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• The retroperitoneum and posterior abdominal wall are key anatomical regions covered in these notes.

Posterior Abdominal Wall Muscles

• The posterior abdominal wall includes the lumbar vertebrae, intervertebral discs, and several muscles.
• Muscles include the psoas, iliacus, quadratus lumborum, transversus abdominis, and obliques.
• The thoracolumbar fascia has anterior, middle, and posterior layers.
• The lumbar plexus, fat, nerves, and vessels are also found in this region.

Psoas Muscle

• The psoas consists of the psoas major and minor.
• The psoas major lies on top of the psoas minor.
• The psoas major crosses the inguinal ligament.
• It acts as a thigh and hip flexor.
• Originates from the transverse processes of the lumbar vertebrae and inserts on the lesser trochanter of the femoral shaft.
• Innervated by the anterior rami of L1-L3.
• Actions include thigh and trunk flexion and lateral flexion of the vertebral column.
• The psoas minor stabilizes the spine, but doesn't act on the hip joint as much because doesn't bypass the joint.

Iliacus Muscle

• The iliacus works with the psoas to flex the thigh.
• It's the strongest hip flexor.
• Inserts on the lesser trochanter, more distally than the psoas, specifically on the femoral shaft inferior to psoas major tendon.
• The iliacus originates from the superior two-thirds of the iliac fossa and the anterior superior iliac spine.
• Innervated by the femoral nerve (L2-L4).
• Its key action is thigh flexion and hip stabilization.

Quadratus Lumborum Muscle

• The quadratus lumborum is a quadrilateral muscle.
• It inserts on the internal lip of the iliac crest.
• Action includes lateral flexion of the vertebral column.
• It also fixes the 12th rib during inspiration to aid the diaphragm in lung expansion.
• Originates from the medial half of the inferior border of the 12th rib and lumbar transverse processes.
• Inserts on the iliolumbar ligament and the internal lip of the iliac crest.
• Innervated by anterior branches of T12-L4.
• Actions include extending and laterally flexing the vertebral column and fixing the 12th rib during inspiration.

Fascia

• Endoabdominal fascia is all fascia inside the abdomen.
• It lies between the parietal peritoneum and back muscles.
• Continuous anteriorly with the transversalis fascia
• Has continuous with the lumbodorsal Fascia.
• Named after the structure it invests or covers.
• Psoas Fascia mentioned.
• Thoracolumbar Fascia (lumbodorsal or thoracodorsal) invests the deep back muscles.
• Anterior to quadratus lumborum fascia.
• Middle over erector spinae/back muscles.
• Posterior under erector spinae
• Has bony posterior attachment into the lumbar vertebrae.
• It acts as a compression of the abdomen, regulating intra-abdominal pressure and aiding digestion.

Ligaments

• The median arcuate ligament forms an arch over the aortic hiatus.
• Medial and lateral arcuate ligaments are paired bilaterally.
• The psoas minor lies on top of psoas major, with its tending inserting inferiorly.
• Anterior to Posterior: rectus abdominis → external oblique (posteriorly has a free edge) → internal oblique → transversus abdominis → transversalis fascia → lumbodorsal/thoracolumbar fascia
• Other muscles: psoas major & minor, quadratus lumborum, latissimus dorsi, erector spinae

Nerves

• Somatic nerves control sensory (afferent) and motor functions in the posterior abdominal wall.
• The subcostal nerve (T12) arises from under the lateral arcuate ligament.

Lumbar Plexus Branches

• Lumbar spinal nerves (posterior rami) innervate the back muscles.
• Femoral nerve (L2-L4) supplies the iliacus (thigh flexors)

Obturator Nerve

• The obturator nerve (L2-L4) is on the medial border of the psoas, supplying the adductors of the medial thigh.

Lumbosacral Trunk

• The lumbosacral trunk is medial to the obturator nerve, contributing to the sacral plexus.

Other Nerves

• Ilioinguinal nerve (L1) is related to the anterior surface of the quadratus lumborum and is more medial.
• Iliohypogastric nerve (L1) is related to the quadratus lumborum, found more lateral.
• Genitofemoral nerve (L1-L2) pierces the psoas near its medial tendon, entering the inguinal canal and dividing into genital and femoral branches.
• Lateral cutaneous nerve of the Thigh (L2-L3) crosses at the ASIS (anterior superior iliac spine) and iliac crest, parallel to the iliac crest
• Accessory Obturator Nerve is also noted
• Visceral nerves innervate abdominal contents.
• Lumbar sympathetic trunks include lumbar spinal nerves anterior rami that white communicating branches.

Abdominal Aorta Branches

• There are various vascular planes to easily trace the vasculature of the posterior abdominal wall

Vascular Planes

• Anterior Midline includes Unpaired Visceral and is for Digestive
• Lateral plane includes Paired Visceral and is for Urogenital and Endocrine
• Posterolateral Includes Paired Parietal and is for Diaphragm Body Wall
• Posterior includes Unpaired Parietal and is for Body Wall and Pelvis

Vertebral Level of Abdominal Aorta Branches

• Celiac is at T12.
• Sup. Mesenteric is at L1.
• Inf. Mesenteric is at L3.
• Suprarenal is at L1.
• Renal is at L1.
• Gonadal is at L2.
• Subcostal is at L2.
• Inf. phrenic is at T12.
• Lumbar is at L1-L4.
• Median sacral is not specified.

Venous Drainage

• Drainage pathway: hepatic portal vein→ liver→ hepatic vein go back to inferior vena cava.
• Anterior Midline drains into the Hepatic Portal Vein and the tributaries are:
  • Celiac
  • Sup. Mesenteric
  • Inf. Mesenteric
• Lateral drains into the Inferior Vena Cava, the tributaries are:
  • Suprarenal
  • Renal
  • Gonadal
• Posterolateral drains into the Inferior Vena Cava, the tributaries are:
  • Subcostal
  • Inf. phrenic
• Posterior drains the Lumbar and Median sacral and the tributaries Lumber, and Median sacral.
• There is a location difference between the left and right lateral branches.
• The right venous branches are located to the right of the midline, while the aorta is to the left of the midline
• The aorta is always left.
• The inferior vena cava is always right.
• Left renal vein is longer since lies left of the midline

Lymphatics

• Lymphatic drainage direction is Inferior → superior and Right → left.
• The lymph flows into the left venous angle.
• Lymphatic drainage pathway:
• Internal & external iliac nodes
• Common iliac node
• Lumbar nodes
• Cisterna chyli
• Thoracic duct

Retroperitoneum

• It's a general area under the posterior reflection of the peritoneum.
• Essentially, the outer room posteriorly in the "room within a room"
• Contents:
  • Kidneys
  • Ureters
  • Adrenals
  • Pancreas
  • Portions of the duodenum
  • Ascending and descending colon (secondary)
  • Mesentery
  • Vasculature
  • Lymphatics
  • Nerves
• Anterior border is the posterior reflection of the peritoneum.
• Posterior border is the posterior abdominal wall.
• The lumbodorsal fascia merges anteriorly with the transversus abdominis muscle and is composed of 3 layers that cover the posterior abdominal wall musculature.
• Anterior layer is over the quadratus lumborum.
• Middle and posterior layers are over and under the erector spinae muscles.
• The primitive mesenchyme is derived from mesoderm.
• The retroperitoneal layer forms three strata in late fetal development:
  • Outer stratum becomes the transversalis fascia and its continuity (lumbodorsal fascia and back muscles).
  • Intermediate stratum becomes the genitourinary area.
  • Inner stratum becomes the gastrointestinal area.
• The layers curve around the ventral side of the body

CT Axial and Cross-Sectional Anatomy

• To determine level, use size of the liver in the body.
  • Right Kidney is RIGHT
  • Inferior Vena Cava is RIGHT
  • Liver RIGHT
  • Left Kidney is LEFT
  • Aorta is LEFT
  • Spleen (appears like a baby liver) is LEFT
• Hollow organs appear black from air or fat and solid organs appear white

Diaphragm

• Caval Opening pierces parts of the diaphragm.
  • Located in the central tendon
  • IVC is adherent to the central tendon and opening margin (blood flow changes during respiration)
• Esophageal Hiatus: contents esophagus, Esophageal branches of left gastric vessels, Anterior and posterior vagal trunks, Lymphatic vessels
  • Pierce parts of the diaphragm
  • Esophagus constricts as diaphragm contracts
• Aortic Hiatus: contents Aorta, Thoracic Duct, Azygos & Hemiazygos
  • Does NOT pierce the diaphragm, diaphragm action does NOT affect flow
• Sternocostal Foramen: Lymphatic Vessels, Superior Epigastric Vessels
• Crural Apertures Right & Left: Greater, Lesser, Splanchnic Nerve
• Medial Arcuate Ligament: Sympathetic trunks, Least Splanchnic n