Posterior Abdominal Wall

1. Describe the skeletal, muscular, and fascial components of the posterior abdominal wall.

Skeletal Components
Superiorly, 12th rib
Medially, Lumbar vertebrae L1-L5
The intervertebral disk between L5 and S1 on the sacrum
Ala of the Sacrum and Ala of the Ilium, joined by the anterior sacral-iliac ligaments
Iliac fossa on the anterior surface of the Ala of the Ilium

Muscular Components
Quadratus Lumborum: a square muscle in the lumbar region that attaches to the 12th rib superiorly, transverse processes of L1-L4 medially, the iliolumbar ligament (transverse process running from L5 to Ala of Ilium) inferiorly, and the iliac crest laterally. Bilateral contraction extends the back with erector spinae muscles. Unilateral contraction causes unilateral bending. Innervated by the ventral rami of T12-L4.

Psoas Major: Attaches to the transverse processes of L1-L4 superiorly, bodies and intervertebral disks medially, the lesser trocanter inferiorly. To reach the lesser trocanter, it passes the pelvis under the inguinal ligament. Contraction flexes the hip joint. It is innervatedby L2-L4 ventral rami.

Iliacus Muscle: Attaches to the superior 2/3 of the iliac fossa superiorly, the Ala of sacrum medially (anterior to the anterior sacro-iliac ligament), and the lesser trocanter inferiorly. Combined with psoas major, the iliopsoas muscle is the primary flexor of the thigh. It is innervated by the L2-L4 ventral rami as it forms the femoral nerve.

Psoas: Superiorly, the fascia of psoas major forms the medial arcuate ligament associated with the diaphragm. Inferiorly, the fascia extends to the thigh.

Quadratus Lumborum: Superiorly, the fascia forms the lateral arcuate ligament of the diaphragm. The quadratus lumborum fascia is located latero-posteriorly to the psoas.

Thoracolumbar: Fascia is anchored on the erector spinae, transverse processes, and the quadratus lumborum.

2. Understand the attachments, innervation and blood supply of the diaphragm.

The dense connective tissue of the central tendon region is surrounded by a muscular periphery that radiates inward. These fibers attach anteriorly to the sternum and costal margins, the 11th and 12th ribs, and posteriory to the lumbar vertebrae.

Blood Supply
The left and right internal thoracic arteries gives off the left and right pericardiophrenic artery which runs on the internal surface of the pericardial sac and onto the diaphragm. It then continues as the musculophrenic artery that supplies the anterior region along the costal margin.

The superior phrenic artery supplies the superior aspect as it comes off the aorta before it penetrates the diaphragm. Inferior phrenic artery supplies the inferior surface of the diaphragm. The superior suprarenal artery is a branch off of the inferior phrenic artery.

The phrenic nerves are the major innervation of the diaphragm, containing inputs from C3-C5. The phrenic nerves contain afferent fibers that can defer pain from the diaphragm to the upper part of the shoulder, and efferent fibers to the muscle.

In the lateral portion of the diaphragm where it attaches to the costal margin, the intercostal/subcostal nerves give afferent innervation to the lateral portion. Inflammation to the lateral diaphragm is perceived as lower rib/intercostal space pain.

3. Be able to describe the relationships of the diaphragm to structures which patch through it.

The right crus fibers anchor into the body of the L1-L3 vertebrae and wrap the esophageal hiatus to form part of the gastroesophageal sphincter. Inferiorly the right crus also attaches to the duodenum to form the ligament of Treitz. The left crus anchors to the L1-L3 vertebrae and has fibers running out into the muscular part of the diaphragm.

Both crus have medial borders that form the aortic hiatus through which the aorta passes. The aorta is bordered by the median arcuate ligament and passes behind the diaphragm rather than through it. Additionally, the thoracic duct, branches of the azygous and greater/lesser splanchnic nerves pass through the diaphragm with the aorta.

The thickening of the fascia of the psoas muscle forms the medial arcuate ligament. This forms a gap in the diaphragm where the psoas muscle passes behind the diaphragm.

Laterally, the thickening of the fascia of the quadratus lumborum forms the lateral arcuate ligament. The median, medial, and lateral arcuate ligaments help support the posterior abdominal wall.

Along the lateral part of the 12th rib is the vertebrocostal triangle where there is a weak spot in the diaphragm with connective tissue fibers rather than skeletal muscle. Hernaition through the diaphragm often occurs here.

4. Describe the attachments and actions of the psoas major, iliacus, and quadratus lumborum muscles.

See muscular components in objective 1.

5. Describe the course and branches of the abdominal aorta and inferior vena cava as they travel through the posterior abdomen.

The abdominal aorta and its paired tributaries supply the posterior abdominal wall and associated organs. The abdominal aorta bifurcates at L4 body into the left and right common iliac which branches further into the internal and external iliac. The external iliac arteries become the femoral arteries. Along the common iliac, the iliolumbar artery branches and splits into the iliac branch to iliacus muscle and ascending lumbar artery to quadratus lumborum. The left and right renal arteries branch off around the L1 level. The inferior phrenic arteries supply the diaphragm and their superior suprarenals supply the adrenal glands. The lumbar arteries supply the vertebrae and muscles of the posterior abdominal wall. Lastly, the gonadal branches travel down to supply the testes or ovaries.

The inferior vena cava is just to the right of the abdominal aorta. It has no valves and is the largest vein in the body. It comes through the vena cava hiatus in the diaphragm and bifurcates into the left and right common iliac veins around L5. Within the liver, the hepatic veins drain into the IVC. Blood also drains from the inferior phrenic, suprarenal, and lumbar branches at L1 and L3. Because the IVC is displaced to the right, the left renal vein is longer because it has to cross the aorta. This longer left renal vein receives blood from vessels that would contribute directly to the IVC on the right side, such as the left gonadal and left suprarenal veins, etc.

6. Understand the system of lymphatic tissues and nodes located along the posterior abdominal wall. What is the pattern of lymphatic drainage.

The lymph nodes of the abdomen and pelvis are divided into visceral lymph nodes, found in association with abdominal organs/mesenteries, and parietal lymph nodes, retroperitoneal and in association with large blood vessels. Lymph generally flows from visceral lymph nodes to parietal lymph nodes and converge at the cisternae chyle around the L1-L2 level. The cisternae chyle is between the inferior vena cava and the psoas muscle and is the site of convergence of lymphatic trunks into a single lymphatic sac, forming the origin of the thoracic duct.

Celiac Lymph Nodes: The gastric lymph nodes along the greater and lesser curvature of the stomach drains the stomach and esophagus. The hepatic lymph nodes along the hepatic artery drain the stomach, duodenum, liver, gallbladder, and pancreas. The pancreaticosplenic lymph nodes along splenic artery drains the stomach, spleen, and pancrease. Gastic, hepatic, and pancreatiosplenic lymph nodes all drain to celiac lymph nodes.

Superior Mesenteric Lymph Nodes: Mesenteric lymph nodes along the superior mesenteric artery drain the jejunum and ilium. The iliocolic lymph nodes along the ileocolic artery drain the terminal ileum, appendix, cecum, and ascending colon. The mesocolic lymph nodes along the middle colic artery drain the transverse and descending colon. Mesenteric, iliocolic, and mesocolic lymph nodes drain to the superior mesenteric lymph nodes.

Inferior Mesenteric Lymph Nodes: Inferior mesenteric lymph nodes along the left colic, sigmoid, and superior rectal arteries drain the descending and sigmoid colon and the rectum.

Parietal Nodes
External Iliac Lymph Nodes: Drains the abdominal wall below the umbilicus, thigh, and pelvic viscera.
Internal Iliac Lymph Nodes: Drains the pelvic viscera, perineum, gluteal region, and posterior thigh.
Common Iliac Lymph Nodes: Drains the pelvic viscera.
Sacral Lymph Nodes: Drains the rectum, prostate gland, uterus, and posterior pelvic wall.
Lumbar Lymph Nodes: Drains gonads, uterine tubes, uterus, kidneys, suprarenal glands, lateral and posterior abdominal wall, and abdominal surface of the diaphragm.

7. Describe the branches of the lumbar plexus, including their courses and innervations.

The nerves of the posterior abdominal wall include the subcostal nerve (T12) and the lumbar plexus (L1-L5).

Subcostal Nerve (T12)
Runs under the 12th rib and travels between the internal oblique and transverse abdominus, giving off cutaneous branches.

Iliohypogastric (L1)
Anterior to the quadradus lumborum. It supplies the muscles of the lower abdominal wall as well as the skin over the pubic region.

Ilioinguinal (L1)
Travels through the inguinal ring and into the scrotum/labia majora

Genitofemoral (L1-L2)
On the anterior surface of psoas major. It travels toward the anterior part of the pelvic and branches into the femoral branch to the anterior surface of the thigh, and the genital branch to the cremaster muscle.

Lateral Femoral Cutaneous (L2-L3)
Travels more laterally on the surface of the iliacus muscle and passes under the inguinal ligament. It supplies the lateral aspect of the thigh.

Femoral (L2-L4)
The largest nerve in the lumbar plexus is found tucked under psoas major and supplies the extensors of the knee.

Obturator (L2-L4)
Found on the medial aspect of psoas major and drops into the true pelvis through the operator canal. It supplies the main adductor muscles on the medial aspect of the thigh.

Lumbosacral Trunk (L4-L5)
It is the most medial nerve of the plexus and runs over the ala of the sacrum. It contributes to the sacral plexus.

8. Describe the anatomy, relations, fascial coverings, internal structure, and vascular supply of the kidneys.

The kidney has a hilum containing its vascular supply, the renal arteries, renal veins, as well as the renal pelvis. The renal pelvis becomes the ureter which travels to the bladder. It is covered by a thin transluscent capsule. The cortex contains glomeruli where blood filtering occurs. Renal columns are regions with glomeruli that extend down to the renal sinus of the renal gland.

The medulla of the kidney is the areas between renal columns and is made up of renal pyramids. At the top of each pyramid is the minor calyx. Two minor calyx come together to form a major calyx. 2-3 major calyx join to form the renal pelvis which drains into the ureter.

The kidney is covered by several coverings, the fibrous capsule, perirenal fat, renal fascia, and pararenal fat. The fibrous capsule is most deep and encloses the kidney. Over the fibrous capsule is the perirenal fat layer. Over that is the renal fascia or Gerota’s capsule which is a fibrous layer continuous with the transversalis fascia laterally and continuous with the fascia of psoas major medially. Above that is the pararenal fat. Finally, the peritoneum is most superficial.

9. Know the course of the ureter as it passes along the posterior abdominal wall on its way to the urinary bladder.

The ureters are the ducts that carry urine from the kidneys to the urinary bladder, passing anterior to psoas major. Their course is 50-70cm long, entering the bladder posteriorly. In the female, the ureters pass through the mesometrium on their course, passing under the uterine arteries ("water under the bridge").

10. Describe the anatomy, relations, internal structure, vascular supply of the suprarenal glands.

The suprarenal glands (adrenals) rest on the superior aspect of the kidneys. Their role is to regulate stress response through hormone synthesis (Corticosteroids, catecholamines). They are superficial to the layer of the renal fascia, and continous with the perirenal renal fat surrounding the kidney. However, it has its won fibrous capsule separate from the fibrous capsule around the kidney.

The suprarenal glands supplied by 3 arteries:

(1) Superior suprarenal artery, a branch off the inferior phrenic artery
(2) Middle suprarenal artery, branch off the abdominal aorta
(3) Inferior suprarenal artery, branch off the renal artery

The left suprarenal gland is drained by the left suprarenal vein into the left renal vein. The right suprarenal gland drains directly into the IVC through the right suprarenal vein.

Innnervation of the suprarenal gland is by the greater splanchnic nerves. Remember, there is no parasympathetic innervation to the suprarenal glands.