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.

 

MUSCLES-

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

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

-        Iliacus

-        Quadratus lumborum

MUSCLE	Origin	Insertion	Innervation	Action
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
Iliacus	Iliac fossa, ala of sacrum, anterior sacroiliac ligaments	Lesser trochanter of femur Psoas major tendon	Femoral Nerve (L2-L4)	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

 

 

FASCIA:

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, 12^th 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 12^th rib to the iliac crest posteriorly; laterally it joins the IO and TA muscles and covers the lat dorsi.

NERVES:

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	Course	Innervation
Femoral Nerve (L2-L4)	Lateral border psoas major Deep to inguinal ligament Anterior thigh	Iliacus 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	pectineus

 

VASCULAR and LYMPHATIC BUNDLES:

1)     ARTERIAL SUPPLY

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	Type	Distribution	Branches	Level
Anterior midline	Unpaired visceral	GI tract	Coeliac SMA IMA	T12 L1 L3
Lateral	Paired visceral	Urogenital and endocrine	Suprarenal renal Testicular/ovarian	L1 L1 L2
Posterolateral	Paired parietal	Diaphragm	Subcostal Inferior phrenic Lumbar	L2 T12 L1-L4
Median Sacral Artery- unpaired parietal branch

 

 

2)     VENOUS DRAINAGE

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).

3)     LYMPHATIC DRAINAGE

 

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)

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. 6^th 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