Musculoskeletal Abdomen

Anatomical features of Abdomen:

 Abdomen: part of the trunk located between the thorax and the pelvis
- boundaries: superiorly by the diaphragm, anterolaterally by the musculoaponeurotic walls and inferiorly by the pelvic inlet (an arbitrary demarcation)
- function: reflex contraction to raise intra-abdominal pressure to aid expulsion of air, fluid, faeces or fetuses from abdominopelvic cavity; expand to accommodate ingestion, pregnancy etc
 Abdominal region: 9 regions of left and right hypochondriac, left and right lumbar, left and right inguinal and epigastric, umbilical and pubic. These are established by:
- two sagittal midclavicular planes that passes from the midpoint of clavicles to mid-inguinal points (in lean people semilunar is used which is the lateral border of the rectus abdominis)
- two transverse planes of subcostal, which passes the inferior border of the 10th costal and transtubercular that passes through the iliac tubercles (transpyloric and interspinous planes are used as well).
 Abdominal quadrants: 4 quadrants of left, right, upper and lower quadrants. Established by
- horizontal transumbilical plane and vertical median plane
 Abdominal walls:
- anterolateral wall: extends from thoracic cage to pelvis and bounded superiorly by the cartilages of 7th to 10th ribs and xiphoid process and inferiorly by inguinal ligament and iliac crest.
- posterior wall: lacking musculoaponeurotic structures

Layer of anterolateral abdominal wall

 Camper fascia: superficial fatty layer beneath the skin
 Scarpa fascia: deep membraneous layer of collagen and elastin beneath Camper fascia
 Investing fascia: extremely thin fascia covering the external aspects of the three muscle layers (superficial, intermediate and deep that covers external oblique, internal oblique and transverse abdominis respectively)
 Endoabdominal fascia: membranous sheet that lines the internal aspect of the abdominal walls that vary in thickness and named different depending on the muscles or aponeurosis it is lining, e.g. transversalis fascia
 Extraperitoneal fat: between the endoabdominal fascia and parietal peritoneum is a variable amount of fat

Muscles of anterolateral abdominal wall

 External oblique: largest, most superficial of three flat anterolateral abdominal muscles. It does not originate from thoracolumbar fascia (investing fascia of the back muscles). Most fibers run inferior medially while most posterior ones run close to vertical and most anterior ones runs close to horizontal. Aponeurosis of the inferiormost fibers of external oblique is thickened forming the inguinal ligament.
- origin: external surfaces of 5th and 12th rib
- insertion: linea alba, pubic tubercle and anterior half of iliac crest
- action: compression of abdominal walls and limited flexion and rotation of trunk
- nerve innervation: thoracoabdominal nerves T7-T11 and subcostal T12
 Internal oblique: the intermediate of three flat abdominal muscles and fans out anteriorly, medially and superiorly. Its fibers become aponeurotic roughly in the same line as external oblique forming rectus sheath.
- ¬origin: thoracolumbar fascia, anterior two third of iliac crest and lateral half of inguinal ligament
- insertion: inferior borders of 10th – 12 the ribs, linea alba and pecten pubis
- action: compress and support abdomen and flex and rotate trunk
- nerve innervation: anterior rami of T7-T12 and first lumbar nerves
 Transverse abdominal: innermost of three flat abdominal muscles, run transversally except for inferior ones which run parallel to internal oblique
- origin: internal surface of 7th – 12th costal cartilages, thoracolumbar fascia, iliac crest and lateral third of inguinal ligament
- insertion: linea alba with aponeurosis of internal oblique, pubic crest and pecten pubis
- action: compresses and support the abdominal viscera
- nerve innervation: anterior rami of T7-T12 and first lumbar nerve
 Rectus abdominis: long hard strap-like muscle. Paired rectus muscles are separated by linea alba lie close together inferiorly. The rectus muscle is anchored transversely by attachment to the anterior rectus sheath at three or more tendinous intersections. Superiorly the rectus abdominis lies on the thoracic wall so no posterior rectus sheath.
- origin: pubic symphysis and pubic crest
- insertion: xiphoid process and 5th and 7th costal cartilages
- action: flexes trunk and compress abdominal viscera and stabilize and control tilt of pelvis
- nerve innervation: thoracoabdominal nerves (anterior rami of T7-T12)
 Pyramidalis: small triangular muscles that lies anterior to the inferior part of the rectus abdominis and attaches to the anterior surface of pubis and pubic ligament

Anatomic feature of the anterolateral abdominal wall

 Aponeurosis of external oblique: muscles fibers become aponeurotic at approximately medial to MCL and spinoumbilical line, forming a sheet of tendinous fiber that decussate at the linea alba. Here two fibers exchange occurs:
- intramuscular: deep fibers of one side of the external oblique becomes superficial with fibers of the contralateral external oblique
- intermuscular: exchange of fibers between aponeurosis of contralateral external and internal oblique (hence the two muscles form a digastric muscle that work as a unit for rotation of trunk etc)
 Rectus sheath: strong incomplete fibrous compartment of the rectus abdominis and pyramidalis. It’s formed from the decussation and interweaving of aponeurosis of the flat abdominal muscles. Anterior sheath formed by external oblique fibers and anterior lamina of internal oblique while posterior sheath formed by posterior lamina of internal oblique and transverse abdominal.
- content: superior and inferior epigastric arteries and veins, lymphatic vessel and distal portion of thoracoabdominal nerves
 Linea semilunaris: the lateral border of the rectus abdominis
 Arcuate line: a crescentic line that demarcates the transition of the posterior covering of rectus abdominis from posterior rectus sheath to transversalis fascia, i.e. aponeurosis of the three flat muscles passes only anteriorly. It occurs in the inferior one-quarter of the rectus abdominis.
 Linea alba: central fusion of the abdominal muscle aponeurosis that forms a band of white shiny collagen and runs down the midline of the abdomen and separates the rectus abdominis muscles. Linea alba transmit small vessels and nerves to the skin.
 Abdominal wall incisions: incisions are made along langer line and primary for adequate exposure and secondarily best possible cosmetic effect. Factors affecting location of incision involves:
- type of operation
- the location of organs and bony or cartilaginous boundaries
- avoidance of nerves, maintenance of blood supply
- minimizing injury to muscle and fascia (muscles are cut longitudinally)
 Type of incisions:
- median: cut through the fibrous tissue of the linea alba superior/inferior to the umbilicus. This incision is relatively bloodless and avoids major nerves.
- Paramedian: made in sagittal plane and extend from costal margin to pubic hairline.
- Macburney: incision at approximately 2.3 cm superomedial to the ASIS on the spinoumbilical line.
- Suprapubic incision: horizontal cut made at the pubic hairline for cesarean section etc.
- Transverse incision: cut through the anterior layer of the rectus sheath and rectus abdominis. Most useful above the umbilicus
- Subcostal incision: incision made parallel but at least 2.5 cm inferior the to costal margin to provide access to gallbladder, biliary duct and spleen
 Clinical significance of abdominal wall
- caput medusae: the distension of anastomising vein of the abdomen when flow in the superior or inferior vena cava or portal is obstructed.

Nerve supply of the anterolateral abdominal wall

 Thoracoabdominal:
- origin: continuation of lower (7th – 11th) intercostal nerves distal to coastal margin
- course: passes inferoanteriorly from the intercostals spaces and run in the neurovascular plane, branches enters subcutaneous tissue as lateral cutaneous branches of T10-T11 and anterior cutaneous branches of T7- T11
- distribution: muscles of anterolateral abdominal wall and overlying skin
 Lateral cutaneous:
- origin: thoracoabdominal nerves T7 –T9
- course: emerge from the musculature of anterolateral wall to enter subcutaneous tissue along the anterior axillary line as anterior and posterior division
- distribution: skin of right and left hypochondriac regions
 Anterior abdominal cutaneous:
- origin: thoracoabdominal nerves T7- T11
- course: pierces the rectus sheath to enter the subcutaneous tissue a short distance from the median plane
- distribution: T7 to T9, T10 and T11 supply skin respectively to superior, around and inferior to umbilicus
 Subcostal:
- origin: T12
- course: runs along inferior border of 12th rib then the neurovascular plane of subumbilical abdominal wall
- distribution: muscles of anterolateral abdominal walls (excluding inferiormost slip of external obliques) and overlying skin above iliac crest and below umbilicus
 Iliohypogastric:
- origin: superior terminal branch of anterior ramus of L1
- course: pierces transverse abdominal muscle to course between neurovascular plane and branches pierce external oblique aponeurosis of most inferior abdominal wall
- distribution: internal oblique and transverse abdominal muscles and skin overlying iliac crest, upper inguinal, hypogastric regions
 Ilioinguinal:
- origin: inferior terminal branch of anterior ramus of L1
- course: passes through neurovascular plane and then traverses inguinal canal
- distribution: internal oblique, transverse abdominal and skin of lower perineal region

Vessels supply of the anterolateral abdominal wall

 Drainage: the skin and subcutaneous tissue of abdominal is drained by an intricate subcutaneous venous plexus, draining superiorly to the internal thoracic vein medially and the lateral thoracic vein laterally. Inferiorly to the superficial and inferior epigastric vein, tributaries of femoral and external iliac vein.
 Anastomoses:
- Paraumbilical: tributaries of the portal vein
- Thoracoepigastric: a relatively direct lateral superficial anastomotic channel between the superficial epigastric vein (a femoral vein tributary) and lateral thoracic vein (am axillary vein tributary)
 Primary arteries of the abdominal wall:

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 Course of the abdominal wall arteries:
- Musculophrenic: descends along costal margin
- Superior epigastric: descends in rectus sheath deep to rectus abdominis
- 10th and 11th posterior intercostal: arteries continue beyond ribs to descend in the neurovascular plane of abdominal wall (same for subscostal)
- Inferior epigastric: runs superiorly and enters rectus sheath, runs deep to rectus abdominis
- Deep circumflex iliac: runs on deep aspect of anterior abdominal wall, parallel to inguinal ligament
- Superficial circumflex iliac: runs in subcutaneous tissue along inguinal ligament
- Superficial epigastric: runs in subcutaneous tissue toward umbilicus
 Axillary lymph nodes: lymph nodes in which most superficial lymphatic vessels superior to umbilicus drain into while a few drain medially and deeply to the parasternal and anterior diaphragmatic lymph nodes.

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