Anterior abdominal wall

Chapter 5

The abdomen

CHAPTER CONTENTS

Anterior abdominal wall 151 Stomach 154 Duodenum 159 Small intestine 162 lleocaecal valve 163 Appendix 164 Large intestine 165 Liver 170 Biliary system 176 Pancreas 181 Spleen 185 Portal venous system 187 The kidneys 189 The ureter 194 The adrenal glands 196 The abdominal aorta 198 The inferior vena cava 199 Veins of the posterior abdominal wall 200 The peritoneal spaces of the abdomen 202 Cross-sectional anatomy of the upper abdomen 207 ANTERIOR ABDOMINAL WALL

Deep to the skin and subcutaneous fat of the lower anterior abdominal wall is a fibrous layer called Scarpa's fascia. This extends to the penis and scrotum (or labia majora) and fuses w i th the deep fascia of the perineum (Colles' fascia).

Muscle layers

The external oblique muscle arises on the anterior surface of the lower ribs and is inserted into the linea alba, pubic crest, inguinal ligament and iliac crest.

The internal oblique muscle arises from the lumbar fascia, the iliac crest and the lateral two-thirds of the inguinal ligament and is inserted into the linea alba and the costal margin.

The transversus abdominis muscle arises from the lower ribs, interdigitating w i th the diaphragm, the lumbar fascia, the iliac crest and the lateral half of the inguinal ligament, and is inserted into the linea alba.

The rectus abdominis muscle is a thick ribbon of muscle on either side of the midline from the costal cartilages to the pubic crest. It has three to four tendinous intersections at intervals along its length, which adhere to the anterior rectus sheath. The rectus sheath is formed by the aponeuroses of the other abdominal wall muscles as they surround the rectus muscle to attach to the linea alba.

Umbilical ligaments

The median umbilical ligament is the fibrous remnant of the urachus which runs from the fundus of the bladder on the deep surface of the anterior abdominal wall to the umbilicus as a cord of variable thickness. It is covered by a fold of peritoneum.

The medial umbilical ligaments are the fibrous remnants of the obliterated umbilical arteries and run from the internal iliac artery (from the origin of the superior vesical branch on each side) to the umbilicus on the deep surface of the anterior abdominal wall. These are also covered by a fold of peritoneum.

Fig. 5.1 Plain film of abdomen.

1. Internal oblique muscle 2. Fat between internal oblique muscle and transversus abdominis muscle 3. Transversus abdominis muscle 4. Properitoneal fat line (extraperitoneal fat layer) 5. Inferior border of right lobe of liver e. Gas in hepatic flexure 7. Right twelfth rib

8. Gas in transverse colon 9. Inferior border of spleen 10. Left kidney 11. Left psoas muscle outline 12. Right psoas muscle outline 13. Left transverse process of L3 14. Spinous process of L5

The term lateral umbilical ligaments is sometimes used to describe the inferior epigastric artery and its covering fold of peritoneum as it passes superomedially from the external iliac artery to enter the rectus sheath posteriorly.

Preperitoneal fat

Only a layer of fat separates the muscles of the anterior abdominal wall from the peritoneum and the abdominal contents. This is referred to as extraperitoneal fatty tissue or preperitoneal fat.

Radiological features of the anterior abdominal wall

Plain films of the abdomen (Fig. 5.1) The muscle layers of the anterior abdominal wall are outlined, especially in obese individuals, between the subcutaneous fat line and the properitoneal fat line. Clearly seen fat lines indicate the lack of oedema in these areas, although in 18% of normal radiographs the properitoneal line is absent.

Free intraperitoneal gas may outline the umbilical ligaments and make them visible, thus making a diagnosis of pneumoperitoneum possible on a supine radiograph.

CT of the abdomen (Figs 5.2-5.4; see also section on Cross-sectional anatomy) The muscle layers of the anterior abdominal wall can be seen in cross-section. Three muscles can be seen anterolaterally: the external oblique is outermost, with the internal oblique and transversus abdominis deep to it. The rectus muscle and its rectus sheath can be seen in the anterior paramedian position. That the rectus sheath is deficient posteriorly in its lowermost part can sometimes be appreciated.

Fig. 5.2 CT scan: level of body of pancreas (T12)

1. Left lobe of liver 2. Right lobe of liver 3. Gallbladder 4. Stomach 5. Portal vein 6. Inferior vena cava 7. Splenic vein

8. Body of pancreas 9. Tail of pancreas 10. Spleen 11. Right adrenal gland 12. Left adrenal gland 13. Left kidney 14. Aorta

Fig. 5.3 CT scan: level of pancreatic head (L1).

1. Right lobe of liver 2. Confluence of splenic and superior mesenteric veins to form portal vein 3. Splenic vein 4. Head of pancreas 5. Second part of duodenum

G. Loops of small bowel 7. Hepatic flexure 8. Descending colon 9. Spleen 10. Aorta 11. Superior mesenteric artery 12. Inferior vena cava 13. Left adrenal gland 14. Right kidney 1 5. Renal cortex 16. Renal pyramid in renal medulla 17. Rectus abdominis muscle 18. Transversus abdominis muscle 19. Latissimus dorsi muscle 20. Erector spinae muscle