Posterior Abdominal Wall
The abdomen is comparable to a roughly cylindrical cavity located inferior to the insertions of the diaphragm and superior to the pelvis. The boundaries of this cavity include an anterior, two lateral, and one posterior abdominal wall. The posterior abdominal wall acts primarily as a scaffold for the retroperitoneal viscera and neurovasculature structures that travel caudally. The muscles and supporting vertebrae that are located in this region also provides postural support.
The posterior abdominal wall is the primary focus of this article. It is roughly defined as the area medial to the lateral abdominal walls and limited anteriorly by the posterior part of the parietal peritoneum. The posterior abdominal is a musculoskeletal structure with numerous neurovascular structures and lymphatics that cannot be ignored during this discussion. The relationship between the posterior abdominal wall and adjacent viscera has great clinical significance that will be discussed as well.
The posterior abdominal wall is supported by 12th thoracic and all the 5 lumbar vertebrae, along with their corresponding intervertebral discs. The transverse processes, body of the vertebrae, as well as the 12th rib, serve as points of attachment for the muscles of the posterior abdominal wall. The lumbar vertebrae have a ventral lordosis or forward convexity that is sandwiched by a left and a right paravertebral gutter. These paravertebral gutters contain both the psoas muscle groups and quadratus lumborum.
Psoas major is a triangular, bilaterally paired muscle that also forms part of the floor of the paravertebral gutter. The muscle can be divided into an anterior and a posterior mass such that the anterior part gives off bands that attach to the bodies and intervertebral discs of adjacent vertebrae, while the posterior part gives a muscular attachment to the bodies of L1-L5 (first to fifth lumbar) vertebrae. It should also be noted that the posterior division of the psoas major muscle has fibers of the lumbar plexus traversing it. There are also attachments at the anteroinferior aspect of the transverse vertebrae of L1-L4. Psoas minor is a bilaterally paired, slender muscle that is immediately superficial to psoas major. Although it is absent in 33% of individuals, when present, it originates from the lateral aspect of T12 and L1 vertebral bodies along with their intervertebral discs. Psoas major, together with iliacus (collectively called iliopsoas) is considered as a member of the muscles of locomotion, owing to their insertion and involvement with the lower limbs. For completion, iliacus has its origins at the inner lip of the iliac crest, the upper two-thirds of the inner iliac fossa, ala of the sacrum and the iliolumbar and sacroiliac ligaments.
As its name suggests, quadratus lumborum is a quadrangular muscle that has medial attachments to the tips of the transverse processes of L1-L4, superior attachments to 12th rib and inferolateral insertions to the aponeurosis of the iliolumbar ligament. The bilaterally paired muscle acts by stabilizing the caudal connections of the diaphragm, thus aiding in inspiration.
The iliolumbar arteries and its branches perfuse the muscles. The quadratus lumborum is supplied by the arteria lumbalis ima (lumbar arteries; branches of the abdominal aorta), subcostal arteries and the lumbar division of the iliolumbar artery. L1-L3 ventral rami pierce and innervate the psoas muscles, while the femoral nerve (L2-L3) gives somatic innervation to iliacus. Innervation of quadratus lumborum muscle is primarily from T12.
Although the diaphragm is not a direct muscle of the posterior abdominal wall, its crura have attachments at the level of T12 and L1. Similarly, the erector spinae muscle is associated with the fascial layers of posterior abdominal wall, but is not directly involved in its formation.
There is a plethora of nerves and vessels coursing through the abdominal cavity. For simplicity, the article will address the abdominal aorta and its major branches, the inferior vena cava and its major tributaries, the sympathetic ganglion chain and sympathetic plexus.
The abdominal aorta enters the abdominal cavity at the level of T12, via the aortic hiatus of the diaphragm. It travels anterior to the bodies of T12 and L1-L3 (L4) on the left hand side of the inferior vena cava, before it bifurcates into the left and right common iliac arteries in the pelvis. It may seem intimidating to recall all the branches of the abdominal aorta initially; but with a little practice it will seem like a second language. Here is a tip – think of the abdominal aorta as a scarecrow:
- As it emerges from the aortic hiatus of the diaphragm, it gives off left and right inferior phrenic arteries. These correspond to the eyes of the scarecrow.
- The celiac trunk is a short, unpaired artery that arises from the anterior surface of the abdominal immediately inferior to the inferior phrenic arteries. This is comparable to the nose of the scarecrow.
- Just inferior to the celiac trunk, and from the lateral surfaces of the abdominal aorta, is a pair of suprarenal arteries – or the scarecrow’s ears.
- The next branch comes off the anterior surface of the abdominal aorta is the superior mesenteric artery. It is analogous to the mouth of the scarecrow.
- The scarecrow’s arms are the left and right renal arteries.
- A pair of gonadal arteries emerges from the anterior surface of the abdominal aorta, caudal to the superior mesenteric artery. These can be compared to the nipples of the scarecrow.
- The inferior mesenteric artery comes off at the point that would be comparable to the umbilicus.
- The left and right common iliac arteries would be the equivalent of the scarecrow’s legs.
- Exactly at the bifurcation of the abdominal aorta is the median sacral artery. It is perfectly located where the scarecrow’s… well, you get the picture.
For completion, there are four pairs of lumbar arteries that arise from the posterolateral surface of the abdominal aorta.
The abdominal inferior vena cava is formed by the fusion of the left and right common iliac veins on the right hand side of the abdominal aorta at the level of L5. It continues cranially, receiving several tributaries along the way, before entering the thoracic cavity via the caval hiatus of the diaphragm. It should be noted that the left common iliac vein is posterior to the right common iliac artery. Consequently, an aneurism at this point may impede venous return via this channel. Fortunately, there are numerous venous anastomoses (that will not be discussed here) that provide alternative pathways for venous drainage of the abdominal walls and lower limbs. The inferior vena cava receives the right gonadal and the right suprarenal veins (the left gonadal and suprarenal veins drain into the left renal vein), and the left and right renal veins. At its distal end (proximal to the caval hiatus), the inferior vena cava also receives three hepatic veins on its anterior surface directly from the liver, and a left and right inferior phrenic vein on its lateral surface. The inferior vena cava also receives the lumbar veins. On the right hand side, the lumbar veins are shorter, and enter the posterior surface of the inferior vena cava directly. However, on the left hand side, the four lumbar veins communicate via the ascending lumbar vein (parallel to the inferior vena cava). The third and fourth left lumbar veins continue to enter the posterior surface of the inferior vena cava, while the first and second left lumbar veins may fuse prior to entering the inferior vena cava. On occasion, the first and second left lumbar veins may drain to the lumbar azygous vein or ascending lumbar vein (communication between common iliac, lumbar and iliolumbar veins to the azygous [right hand side] and hemiazygous veins [left hand side]).
The anterior and posterior components of psoas major muscle mentioned earlier insert along the lumbar vertebrae around the intervertebral foramina. As a result, the fibers of the lumbar plexus leave the spinal cord and pierce the substance of the muscle as it gives rise to its six branches – iliohypogastric, ilioinguinal, genitofemoral, lateral femoral cutaneous nerve of the thigh, obturator and the femoral nerve and lumbosacral trunk. There are two sets of mnemonics that are really helpful with learning about this plexus:
- I I G L O F(or) (a)L(l) – pronounced Igloo for aLl; helps with recalling the names of the nerves. The bold letters corresponds to the first letter of each nerve.
- 2 from 1, 2 from 2 and 2 from 3 – meaning that 2 nerves (iliohypogastric and ilioinguinal) have one root value (L1); 2 nerves (genitofemoral and lateral femoral cutaneous) have 2 root values (L1-L2 & L2-L3, respectively) and 2 nerves (obturator and femoral) have 3 root values (L2, L3 and L4).
- The lumbosacral trunk involves nerve roots L4, L5, S1, S2 and S3.
There are numerous lymphatic channels and nodes adjacent to the posterior abdominal wall. These include the lumbar or lateral aortic nodes, the pre-aortic lymph node group (celiac, superior mesenteric and inferior mesenteric nodes) and the inferior phrenic nodes. Their primary responsibility is drainage of the abdominal viscera. These nodes will eventually drain to the cisterna chyli via the left and right lumbar trunks, along with the intestinal trunk. The cisterna chyli is the abdominal part of the thoracic duct that travels along the right hand side of the vertebral column.
The tissue and musculoskeletal components of the posterior abdominal wall drain to the retro-aortic and lateral aortic lymph nodes. Each side of the upper half of the posterior abdominal wall drains to the axillary lymphatics of the same side.
The muscles of the posterior abdominal wall are all invested with their own fascial layers, deep to the parietal peritoneum. They can be divided into three groups: the anterior, middle and posterior fascial layers. The posterior fascial layer is formed by the investing fascia of latissimus dorsi. The anterior fascia is derived from the fascia of quadratus lumborum and the middle fascia is the thoracolumbar fascia. The thoracolumbar fascia is also made up of several layers. Its posterior, middle and anterior layers eventually fuse to give transversus abdominis its aponeurotic origin. It should also be noted that deep to the latissimus dorsi muscle, the thoracolumbar fascia also invests the quadratus lumborum muscle.
The posterior portion of the parietal peritoneum helps in the formation of the retroperitoneal space. This area between the posterior abdominal wall and the peritoneum contains visceral structures that are typically fixed to the posterior abdominal wall. The pancreas, parts of the ascending and descending colon, kidneys, ureter and adrenal glands are deep to the posterior parietal peritoneum. The perirenal and pararenal fascia further subdivides the retroperitoneum around the kidneys.
There are also several fascial layers that encircle the suprarenal glands, kidneys, perinephric fat and parts of the ureters known as the perirenal fascia. There are anterior and posterior layers that can be distinguished from each other, but become congruent with each other laterally. The posterior sheath is in contact with the psoas major, iliac and anterior thoracolumbar fasciae. The perirenal fascia continues inferiorly along the ureters, gradually becoming thinner until it is indistinguishable from the connective tissue in the area.
The psoas fascia covers the corresponding muscle anteriorly and along its points of insertion medially. Cranially, the fascia integrates with the medial arcuate ligament, laterally it blends with the superior parts of the quadratus lumborum fascia and caudally it fuses with the iliac fascia. The psoas fascia also provides a barrier between the anterior component of psoas major and the retroperitoneal structures that abuts it.
As was previously mentioned, the iliac and psoas fasciae cannot be identified as separate entities after they meet. The iliac fascia also becomes indistinguishable from the thoracolumbar fascia of the quadratus lumborum and it inserts in the inner part of the iliac crest.