Anatomy of Anterior Abdominal wall in a Nutshell

Anatomy of Anterior Abdominal wall

• Separated from the thoracic cavity by the thoracic diaphragm.
• There is no clean separation between abdominal and pelvic cavities.
• The artificial boundary is the pelvic inlet.
• These are considered as one functional cavity since many organs are present in both (intestines).
• Abdomen: portion of trunk between thorax and pelvis.
• Abdominal cavity: space enclosed by the abdominal wall, continuous with a pelvic cavity (forms abdominopelvic cavity). Contains the abdominal viscera and peritoneal cavity.

General characteristics of the Anterior abdominal wall (anterolateral)

• Musculocutaneous sheet anchored to the skeleton (ribs, lumbar vertebrae, pelvis).
• Features:

1. 1. Umbilicus = belly button.
   2. Skin attaches close to the anterior superior iliac spines.
   3. Depression down midline = linea alba (white line).
   4. Lateral on either side to linea alba = linea semilunaris (on sides of 6 pack).

• Major abdominal muscles are located laterally.
• The muscles form a network at diagonals across the abdomen.
• Contain both contractile portions and aponeurosis.
• Major attachment points = iliac crest, superior pubic ramus (with pubic crest), lower ribs.
• Functions of the abdominal wall:

1. 1. Bend and rotate the trunk (anteriolaterally)
   2. Support trunk
   3. Raise abdominal pressure (Valsalva maneuver), loud speech, vomiting, defecation, childbirth.

• Clinical relevance:

1. 1. Diagnostic
      • palpate abdominal organs
      • rigidity and/or rebound tenderness indicates irritation of the deep surface
      • enlarged – 5 Fs and 1 T: Fetus, fluid (ascites) especially in peritoneal space, fat (superficial or deep around organs), flatus (gas), feces (constipation), and tumor.
   2. Surgical
      • common site of entry into the abdominal cavity
      • repair of inguinal hernia

4 quadrants to the abdomen

• This is a coarser grid of separation.
• Useful for localizing visceral pain.
• 2 planes subdivide the abdomen:

1. 1. A horizontal plane through the umbilicus (transumbilical plane) = TUP. Passes through IV disc between L3 and L4.
   2. A vertical plane through midline = median plane.

• Gives rise to 4 quadrants:

1. 1. Right upper quadrant
   2. left upper quadrant
   3. right lower quadrant = appendicitis
   4. left lower quadrant

9 regions

• More defined divisions = 9 regions: uses 4 planes.

1. 1. Midclavicular lines divide on either side vertically
   2. Subcostal plane = passes along the lower extent of ribs, corresponds with L2 vertebrae.
   3. Transtubercular plane = passes along the tubercles of the ilium.  On the iliac crest, have thickening as move laterally, these are the tubercles.  This is at the level of L5 vertebrae.

• 9 regions:

1. 1. at midline – epigastric, umbilical, hypogastric
   2. laterally on either side = R/L hypochondriac, Lumbar, inguinal

Abdominal wall layers

1. Skin on outside
2. 2 layers of superficial fascia:
   1. 1^st layer= fatty superficial layer AKA Camper’s fascia.  This thickness varies a great deal.
   2.  2^nd layer = membranous deep layer AKA Scarpa’s fascia.  Has no fat in it. This fuses with the deep fascia of the thigh as extends into the lower extremities.  Near midline, it continues into scrotum/labia majora.
3. Deep to the membranous layer = 3 layers of muscle, each with a deep fascia around it.
4. Deep to the muscles lies transversalis fascia which will continue in areas where lack abdominal muscles.
5. Deep to the transveralis fascia lies endoabdominal fat (extraperitoneal)
6. Deepest layer = parietal peritoneum which is the lining of the abdominal cavity.

Muscles of the abdominal wall

• Innervation: anterior rami of T7-T12 spinal nerves, called thoracoabdominal nerves.
  1. They pass the costal margin and travel between the 2^nd and 3^rd layers of muscles as thoracoabdominal muscles.
  2. L1 may / may not participate.
• Combined action = compress abdominal contents.  May act individually to rotate/bend the trunk.

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1 – fascia thoracolurnbalis (posterior layer); 2 – m. erector spinae; 3 – m. quadratus lumborum; 4 – m. psoas major; 5 – peritoneum; 6 – fascia transversalis; 7 – m. transversus abdominis; 8 – m. obliquus internus abdominis, 9 – m. obliquus externus abdominis; 10 – m. rectus abdominis; 11 – linea alba; 12 – the front wall of the vagina is a straight abdominal muscle; 13 – the back wall of the vagina; 14 – corpus vertebrae; 15 – spinal cord; 16 – m. multifidus

• Derivatives of the External Oblique muscle:

1. 1. External oblique forms inguinal ligament = from anterior superior iliac spine to pubic tubercle.
   2. Rotates trunk, lateral bending. Has aponeurosis along anterior region = forms Rectus Sheet.

• Derivative of Internal Oblique muscle:

1. 1. Cremaster muscles = Some fibers go down to form network around testes
      • These are skeletal muscles that are under voluntary control to elevate testes.
      • Innervated by the genital branch of the genitofemoral nerve (Cremaster reflex causes contraction = stimulation of L1 in medial aspect of thigh).
   2. Part of fibrous portion = forms conjoined tendon AKA Falx Inguinalis

• Pyramidalis muscle: located posterior to rectus abdominus, attaches pubic crest and superiorly at the linea alba.  Only 80% of the population have a pyramidalis muscle.

Rectus sheath

• A sleeve that encases ¾ of the rectus abdominus
• Along the midline, fibers of the rectus sheath come together to form the linea alba
• Linea Semilunaris: 2 lateral depressions that show the lateral borders of the rectus sheath and rectus abdominus
• Cross-section above umbilicus:

1. 1. External oblique aponeurotic fibers run anterior to rectus abdominus and form the anterior portion of rectus sheath.
   2. Internal oblique has fibers that split into 2 portions that go either anterior / posterior to rectus abdominus.
   3. Transverse abdominal muscle has aboneurotic fibers that all go posterior to rectus abdominus muscle.  Have even splitting of aponeurotic layers.

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1 – fascia of the abdomen; 2 – transversus abdominus; 3 – Internal oblique muscle; 4 – external oblique muscle; 5 – rectus abdominus

• Cross-section ½ between umbilicus and iliac crest:
  1. external oblique has aponeurotic fibers anterior to rectus abdominus
  2. internal oblique and transverse abdominal muscle as well give off fibers that run anteriorly to rectus abdominus.
  3. Thus, there is no posterior portion from muscle, only from parietal peritoneum and extraperitoneal fat.
  4. Arcuate line: the crescent-shaped inferior border of the posterior layer of the rectus sheath located approximately 1/3 of the distance from the umbilicus to the pubic crest.
  5. Below this line, no posterior layer of the rectus sheath (lack of posterior sheath allows blood supply easier access).

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1 – fascia of the abdomen; 2 – transversus abdominus; 3 – Internal oblique muscle; 4 – external oblique muscle; 5 – rectus abdominus

Innervation to the anterior abdominal wall

• Sources:
  1. Anterior rami of spinal nerves.
  1. Motor: abdominal muscles.
  2. Sympathetics: sweat glands in skin and blood vessels.
  3. Sensory fibers: in skin, proprioreceptors in muscle, and parietal peritoneum (somatic innervation).
• These travel between the internal oblique and the transverse abdominus muscles, pierce the rectus sheath to supply the rectus abdominus muscle and then provide anterior cutanous branches.
• Specific sources:

1. 1. T7-T11 = thoracoabdominal nerves.
   2. T12 = subcostal nerve
   3. L1 = iliohypogastric nerve which runs above iliac crest to supply skin there → terminates to supply skin above pubis.
   4. L1 = ilioinguinal nerve → supplies scrotum / labia majora.

• These nerves create a dermatome pattern:

1. 1. At midline: below xyphoid  = T7.
   2. umbilicus = T10,
   3. Pubic area = L1.

Blood supply to the anterior abdominal wall

• Arteries distributed similarly to nerves.
• Many are continuations of intercostal arteries.
• Superiorly: continuation of the internal thoracic artery → 2 terminal branches
  1. Superior epigastric which is on the deep surface of rectus abdominus to supply it.
  2. Musculophrenic artery – travels laterally along the costal margin to supply the lateral muscles superiorly.
• Posterior intercostal arteries (10^th, 11^th).
• Subcostal artery.
• Inferior epigastric (largest in the abdominal wall):
  1. Travels on the deep surface of rectus abdominus.
  2. This is a branch off of the external iliac → also has deep circumflex iliac → along the deep surface of the iliac crest to supply lateral muscles.
  3. External iliac becomes femoral artery when it crosses the inguinal ligament.
• Femoral artery gives off:
  1. 1. superficial epigastric
     2. superficial circumflex iliac (travel on top of muscles).

• Each artery has a matching vein with the same name which drains blood from the same region.

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1 – a. intercostalis post. IV; 2 – a. musculophreniea; 3 – a. epigastrica inf.; 4 – a. thoracica interna; 5 – a. epigastrica sup; 6 – a. epigastrica superficialis; 7 – aa. dorsales penis​​​

Appropriate incisions of the anterior abdominal wall based on avoiding damage

1. Along linea alba at the midline.  Problem is that it doesn’t heal well due to low blood supply.
2. Paramedian = just laterally to linea alba, heal faster.
3. Above the pubic crest = suprapubic incision.
4. Subcostal = parallel to subcostal margin.  Used to remove gallbladder.
5. McBurney’s point: point at skin that overlies the appendix.  1/3 distance from anterior superior iliac spine to umbilicus.

Also read:

1. Anterolateral Abdominal Wall @ 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:

• Anterolateral Abdominal Wall @ Lecturio