagnose clinically. The typical triad of back pain, limp and fever is only seen in about one third of patients, so symptoms can be quite non-specific. On examination, a psoas sign may be present (see next section). Psoas abscesses may be primary (immunosuppression, IVDU, diabetes etc) or secondary (underlying inflammatory or infective conditions). The pathogen responsible is commonly s.aureus, but infection by m.tuberculosis is still common in developing countries where TB of the spine is still seen. Management will depend on the underlying cause but it usually involved antibiotics and drainage (surgical or percutaneous depending on the case).

2)     Psoas Sign

A positive psoas sign may indicate inflammation of either the muscle or the peritoneum that overlies it (such as in appendicitis).

On way of eliciting it is to get the patient lying on their unaffected side and to hyperextend the hip on the contralateral side. This will stretch the iliopsoas and cause pain when inflammation is present.


3)     Lumbar Hernias


These are very rare, but may sometimes occur iatrogenically in nephrectomy patients. They occur through areas of weakness such as the superior and inferior lumbar triangles (Grynfeltt-Lesshaft and Petit Triangles respectively).

Quick Anatomy

Key Facts

A quick-look box with the structure’s developmental precursor (e.g. limb bud, foregut, hindgut) and blood – arterial and venous and lymph and nerve supply (please be specific with the nerve and vessel)

For muscles: also include origin, insertion, action and antagonistic muscle

For vessels: also include origin (i.e. proximal branch), branches and corresponding vein(s)

For nerves: also include origin, branches, dermatome and muscles supplied/myotome.


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In summary, the posterior abdominal wall is a crucial structure as it shares such close proximity with vital organs, such as the kidneys, the main abdominal vessels and a complex nervous plexus. Knowing the contents of such a space, the path they take and their anatomical relationship, can help put signs into context and help problem solve anatomically.


Moore KL, Dalley AF, Agur AMR. Clincally Oriented Anatomy. 6th ed, Philadelphia. Lippincott Williams & Wilkins, 2010

Mallick IH, Thoufeeq MH, Rajendran TP. Iliopsoas Abscesses. Postgrad Med J 2004;80:459-462

Moreno-Egea A et al, Controversies in the Current Management of Lumbar Hernias. Arch Surg. 2007; 142(1):82-88