Anatomy of Posterior abdominal wall in a nutshell

Anatomy of Posterior abdominal wall

Skeletal components:

1. Superiorly 12^th rib
2. Medially, lumbar vertebrae L1-L5
3. The disc of the L5 / S1 vertebrae is on the sacrum.
4. Ala (2)
   1. Ala of the sacrum
   2. Ala of the ilium
   3. These are joined by the sacral iliac joint.
   4. Iliac fossa = the anterior surface of the ala of the ilium.

Muscular component of Posterior abdominal wall

• Iliacus combined with psoas major forms the iliopsosas muscle which is the chief flexor of the thigh.
• The Iliopsoas test:

1. 1. abdominal viscera will rest on these muscles (such as cecum, appendix on right side pancreas is superior / on left side, sigmoid colon).
   2. Inflammation of these associated organs can be checked for by flexing the iliacus / psosas.
   3. A positive psoas sign is pain when flexing the iliopsoas, thus showing irritation of the muscle by some entity in the abdominal cavity.
   4. Acute appendicitis will have right side positive psoas test.

The fascia of the posterior abdominal wall (cross-section at L2)

• At midline, have crus of diaphragm right against the vertebrae.  These Right and Left crus give rise to the median arcuate ligament (behind which the Aorta passes)
• Just lateral, find the psoas muscle
  1. fascia of psoas superiorly forms the medial arcuate ligament associated with diaphragm
  2. inferiorly, the fascia extends into the thigh.
     1. A psoas abscess is an infection that has traveled from the lumbar vertebrae to the psoas (such as tuberculosis).
     2. This abscess may end up draining through skin in upper thigh region.
• Laterally / posteriorly to psoas, have Quadratus lumborum.  The fascia superiorly forms the lateral arcuate ligament of the diaphragm.
• Thoracolumbar fascia:

1. 1. The posterior layer is the strongest and passes from the lumbar spines, superficial to the erector spinae, to fuse with the middle layer lateral to that muscle.
   2. The middle layer separates the erector spinae from the quadratus lumborum. The erector spinae contributes to this layer.
   3. Anterior “quadratus” layer:
      1.  is thin but strong and covers the anterior aspect of the quadratus lumborum (which has fascia contributing to this layer)
      2. Superiorly it is thickened to form the lateral lumbocostal arch which gives attachment to the diaphragm.
   4. Latissimus dorsi has attachment on thoracolumbar fascia.
   5. The thoracolumbar fascia is anchored on erector spinae, transverse processes, quadratus lumborum.

​​​​​

Retroperitoneal Space

• Location: on the posterior abdominal wall behind the parietal peritoneum.
• Extent: twelfth thoracic vertebra and the twelfth rib to the sacrum and the iliac crests below.
• Contents:
  1. Variable amount of fatty connective tissue
  2. Suprarenal glands & the kidneys
  3. ureters and the renal and gonadal blood vessels.
  4. Pancreas, Duodenum +/-ascending and descending parts of the colon

Abdominal aorta Paired tributaries

• Supply posterior abdominal wall and organs associated there.

1. Aorta eventually bifurcates at L4 body.  The highest point of iliac crest is just above this bifurcation.  Also corresponds with umbilicus (slightly to the left of the umbilicus).
   1. Branches into R and L common iliac → branch into internal / external iliac.  The external iliac becomes the femoral.
   2. Along common iliac: have branch = iliolumbar artery. Comes off and branches
      • Branch to Iliacus muscle
      • Another branch travels superiorly and supplies psoas and quadratus lumborum (sometimes called ascending lumbar artery).
2. Renal arteries at L1 level.
3. Inferior phrenic arteries:
   1.  supply diaphragm
   2. have branch = superior superrenals
4. Lumbar arteries – supply vertebrae and muscles of posterior abdominal wall.
5. Gonadal branches: are either testicular or ovarian,  Travel down to supply those structures.

• Clinical:

1. 1. Aortic aneurysm: associated with abdominal aorta.
      1. Refers to a weakness in the wall of an artery that is worked on by pressure.  The wall will balloon out.
      2. May grow to the size of a softball. It pulses along with aorta, may palpate it.
   2. When people fall from great heights, have a ruptured abdominal aorta.  Aortic aneurysms are 90% lethal.

​​​​

Inferior vena cava

• Just to the right of the abdominal aorta.  Has no valves, is the largest vein in the body.
• Comes through vena cava hiatus in the diaphragm.
• Bifurcates at L5 into 2 common iliac veins.
• Lacks unpaired tributaries instead:
  • Has hepatic veins which enter the IVC & are short and are within the liver.
  • Right gonadal vein.
• Has inferior phrenic veins, suprarenal veins lumbar veins (at L1 and L3) as tributaries
  • Because it is Right displaced, the left renal vein is longer.
  • This longer left vein will receive inputs from vessels that would contribute to the IVC on the right side (such as left gonadal, left suprarenal etc.).

lymphatic drainage

1. Visceral nodes – found on organs and mesentery.
   • Pass from organs to mesentery to celiac nodes on either side of celiac trunk or to superior mesenteric nodes (terminal nodes)
   • Inferior mesenteric nodes
2. Parietal nodes:
   • Associated with abdominal wall and large arteries
   • Drain into external iliac b into common iliac → into cysternae chyli.
3. Lymph flows from visceral → parietal lymph nodesà cysterna → chyle (larger dilated lymphatic vessel behind the IVC).  It is continuous with thoracic duct.

Innervation of posterior abdominal wall (somatics)

1. Subcostal nerve – T12
   1. travels under rib 12. travels between internal oblique and transverse abdominus.
   2. Gives off cutaneous branches.
2. Lumbar plexus – L1-L5 ventral rami. Located in the psoas.

1. 1. Iliohypogastric – L1- supplies muscles of lower abdominal wall as well as skin of lower abdominal wall, hip, upper thigh.
   2. Ilioinguinal – L1  – travels into scrotum / labia majora
   3. Genitofemoral – L1-2 – on anterior surface of psoas major.  Travels toward anterior part of pelvis and branches
      1. Anterior surface of thigh
      2. Medial branch into inguinal canal = genital branch to cremaster muscles.
   4. Lateral femoral cutaneous – L2-3. travels more laterally on surface of iliacus muscle.  Passes under the inguinal ligament close to origin on ASIS. Supplies lateral aspect of thigh.
   5. Femoral – L2-4- largest nerve in plexus.  Tucked under psoas major.  Supplies extensors of knee.
   6. Obturator – L2-4. on medial aspect of psoas major.  Drops into true pelvis and through a foramen = operator canal.  Supplies muscles on medial aspect of thigh = major adductors of thigh.
   7. Lumbosacral trunk – L4-5- most medial of plexus.  Contributes to sacral plexus. Runs over ala of the sacrum.

Also read:

1. The Posterior Abdominal Wal @ TeachMeAnatomy
2. Practice Anatomy tests for free!
3. Best Anatomy books for Medical students
4. Best Anatomy websites & journals
5. Anatomy syllabus in (CBME) in India
6. Anatomy syllabus in Russia & Other CIS
7. Anatomy syllabus in China

Also Watch:

• Posterior Abdominal Wall @ Lecturio