Posterior Musculature and Lumbodorsal Fascia

See Figures 1-3 to 1-6 and Table 1–1. The lumbodorsal fascia surrounds the sacrospinalis and quadratus lumborum, which together comprise the posterior abdominal wall. The lumbodorsal fascia originates from the spinous processes of the lumbar vertebrae and extends anteriorly and cranially. As it progresses upward, it separates into three layers: posterior, middle, and anterior.

Figure 1–3 Posterior abdominal wall musculature, superficial dissection. A section of the latissimus dorsi muscle has been removed. The location of the right kidney within the retroperitoneum is shown by dashed outline.

Figure 1–4 Posterior abdominal wall musculature, intermediate dissection. The sacrospinalis muscle and three anterolateral flank muscle layers are seen in cut section, and the three layers of the lumbodorsal fascia posteriorly can be appreciated.

Figure 1–5 Posterior abdominal wall musculature, deep dissection. The lumbodorsal fascia and costovertebral ligament are visualized, arising from the transverse processes of the lumbar vertebrae. The relation of the kidney and pleura is also shown.

Figure 1–6 Muscles of the posterior abdominal wall.

(From Drake RL, Vogl W, Mitchell AWM. Gray’s anatomy for students. Philadelphia: Elsevier; 2005. p. 316.)

Table 1–1 Musculature of the Posterior and Lateral Abdominal Wall

The posterior layer provides the posterior covering for the sacrospinalis muscle and is the origin of the latissimus dorsi muscle. The middle layer forms the fascial layer separating the anterior aspect of the sacrospinalis muscle from the posterior aspect of the quadratus lumborum. The anterior layer of the lumbodorsal fascia provides the anterior covering to the quadratus lumborum muscle and forms the posterior margin of the retroperitoneum. As one moves laterally away from the sacrospinalis and quadratus lumborum muscles, the lumbodorsal fascial layers fuse together and then connect with the transversus abdominis muscle.

The quadratus lumborum and sacrospinalis muscles (see Figs. 1-6 and 1-7) form the muscular portion of the posterior abdominal wall, filling the space among the 12th rib, spine, and iliac crest. The quadratus lumborum serves a number of functions. It supports the 12th rib, thus improving diaphragmatic contraction and inspiration, as well as aiding intercostal muscle function during forced expiration. Finally, it controls lateral bending of the trunk. The sacrospinalis also controls movement of the trunk by promoting extension of the spine. These muscular and fascial relationships become important clinically when performing a dorsal lumbotomy incision. As seen in Figure 1–7, this is a vertical incision lateral to the border of the sacrospinalis and quadratus lumborum. This approach allows entrance to the retroperitoneum without violation of the musculature.

Figure 1–7 Transverse section through the kidney and posterior abdominal wall showing the lumbodorsal fascia incised. Note that through such a lumbodorsal incision, the kidney can be reached without incising muscle.

(After Kelly and Burnam, from McVay C. Anson & McVay surgical anatomy. 6th ed. Philadelphia: WB Saunders; 1984.)

Lateral Flank Musculature

See Figure 1–8 and Table 1–1. Three muscular layers comprise the lateral flank musculature. From superficial to internal, these are the external oblique, internal oblique, and transversus abdominis muscles. The most superficial structure is the external oblique muscle. This muscle arises from the lower ribs and moves from lateral to medial as it progresses caudally. Final attachment is to the iliac crest caudally and the rectus sheath anteriorly. The posterior border remains free as it terminates before reaching the lumbodorsal fascia. Next is the internal oblique muscle. Again, this muscle arises from the lower rib cage, but the orientation of the fibers is from medial to lateral as they move caudally. Final attachment is to the iliac crest and lumbodorsal fascia. The final structures are the transversus abdominis muscle and transversalis fascia. The transversus abdominis muscle arises from the lumbodorsal fascia with fibers running directly transversely until it attaches anteriorly and medially onto the rectus sheath. Immediately deep to the transversus abdominis muscle is the transversalis fascia and then the retroperitoneal space. The function of the lateral flank musculature is to compress and stabilize the abdomen and trunk. This provides controlled movement and protection for the abdominal organs.

Figure 1–8 Transverse section showing layers of the lateral flank musculature.

(From Drake RL, Vogl W, Mitchell AWM. Gray’s anatomy for students. Philadelphia: Elsevier; 2005. p. 252.)

Psoas and Iliacus Muscles

The psoas major muscle originates on the 12th thoracic through the 5th lumbar vertebrae (see Fig. 1–6). A smaller psoas minor is identifiable in about one half of the population and resides medial to the psoas major. The psoas muscle(s) is covered by the psoas fascia. In close proximity to the psoas muscle is the iliacus muscle, which attaches to the inner aspect of the iliac pelvic wing. As the iliacus progresses caudally it joins with the psoas muscle to form the iliopsoas muscle. This combined muscle then joins to the lesser trochanter of the femur and controls flexion of the hip.

Lower Rib Cage

See Figure 1–9. In addition to the protection provided by the muscular layers of the posterior and lateral abdominal wall, the 10th, 11th, and 12th ribs safeguard the upper retroperitoneal space and are intimately related to the adrenal glands and kidneys. Given the close proximity, injury to these ribs can be associated with significant retroperitoneal injury. While providing protection, the lower ribs and the accompanying pleura and lung limit surgical exposure to the upper retroperitoneum. The limits of the pleura are the 8th rib anteriorly, the 10th rib in the midaxillary line, and the 12th rib posteriorly. Given this location of the pleura, flank incisions at or above the 11th or 12th ribs risk pleural violation.

Figure 1–9 Structures related to the posterior surface of the kidney.

(From Drake RL, Vogl W, Mitchell AWM. Gray’s anatomy for students. Philadelphia: Elsevier; 2005. p. 322.)

Abdominal Aorta

The aorta enters the abdomen via the aortic hiatus found between the diaphragmatic crura in the posterior diaphragm at the level of the 12th thoracic vertebrae (see Fig. 1–2). It continues caudally to the 4th lumbar vertebrae, where it bifurcates into the common iliac arteries. During its course through the abdomen the aorta gives off a number of large branches (Table 1–2). The paired inferior phrenic arteries are first. They supply the inferior diaphragm and the superior portion of the adrenal gland (see Fig. 1–2). Next is the celiac trunk, which is the origin for the common hepatic, left gastric, and splenic arteries that supply the liver, stomach, and spleen, respectively. The paired adrenal arteries follow with an artery going to each adrenal gland. The superior mesenteric artery leaves the aorta on the anterior side and supplies the entire small intestine and majority of the large intestine. Also of note, this artery communicates with the celiac trunk vasculature via the pancreaticoduodenal artery. Overlying the 2nd lumbar vertebrae, the paired renal arteries are the next branching point of the aorta. To the urologist, renal artery anatomy is obviously of great importance and is discussed in detail in the kidney section.

Table 1–2 Branches of the Abdominal Aorta

Moving distally on the aorta, the paired gonadal arteries are encountered. In the male this artery is also called the testicular artery and in the female it is the ovarian artery. The initial course in both the male and female is similar, with the artery moving caudally and laterally from the aorta, with the right gonadal artery crossing anterior to the inferior vena cava. In men, the gonadal artery then crosses over the ureter and exits the retroperitoneum at the internal inguinal ring. In women the course is different: Instead of exiting the pelvis, the artery crosses medially back over the external iliac vessels and enters the pelvis. It then proceeds via the suspensory ligament to the ovary. The destination of the gonadal artery (the testis in the male and the ovary in the female) has significant collateral sources of arterial blood, from the deferential and cremasteric arteries in the male and the uterine artery in the female. Thus the gonadal artery can generally be ligated during retroperitoneal surgery without detrimental effect.

After the gonadal arteries, the inferior mesenteric artery is found on the anterior side of the aorta before its bifurcation into the common iliac vessels. This vessel provides vascular supply to the left third of the transverse colon, descending colon, sigmoid colon, and rectum. In patients without significant vascular disease, this artery can be sacrificed without ill effect because there is collateral circulation to these bowel segments from the superior mesenteric, middle hemorrhoidal, and inferior hemorrhoidal arteries.

In addition to the listed arteries that exit the aorta from its anterior or lateral aspect, there are a number of small branches from the posterior side of the aorta. Lumbar arterial branches are found at regular intervals along the length of the aorta, with generally four pairs located within the retroperitoneum. These branches supply the posterior body wall and spine. Again these arteries can generally be ligated without detrimental effects, although spinal ischemia and paralysis has occurred after ligation at multiple levels. The final posterior branch from the aorta is the middle sacral artery, which exits the aorta immediately before the branching of the common iliac arteries and then sends branches to the rectum and anterior sacrum. The common iliac arteries then proceed into the pelvis, thus completing the arterial course through the retroperitoneum.

Inferior Vena Cava

The inferior vena cava (IVC) arises from the confluence of the common iliac veins at the level of the fifth lumbar vertebra (see Fig. 1–10). Because the common iliac veins lie medial and posterior to the iliac arteries, the confluence of the iliac veins is posterior and to the right of the aortic bifurcation. As the IVC progresses cranially through the abdomen, tributaries include the gonadal, renal, adrenal, and hepatic veins. In addition, the middle sacral vein enters the inferior vena cava posteriorly and the lumbar veins enter throughout the length of the abdominal vena cava.

The first tributary encountered along the IVC is the middle sacral vein, which enters at the junction of the common iliac veins. Also entering along the posterior aspect of the IVC throughout its course are lumbar veins. These veins course anterior to the spinal transverse processes and generally parallel the lumber arteries. In addition to providing vascular drainage, the lumbar veins connect the IVC to the azygous venous system on the right side and hemiazygos venous system on the left side of the thorax. This provides alternate routes of venous drainage within the retroperitoneum (Fig. 1–12).

Figure 1–12 Lumbar, azygos, and hemiazygos veins.

(From Drake RL, Vogl W, Mitchell AWM. Gray’s anatomy for students. Philadelphia: Elsevier; 2005. p. 332.)

The next tributaries to the IVC are the gonadal veins, whose course is analogous to the gonadal arteries until approaching the IVC. During the cranial portion of their course these veins are more lateral and closer to the ipsilateral ureter. Of surgical importance is their terminal drainage because the right gonadal vein drains directly into the IVC and the left empties into the inferior aspect of the left renal vein (see Fig. 1–10).

After the gonadal veins, the renal veins are encountered. The renal veins are generally directly anterior to the accompanying renal artery, but it is not unusual for them to be separated by 1 to 2 cm in the craniocaudal direction. The right renal vein typically is short and without branches, but in a small minority of patients the right gonadal vein can enter the right renal vein as opposed to the IVC. In a second anatomic variation, a lumbar vein will enter on the posterior aspect of the right renal vein as opposed to entering the IVC directly. The left renal vein is significantly longer than the right and receives additional branches before entering the IVC. Typically after exiting the renal hilum, the left renal vein receives a lumbar vein posteriorly, the left gonadal vein inferiorly, and the adrenal vein superiorly. Next, the left renal vein crosses anterior to the aorta and under the caudal edge of the superior mesenteric artery before draining into the IVC. Rarely the left renal vein crosses the aorta to the IVC in a retroaortic or circumaortic path.

Proceeding cranially, the posterior aspect of the IVC receives the right adrenal vein. This short vein is located posteriorly on the IVC, making it challenging to expose during right adrenal or renal surgery. As noted already, the left adrenal vein drains into the left renal vein as opposed to the IVC. The inferior phrenic vein on the right side enters along the posterior or posterior lateral aspect of the IVC, with the left inferior phrenic vein typically entering the left renal vein. The final tributaries to the IVC before it leaves the retroperitoneum are the short hepatic veins draining the liver. Inferiorly these veins are small, but superiorly three large hepatic trunks are encountered.

Autonomic System

The autonomic system is further divided into sympathetic and parasympathetic fibers. The origin of these two nerve types is quite different, with the sympathetic preganglionic fibers originating from the thoracic and lumbar portions of the spinal column and the parasympathetic preganglionic fibers beginning in the cranial and sacral spinal column segments. Preganglionic sympathetic fibers enter the retroperitoneum through both the paired sympathetic chains and input from the lumbar spinal nerves (Fig. 1–14). The lumbar portion of this sympathetic chain then sends preganglionic fibers to autonomic plexuses associated with the major branches of the abdominal aorta. Within these aortic plexuses the preganglionic fibers synapse and postganglionic fibers are then distributed to the various abdominal viscera and organs. Parasympathetic input from the vagus nerve also supplies these ganglia.

Figure 1–14 Sympathetic chain and splanchnic nerves.

(From Drake RL, Vogl W, Mitchell AWM. Gray’s anatomy for students. Philadelphia: Elsevier; 2005. p. 309.)

In more detail, the thoracic and lumbar portions of the sympathetic chain originate from preganglionic sympathetic fibers arising from the first thoracic through the third lumbar spinal nerves (see Fig. 1–14). This chain then courses vertically along the anterolateral aspect of the spine just medial to the psoas muscle. Within the retroperitoneum, lumbar arteries and veins are closely associated with the lumbar sympathetic chain, in some instances even splitting the fibers as they cross the chain perpendicularly. From this sympathetic chain preganglionic fibers follow one of three courses. First, preganglionic fibers are sent to the various autonomic plexuses (splanchnic nerves). Once in the plexus, the preganglionic fibers synapse within a ganglion to postganglionic fibers, which in turn proceed to the abdominal viscera. Second, preganglionic fibers can synapse within the sympathetic chain ganglia and send postganglionic fibers to the body wall and lower extremities. Finally, preganglionic sympathetic fibers can proceed directly to the adrenal gland without synapsing. Within the adrenal medulla, these preganglionic fibers control release of catecholamines.

The major autonomic nerve plexuses are associated with the primary branches of the aorta. These plexuses include the celiac, superior hypogastric, and inferior hypogastric plexuses (Fig. 1–15). These plexuses receive sympathetic input from the sympathetic chains via the greater, lesser, and least thoracic splanchnic nerves originating from the 5th through 12th thoracic spinal nerves. They also receive input from the lumbar portion of the sympathetic chain via the lumbar splanchnic nerves, as well as parasympathetic input via the vagus nerve.

Figure 1–15 Autonomic plexuses associated with branches of the aorta.

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