OS 206: Abdomen and Pelvis PDF

OS 206: ABDOMEN AND PELVIS RETROPERITONEUM, POSTERIOR ABDOMINAL WALL UPCM 2029 | Dr. Sylvia Alip | LU3 A.Y. 2024-2025 OUTLINE I.​ Posterior abdominal wall II.​ Retroperitoneum A.​ Muscle...

OS 206: ABDOMEN AND PELVIS RETROPERITONEUM, POSTERIOR ABDOMINAL WALL UPCM 2029 | Dr. Sylvia Alip | LU3 A.Y. 2024-2025 OUTLINE I.​ Posterior abdominal wall II.​ Retroperitoneum A.​ Muscle A.​ Boundaries and B.​ Fascia Contents C.​ Nerves B.​ Embryology D.​ Vasculature C.​ Axial cross-sectional E.​ Lymphatics anatomy III.​ Clinical Correlates IV.​ Review: Diaphragm I.​ POSTERIOR ABDOMINAL WALL A.​ MUSCLE OVERVIEW Figure 3. Quadratus Lumborum Muscle 1.​ Lumbar vertebra and intervertebral discs 2.​Posterior abdominal wall muscles Table 1. SUMMARY OF MUSCLES OF POSTERIOR ABDOMEN a.​Psoas b.​Iliacus QUADRATUS MUSCLE PSOAS ILIACUS c.​Quadratus lumborum LUMBORUM d.​Transversus abdominis Medial half of e.​Obliques Superior 2/3 of Transverse inferior border 3.​Thoracolumbar Fascia iliac fossa, processes of of 12th rib, a.​Anterior ORIGIN Anterior the lumbar lumbar b.​Middle Superior Iliac vertebra transverse c.​Posterior Spine processes 4.​Lumbar plexus 5.​Fat, nerves, vessels Lesser trochanter and Iliolumbar PSOAS Lesser femoral shaft ligament and INSERTION trochanter of ​ Consists of the psoas major and minor (inferior to internal lip of femoral shaft ○​ The psoas major lies on top of the psoas minor psoas major iliac crest ○​ The psoas major crosses the inguinal ligament to act as a thigh tendon) and hip flexor Anterior ○​ The psoas minor stabilizes the spine but does NOT act on the hip NERVE Anterior rami Femoral nerve branches of so much because it does NOT bypass the joint INNERVATION of L1-L3 (L2-L4) T12-L4 ​ Most back muscles act on the hip joint Extends and Thigh and laterally flexes trunk flexor, Thigh flexor, hip vertebral ACTION lateral flexion stabilizer column, fixes of vertebral 12th rib during column inspiration B.​ FASCIA ​ Endoabdominal fascia ○​ All fascia inside the abdomen ○​ Lies between the parietal peritoneum and back muscles ○​ Continuous anteriorly with the transversalis fascia ​ Continuous with the lumbodorsal Fascia ○​ Named after the structure it invests or covers ​ Psoas Fascia Figure 1. Psoas Muscle ​ Thoracolumbar Fascia (lumbodorsal or thoracodorsal) ○​ Invests the deep back muscles ILIACUS ​ Anterior ​ Collectively flexes the thigh working in conjunction with psoas ​quadratus lumborum fascia ○​ Iliopsoas: the strongest hip flexor ​ Middle ​ Iliacus inserts on the lesser trochanter, more distally compared to ​over erector spinae/back muscles the psoas at the femoral shaft, inferior to the psoas major tendon ​ Posterior ○​ Similar insertion as psoas, thus the similarity in action (refer to ​under erector spinae Table 1) ○​ If the rectus sheath covers the anterior of the body, the thoracolumbar fascia covers the posterior ​ Thoracolumbar fascia is even more formidable than the anterior rectus sheath because of bony posterior attachment into the lumbar vertebrae ​ They collectively serve to act as compression of the abdomen regulating intra-abdominal pressure helping with digestion Figure 2. Iliacus Muscle QUADRATUS LUMBORUM ​ Quadrilateral muscle ​ Inserts on the internal lip of the iliac crest ​ Action includes lateral flexion that laterally flexes the vertebral Figure 4. Thoracolumbar Fascia in blue column, fixes 12th rib during inspiration, and helps diaphragm in lung expansion Trans 10 TG23: Petel, Pitogo, Prudenciado, Punongbayan, Quino, Ranollo, Reyes, A. TH: Punongbayan 1 of 7  C.​ LIGAMENTS ​ Median arcuate ligament (1): Forms an arch over the aortic hiatus ​ Medial arcuate ligaments (2): Paired bilaterally ​ Lateral arcuate ligaments (2): Paired bilaterally ​ Psoas major and minor: Psoas minor lies on top of psoas major, with its tendon inserting inferiorly Figure 5. Ligaments in the Posterior Abdominal Wall ​ Figure 6: ○​ 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, Figure 7. Nerves of the Posterior Abdominal Wall latissimus dorsi, erector spinae E.​ ABDOMINAL AORTA BRANCHES ​ There are various vascular planes to easily trace the vasculature of the posterior abdominal wall Table 2. Vascular Planes of Abdominal Aorta Branches PLANE CLASS DISTRIBUTION Anterior Midline Unpaired Visceral Digestive Urogenital Lateral Paired Visceral Endocrine Posterolateral Paired Parietal Diaphragm Body Wall Posterior Unpaired Parietal Body Wall Pelvis Table 3. Vertebral level of each Abdominal Aorta branches PLANE ABDOMINAL BRANCH VERTEBRAL LEVEL Figure 6. L3 cross-section, just past umbilicus, viewed from feet (supine), anterior top, posterior bottom Celiac T12 D.​ NERVES Anterior Midline Sup. Mesenteric L1 ​ Somatic – innervates everything under voluntary control, sensory Inf. Mesenteric L3 (afferent), and motor (efferent) functions in the posterior abdominal wall Suprarenal L1 ○​ Subcostal Nerve (T12) – only nerve arising from under the Lateral Renal L1 lateral arcuate ligament ○​ Lumbar Spinal Nerves (Posterior Rami) – innervate the back Gonadal L2 muscles Subcostal L2 ○​ Lumbar Plexus of Nerves (Anterior Rami of L1-L4) Branches: ​ Femoral Nerve (L2–L4)* Posterolateral Inf. phrenic T12 ​Lateral border of the psoas Lumbar L1-L4 ​Supplies the iliacus (thigh flexors) ​ Obturator Nerve (L2-L4)* Posterior Median sacral not specified ​Medial border of the Psoas ​Supplies the adductors of the medial thigh ​ Lumbosacral Trunk* ​Found beside (medial to) the obturator nerve ​Big mass of nerves the contribute to the sacral plexus ​ Ilioinguinal Nerve (L1) ​Related to anterior surface of quadratus lumborum ​Found more medial ​ Iliohypogastric Nerve (L1) ​Related to anterior surface of quadratus lumborum ​Found more lateral ​ Genitofemoral Nerve (L1-L2) ​Pierces the psoas near its medial tendon Figure 8. Classifications of Abdominal Aorta Branches ​Only main nerve that enters the inguinal canal ​ Anterior midline branches go to the GI ​Divides into the (1) genital and (2) femoral branch, wherein ​ Lateral branches go to the urogenital the latter passes deep to the inguinal ligament ​ Posterolateral branches go to the body wall ​ Lateral Cutaneous Nerve of the Thigh (L2-L3) ​Crossing at the ASIS (anterior superior iliac spine) and iliac crest ​Parallels the iliac crest ​ Accessory Obturator Nerve ​ Visceral – innervates abdominal contents ○​ Lumbar (abdominal) sympathetic trunks (lumbar spinal nerves anterior rami white communicating branches) Remember the relationship between nerves and the muscles they innervate as this will help determine what nerve it is *Nerves marked with an asterisk are tagged as some of the largest nerves. OS 206 Retroperitoneum, Posterior Abdominal Wall 2 of 7  ANTERIOR MIDLINE PLANE F.​ VENOUS DRAINAGE ​ Drainage pathway: ○​ hepatic portal vein→ liver→ hepatic vein → go back to inferior vena cava Table 4. Venous Drainage Source and Tributaries PLANE SOURCE TRIBUTARIES Celiac Anterior Midline Hepatic Portal Vein Sup. Mesenteric Inf. Mesenteric Suprarenal Lateral Renal Gonadal Figure 9. Arteries in Anterior Midline Plane ​ Celiac artery Inferior Vena Cava Subcostal ○​ Contributes a lot to liver & stomach Posterolateral Inf. phrenic ​ Superior mesenteric artery ○​ Behind the pancreatic head, intimately related to the uncinate Lumbar process Posterior Median sacral ​ Inferior mesenteric artery ○​ Found at the root of the small bowel mesentery LATERAL PLANE Figure 13. Venous Drainage of Posterior Abdominal Wall ​ Note: there is a difference between the left and the right lateral Figure 10. Arteries in Lateral Plane branches ​ Suprarenal/adrenal artery ○​ Right venous branches are located to the right of the midline, ​ Renal artery while the aorta is to the left of the midline ​ Gonadal artery ​ The aorta is always left ○​ Ovarian (female) ​ The inferior vena cava is always right ○​ Testicular (male) ​ This is the reason why the left renal vein is longer than the right renal vein. POSTEROLATERAL PLANE ○​ Because of the asymmetric body development wherein right and left veins develop at the same week of age of gestation: ​ The left renal vein will receive the left gonadal and left adrenal veins before draining into the IVC ​ The right gonadal and adrenal veins will drain directly into the IVC G.​ LYMPHATICS ​ Lymphatic drainage direction ○​ Inferior → superior ○​ Right → left ​ because the entire drainage is in the left venous angle ​ Lymphatic drainage pathway (general route) ○​ Internal & external iliac nodes ○​ ↓ Common iliac node ○​ ↓ Lumbar nodes Figure 11. Arteries in Posterolateral Plane ​ Paracaval for right lumbar nodes ​ Vasculature that enters into the muscles ​ Paraaortic for left lumbar nodes ​ Inferior phrenic artery ○​ ↓ Cisterna chyli ​ Subcostal artery ○​ Thoracic duct ​ Lumbar artery ○​ Everything that arises like horizontal lines over the aorta ○​ Hard to see in the cadaver because these traverses to the back muscles POSTERIOR PLANE Figure 12. Arteries in Posterior Plane ​ Median sacral artery ○​ Single midline artery over the bony prominence as the last branch of the aorta Figure 14. Lymphatic Drainage of Posterior Abdominal Wall OS 206 Retroperitoneum, Posterior Abdominal Wall 3 of 7 ​ If you have enlarged inguinal lymph nodes, there are specific C.​ AXIAL AND CROSS-SECTIONAL ANATOMY pathologies that you should look out for ​ To determine the CT scan level, pick a structure that is always there ○​ Usually seen in the skin and subcutaneous part of the genital ○​ Dra. Alip’s pick: use size of the liver to determine location in the area body ​ infected foreskin ​ wounds in the genital area Table 5. Axial and Cross-sectional Anatomy ​ syphilis ORGAN ABBREVIATION LOCATION ​ Scrotum lymphatic drainage is different ○​ Testes and the scrotum have different embryological origins Right kidney RK RIGHT ○​ Testicular cancer drains into the retroperitoneal nodes, NOT inguinal lymph nodes, because they follow the force of the Inferior vena cava IVC RIGHT testicular vein Liver RIGHT II.​ RETROPERITONEUM ​ Essentially a general area under the posterior reflection of the Left kidney LK LEFT peritoneum ​ The outer room posteriorly in the “room within a room” analogy Aorta AO LEFT A.​ BOUNDARIES AND CONTENTS Spleen (appears like a baby liver) LEFT ​ Contents: ○​ Kidneys ○​ Ureters ○​ Adrenals ○​ Pancreas ○​ Portions of the duodenum ○​ Ascending and descending colon (secondary) ​ Can be seen from the anterior abdominal wall but are plastered posteriorly ○​ Mesentery ○​ Vasculature ○​ Lymphatics ○​ Nerves ​ Anterior border ○​ posterior reflection of the peritoneum ​ Posterior border ○​ posterior abdominal wall ​ Lumbodorsal fascia merges anteriorly with the transversus abdominis muscle and is composed of 3 layers that cover the Figure 16. CT Scan [L] and Diagram [R] of Abdomen posterior abdominal wall musculature ​ Hollow organs will usually appear black from air or fat ○​ Anterior layer ​ Solid organs appear more white ​ over the quadratus lumborum ​ Image A is mid-liver, while Image B is lower ○​ Middle and posterior layers ​ over and under the erector spinae muscles B.​ EMBRYOLOGY ​ Primitive Mesenchyme ○​ Derived from mesoderm ○​ Divided into subcutaneous layer, body layer, retroperitoneal layer ​ 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 ​ Located laterally ○​ Inner stratum ​ Becomes the gastrointestinal Figure 17. Retroperitoneum (below the blue line) ○​ The layers curve around the ventral side of the body ​ When performing surgery, all structures above the blue line will be visible (refer to Figure 17) ​ Posterior leaflet of the peritoneum ○​ covers all structures under the blue line ○​ goes under the colon (secondary retroperitoneal) Figure 15. Retroperitoneum at 5 weeks of development Figure 18. Retroperitoneum (below the blue line) at a lower level in the body ​ Successively lower layers = liver continues to appear smaller ​ Important to note the superior mesenteric artery (SMA) and superior mesenteric vein (SMV) Question: Where do we incise to operate on retroperitoneal organs? ​ Surgeons must design operations for the best outcome ○​ May enter through the back, with patient in decubitus position ​ Sweep intraperitoneal area away so that intraperitoneal area will not be opened ○​ If approaching ventrally, surgeon must dig through the anterior to get to the retroperitoneum ○​ Laparoscopic option for intraperitoneal ​ Retroperitoneum can just be opened OS 206 Retroperitoneum, Posterior Abdominal Wall 4 of 7  III.​ CLINICAL CORRELATES IV.​ REVIEW OF THE DIAPHRAGM A.​ RENAL MASS A.​ APERTURES/OPENINGS Figure 19. [L] Normal [R] Renal Mass ​ Robot Partial Nephrectomy: took away a portion of the mass ○​ Very small so it is useless to dig through the anterior portion of the abdominal cavity Figure 23. Diaphragm Apertures ​ Caval Opening ○​ Pierces parts of the diaphragm ○​ Located in the central tendon Figure 20. [L] Normal [M] Renal Mass [R] IVC in blue ○​ IVC is adherent to the central tendon and opening margin (blood flow changes during respiration) ​ Mass crosses the midline ○​ Contents: ○​ Do an exploratory laparotomy or midline incision and secure the ​ Inferior Vena Cava vessels ​ Right Phrenic Nerve ○​ If you open the back muscles only, you will only see the mass = ​ Lymphatic Vessels not ideal ​ Esophageal Hiatus ​ Aorta is usually in the LEFT of the midline but now it became RIGHT ○​ Pierce parts of the diaphragm ​ Flattened IVC in blue due to the renal mass ○​ Esophagus constricts as diaphragm contracts ○​ Need to be careful when removing the mass since under it is the ○​ Contents: IVC where excessive bleeding may occur ​ Esophagus ​ Doc likes blood ​ Esophageal branches of left gastric vessels ​ Anterior and posterior vagal trunks ​ Lymphatic vessels ​ Aortic Hiatus ○​ Does NOT pierce the diaphragm, diaphragm action does NOT affect flow ○​ Between left and right crus posterior to median arcuate ligament ○​ Contents: ​ Aorta ​ Thoracic Duct ​ Azygos & Hemiazygos Figure 21. Title here SMALLER APERTURES AND OPENINGS ​ Surgically unresectable tumor ​ Sternocostal Foramen ​ Mass ate up 75% of the aortic circumference ○​ Lymphatic Vessels ○​ Aortic segment can be grafted ○​ Superior Epigastric Vessels ○​ Patients need to be in cardiopulmonary bypass ​ Crural Apertures Right & Left ​ Treatment: Biopsy → Chemotherapy ○​ Greater, Lesser, [X] Splanchnic Nerve ○​ To make it smaller and have it be taken out surgically ○​ Least splanchnic n. B.​ PANCREATIC HEAD MASS (CANCER) ​ Medial Arcuate Ligament ○​ Sympathetic trunks ○​ Least splanchnic n. (occasionally) Figure 22. Venous Drainage of Posterior Abdominal Wall ​ Also present is dilatation of the ureter ○​ Posterior to the pancreatic head mass ○​ Enlarged lymph nodes causing ureteral obstruction Figure 24. Summary of Diaphragm V.​ REFERENCES Alip, S. (2025).Posterior Abdominal Wall, Retroperitoneum, Diaphragm. Cochard, L. R., & Netter, F. H. (2002). Netter's atlas of human embryology. Icon Learning Systems. Moore’s Clinically Oriented Anatomy, 9th Edition (2023). Netter, F. H. (2022). Netter’s atlas of human anatomy (8th ed.). Elsevier. OS 206 Retroperitoneum, Posterior Abdominal Wall 5 of 7  APPENDIX Figure 7. Nerves of the Posterior Abdominal Wal OS 206 Retroperitoneum, Posterior Abdominal Wall 6 of 7  Figure 16. CT Scan [L] and Diagram [R] of Abdomen Figure 17. Retroperitoneum (below the blue line) Figure 18. Retroperitoneum (below the blue line) at a lower level in the body OS 206 Retroperitoneum, Posterior Abdominal Wall 7 of 7