Posterior Abdominal Wall

PAW: Overview & Boundaries - Backstage Pass

The Posterior Abdominal Wall (PAW): Musculo-skeletal & fascial structure, the posterior limit of abdominal cavity.

• Superior: Diaphragm (posterior part), 12th rib.
• Inferior: Iliac crests, pelvic brim.
• Lateral: Lateral abdominal wall muscles (e.g., TA), quadratus lumborum.
• Posterior: Psoas major, iliacus, quadratus lumborum (posterior aspect), associated fascia.
• Anterior (Central bony support): Lumbar vertebrae (L1-L5) and intervertebral discs.

⭐ The vertebral bodies of L1-L5 vertebrae form the central part of the posterior boundary.

PAW: Muscles - Core Strength Crew

⭐ Psoas major + iliacus = iliopsoas (main thigh flexor).

PAW: Fascia - Supportive Wraps

• Psoas Fascia: Covers psoas major; forms medial arcuate ligament. Continuous with iliac fascia.
• Quadratus Lumborum Fascia: Anterior layer of thoracolumbar fascia; forms lateral arcuate ligament.
• Thoracolumbar Fascia:
  • Anterior: Covers quadratus lumborum (QL).
  • Middle: Posterior to QL, anterior to erector spinae.
  • Posterior: Covers erector spinae; thickest layer.
  • Fuse laterally; origin for transversus abdominis.

⭐ The anterior layer of thoracolumbar fascia covers Quadratus Lumborum.

PAW: Nerves & Vessels - Vital Conduits

Nerves:

• Subcostal N. (T12): Supplies ext. oblique, overlying skin.
• Lumbar Plexus (Ventral rami L1-L4): Forms in Psoas Major. 📌 "I Invariably Get Lazy On Fridays"
  • Iliohypogastric N. (L1): Abd. muscles; skin: iliac crest, hypogastrium.
  • Ilioinguinal N. (L1): Abd. muscles; skin: groin, upper medial thigh, ext. genitalia.
  • Genitofemoral N. (L1, L2): Genital & Femoral (upper anterior thigh skin) branches.
  • Lateral Femoral Cutaneous N. (L2, L3): Sensory to anterolateral thigh.
  • Obturator N. (Ant. Div. L2, L3, L4): Medial thigh adductors.
  • Femoral N. (Post. Div. L2, L3, L4): Ant. thigh (quadriceps), hip flexors.
• Sympathetic Trunks: Medial to Psoas Major; ganglia, rami communicantes.

Vessels:

• Abdominal Aorta:
  • Lumbar Arteries (4 pairs): Supply PAW muscles, vertebrae, spinal cord.
• Inferior Vena Cava (IVC):
  • Lumbar Veins: Drain PAW to IVC/ascending lumbar veins.

⭐ The genitofemoral nerve (L1, L2) pierces the Psoas major muscle.

PAW: Clinical Significance - Practice Pearls

• Psoas Abscess: Often from TB (Potts spine) or Crohn's. Spread inferiorly along psoas sheath.

  ⭐ Psoas abscess can track down the psoas sheath into the femoral triangle.

• Referred Pain: Kidney/ureteric colic radiates from loin to groin, scrotum/labia, or inner thigh.
• Nerve Injuries:
  • Lateral femoral cutaneous nerve: Meralgia paresthetica (anterolateral thigh burning/numbness).
  • Ilioinguinal/iliohypogastric nerves: Risk during inguinal hernia repair or appendectomy.
• Abdominal Aortic Aneurysm (AAA): Pulsatile epigastric mass; rupture risk ↑ with size.
• Lumbar Hernias: Rare; through Grynfeltt-Lesshaft (superior) or Petit's (inferior) triangles.

High‑Yield Points - ⚡ Biggest Takeaways

• Key Muscles: Psoas major, quadratus lumborum, iliacus form the primary muscular layer.
• Lumbar Plexus Nerves: Subcostal (T12), iliohypogastric & ilioinguinal (L1), genitofemoral (L1-L2), lateral femoral cutaneous (L2-L3), femoral & obturator (L2-L4) are major nerves.
• Psoas Major: Originates T12-L5 vertebrae; psoas sign indicates retroperitoneal irritation (e.g., abscess).
• Quadratus Lumborum: Extends from 12th rib to iliac crest; fixes 12th rib, aids lateral trunk flexion.
• Key Retroperitoneal Structures: Kidneys, ureters, and adrenal glands are located on this wall.
• Major Vessels: Abdominal Aorta and Inferior Vena Cava (IVC) lie anterior to the vertebral column here.
• Diaphragmatic Crura: Contribute to the superior aspect of the posterior abdominal wall near the midline.