Posterior Abdominal Wall

Posterior Abdominal Wall

Posterior Abdominal Wall

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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.

Muscles of the Posterior Abdominal Wall

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

PAW: Muscles - Core Strength Crew

Posterior abdominal muscles (anterior view)

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

MuscleOriginInsertionInnervationMain Action
Psoas majorT12-L5 vertebraeLesser trochanter (femur)Ventral rami L1-L3Thigh flexion, trunk lat. flexion
Quadratus lumborumIliac crest12th rib, L1-L4 trans. proc.Ventral rami T12, L1-L4Extends, lat. flexes vert. column; fixes 12th rib
IliacusIliac fossaLesser trochanter (femur) with Psoas majorFemoral n. (L2-L4)Thigh flexion
Psoas minor (variable)T12-L1 vertebraePectineal line (pubis)Ventral rami L1Weak trunk flexor
Transversus abdominis (lat. boundary)Iliac crest, costal cart. 7-12Xiphoid, linea alba, pubic crestVentral rami T7-T12, L1Compresses abdomen
Diaphragm (sup. boundary)Xiphoid, costal margin, lumbar vert.Central tendonPhrenic n. (C3-C5)Inspiration

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.

Lumbar Plexus and Nerves

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.

Psoas muscle and femoral nerve variations

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.

Practice Questions: Posterior Abdominal Wall

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A building contractor suddenly complains of lower backache which increases on bending down. He has:

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Flashcards: Posterior Abdominal Wall

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The _____ nerve enters the inguinal canal by piercing its posterior wall

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The _____ nerve enters the inguinal canal by piercing its posterior wall

ilioinguinal

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