Colectomy, Partial – Open

Relationships of the Colon and Rectum in the Abdominal Cavity and Pelvis

The colon represents the terminal segment of the digestive tract, and is approximately 1.50 m (5 to 6 ft) in length. The proximal colon is larger in diameter than the distal colon (7.5 cm in the cecum to 2.5 cm in the sigmoid) ( Figure 28-1-1 ).

The cecum represents the beginning of the large bowel. The ileocecal valve is located in the posteromedial surface of the cecum, and is sustained in place by the superior and inferior ileocecal ligaments, which help maintain the angulation between the ileum and cecum, preventing cecal reflux. The appendix arises also from the posteromedial surface of the cecum approximately 3 cm below the ileocecal valve. It ranges from 2 to 20 cm in length and, due to its mobility, can be in different positions: retrocecal (65%), pelvic (31%), subcecal (2.3%), preileal (1.0%), and postileal (0.4%). The blood supply of the appendix is provided by vessels located at the mesoappendix originating from 16 ileocolic vessels (see Figure 28-1-1 ). The cecum lies down on the lumbar musculature and usually has a short mesocecum that gives little mobility to this portion of the colon. However, an abnormally mobile cecum and ascending colon can be found in 10 to 22% of cases and predisposes to volvulus.

The ascending colon extends from the ileocecal valve proximally to the hepatic flexure distally. It is usually covered by peritoneum on its anterior and lateral surfaces and has little or no mobility. Mobilization of the right colon should occur through Toldt's fascia, an avascular areolar tissue derived from coalescence of the posterior parietal peritoneum and the serosa that reflects off the posterior wall of the ascending and descending colon posteriorly and laterally. The right ureter lies over the psoas muscle, and is anteriorly crossed by the spermatic vessels and genitofemoral nerve. It is also lateral to the inferior vena cava and anteriorly crossed by the right colic and ileocolic arteries, the mesenteric root, and the terminal ileum. At the pelvis, anterior and slightly lateral to the common iliac artery bifurcation, the ureter descends abruptly between the peritoneum and the internal iliac artery. In females, it traverses the posterior layer of the broad ligament and parametrium, then runs alongside the neck of the uterus and upper third of the vagina. Here, the uterine artery crosses the ureter above and lateromedially.

As the colon ascends, it reaches the undersurface of the right lobe of the liver, lateral to the gallbladder, where it angulates acutely medially, downward, and anteriorly to the hepatic flexure. This angle is supported by the nephrocolic ligament anterior to the right kidney, and covering the second part of the duodenum. The second portion of duodenum and right kidney are exposed during mobilization of this flexure.

The transverse colon is the longest segment of the colon, very mobile, and enveloped by both layers of the transverse mesocolon attaching the posterosuperior border of the colon to the lower border of the pancreas. Moreover, the posterior and inferior layers of the greater omentum are fused on the anterosuperior aspect of the transverse colon. A dissection between the omentum and the mesentery is required to mobilize the transverse colon. This dissection is easy on the left, where the structures are naturally separated by the lesser sac of the peritoneum. It is harder on the right, where the omentum and the transverse mesocolon are fused. These peculiarities make laparoscopic dissection of the transverse colon difficult, increasing the risk of injury to important nearby structures.

The splenic flexure, the highest and deepest segment of the colon, is attached to the undersurface of the diaphragm at the level of the 10th and 11th ribs by the phrenocolic ligament and represents the distal limit of the transverse colon. Some attachments of mesentery of appendices epiploicae to the splenic capsule make traction on the splenic flexure potentially dangerous. At the time of mobilization, the surgeon should take care to avoid inadvertent splenic injury.

The descending colon passes over the lateral border of the left kidney, then descends medially between the psoas and the quadratus lumborum muscles to the junction with the sigmoid, which usually begins at the pelvic brim and the transversus abdominal muscle. Similar to the ascending colon, the descending colon is covered by peritoneum in the anterior, lateral, and medial surface.

The length of the sigmoid colon varies from 15 to 50 cm. It is mobile, with a generous inverted V-shaped mesosigmoid, creating a recessed intersigmoid fossa. The left ureter lies immediately beneath this fossa and is crossed anteriorly by the spermatic vessels and left colic and sigmoid vessels. The mesosigmoid recess on its lateral surface gives guidance to the left ureter, which is situated posteriorly. The ureter should be seen before any ligation of colonic vessels is attempted. If distorted anatomy is suspected preoperatively, the placement of ureteric stents can help to localize the ureters.

The rectum is 12 to 15 cm in length and is divided into an upper third, middle third, and lower third. The anterolateral surface of the upper third is covered by peritoneum, while the middle third is covered only anteriorly; the lower third is completely extraperitoneal. The rectum descends, following the curvature of the sacrum and coccyx, and ending as it passes through the levator muscle ( Figure 28-1-2 ). In males, the proximal two thirds are related to the small bowel loops and sigmoid colon while the lower third is related anteriorly to the prostate and seminal vesicles, vasa deferens, ureters, and urinary bladder. In females, the distal third is related anteriorly to the posterior vaginal wall. The upper and middle thirds are related to the upper part of the vagina, uterus, fallopian tubes, ovaries, small bowel, and sigmoid colon. Below the peritoneal reflection, lateral relations of the rectum are the ureters and iliac vessels. The rectal lumen shows three folds known as the valves of Houston; the upper and lower with the convexity to the right and the middle to the left, this being a landmark of the anterior reflection of the peritoneum. After complete mobilization, the rectum straightens and its length increases by 4 cm.

The rectum is surrounded by a fatty mesentery, which is most prominent posterolaterally. Superiorly, it is continuous with the sigmoid mesentery. In the pelvis, it is encapsulated by a fascial layer, the fascia propria. A lateral condensation of this fascia, known as lateral stalks or lateral ligaments, which attaches the rectum to the endopelvic fascia, has been described by Goligher. However, this is controversial. Although the middle rectal artery does not traverse the lateral stalks, it can send minor branches along one or both sides in approximately 25% of cases. Therefore, there is a 1:4 chance of minor bleeding when the lateral ligaments are cut. A strong presacral fascia covers the sacrum and coccyx, with the middle sacral artery, nerves, and the presacral veins underneath. Intraoperative rupture of the presacral fascia may cause a hemorrhage from these veins that is difficult to control. Waldeyer's fascia runs from the presacral fascia over the fourth sacral segment forward and down to the fascia propria of the rectum. During rectal mobilization, this avascular tissue should be deliberately divided. Anteriorly, the extraperitoneal portion of the rectum is covered by Denonvilliers' fascia, a layer of connective tissue which extends from the anterior peritoneal reflection to the urogenital diaphragm, separating the rectum from the vagina in females and the seminal vesicles in males.

The proximal margin of the anatomic anal canal is the dentate line at 2 cm from the anal verge. The surgical anal canal extends 2 cm above the dentate line to the level of the anorectal sling ( Figure 28-1-3 ). The anal canal is surrounded by two distinct muscle tubes, which completely collapse the lumen. The inner layer, which has autonomic innervation, is the internal sphincter, a continuation of the circular muscle of the rectum that becomes thickened and rounded at its lower end. The outer layer is composed of skeletal muscle: the puborectalis and the external sphincter. Between these two muscles is the conjoined longitudinal muscle, which is a continuation of the longitudinal muscle layer of the rectum. As it passes downward, some of its terminal fibers will cross the external sphincter and gain insertion on the perianal skin; this muscle is referred to as the corrugator cutis ani (see Figure 28-1-3 ). The intersphincteric space is the preferable plane of dissection during an abdominal perineal resection for a patient with Crohn's disease, as it will facilitate wound healing. Moreover, it is the same plane that should be entered while a closed sphincterotomy is performed in the treatment of anal fissure. The lining of the anal canal corresponds to the different anatomic structures. A monostratified columnar epithelium in the rectum is followed by 0.5 to 1 cm of transitional epithelium. This includes columnar, transitional, and squamous epithelium, with a predominance of stratified columnar epithelium. Above the dentate line are 6 to 14 columns of Morgagni. Between the dentate line and the distal portion of these columns, there are crypts that can be obstructed by foreign material causing sepsis. The anoderm is a squamous epithelium below the dentate line that in its proximal half is devoid of hair, sebaceous glands, and sweat glands (see Figure 28-1-3 ).

Blood Supply

The blood supply of the right colon to the middle of the transverse colon is provided by the superior mesenteric artery, while the distal transverse colon, left colon, and superior third of the rectum are supplied by the inferior mesenteric artery (see Figure 28-1-1 ).

The pattern of vascular distribution is variable. The superior mesenteric artery arises from the aorta, below the celiac trunk. Its first branch is the middle colic artery, which subdivides into two or three arcades in the mesentery of the transverse colon. The next branch is the right colic artery, which divides into an ascending branch joining the right branch of the middle colic and a descending branch that will anastomose with the arcades of the ileocolic artery. The ileocolic artery is the last colonic branch of the superior mesenteric artery, although sometimes it can arise from the right colic artery. It divides into four terminal branches: anterior and posterior cecal branch, the appendicular artery, and an ileal branch that forms arcades with the ileal branches from the superior mesenteric artery. This particular arcade pattern allows the construction of an ileal J-pouch after division of the ileocolic artery, because there are patent ileal branches.

The inferior mesenteric artery arises from the aorta below the third part of the duodenum. The first branch is the left colic artery, which divides into a descending and an ascending branch directed toward the splenic flexure. This ascending branch of the left colic artery joins with the left branch of the middle colic, while the descending branch of the left colic anastomoses with the sigmoid arteries. Three to four sigmoid branches of the inferior mesenteric artery give continuity to a marginal artery known as the marginal artery of Drummond. This marginal artery can provide blood supply to the entire left colon in case of occlusion of the inferior mesenteric artery. In about 7% of the population, another artery connects the middle colic artery with the left colic. This is the arc of Riolan (meandering mesenteric artery), and, on occasion, it may be the main blood supply to the left colon. The inferior mesenteric artery becomes the superior rectal artery as it crosses the pelvic brim, running downward in the mesorectum and dividing into the right and left branches. These branches subdivide into anterior and posterior arteries supplying the upper third of rectum. The lower two thirds of the rectum are supplied by the middle rectal artery and inferior rectal artery; the first is a direct branch of the internal iliac and the second derives from the pudendal artery, a subsidiary of the anterior division of the internal iliac artery. The inferior rectal artery passes downward surrounded by the endopelvic fascia as it exits the pelvis through the greater sciatic foramen. After a short distance, it reenters the pelvis through Alcock's canal in the lateral wall of the ischiorectal fossa. As it crosses this space, it can cause bleeding during abdominal perineal resection.

Venous drainage generally follows the arterial pattern, with three exceptions: (1) the contribution of the middle rectal vein to the drainage of the rectum is minimal; (2) there is an intense communication between the superior and inferior rectal vein through the rectal venous plexus; and (3) the inferior rectal vein drains to the systemic circulation through the internal iliac vein while the inferior mesenteric vein unites with the splenic vein at the lower border of the pancreas to join the portal vein.

Lymphatic Drainage

The lymphatic drainage of the colon and rectum starts at the level of the lamina propria, becomes more significant at the level of submucosa and muscle wall, and finally reaches the extramural lymphatics. This network explains why tumors confined to the mucosa will not spread. Lymphatic vessels and nodes pair with the regional arteries. The lymph nodes can be classified into four types: epicolic, paracolic, intermediate, and main nodes (see Figure 28-1-1 ). The first group represents the extramural nodes under the peritoneum, usually at the appendices epiploicae. Paracolic nodes are at the mesenteric border of the colon. Intermediate nodes lie around the main colic arteries, before their point of division. Main lymph nodes are located at the origin of the superior and inferior mesenteric arteries, in front of the aorta.

The pararectal lymph nodes are contained in the mesorectal tissue mainly at the posterior and lateral surfaces of the rectum. The proximal two thirds of the rectum drain to the inferior mesenteric lymph nodes, following the course of the superior rectal artery. The lower third of the rectum drains in two directions: cephalad, through the same lymphatics as the upper and middle thirds, and laterally, through the middle rectal lymphatic to the internal iliac nodes ( Figure 28-1-4A ).

Total mesorectal excision can be safely performed, even through the lateral stalks, where occasionally some small vessels can cause minor bleeding. Posterior dissection of the rectum should be between the presacral fascia and fascia propria of the rectum through Waldeyer's fascia, to the level of the levator ani muscle.

The anal canal lymphatics drain in all directions: upward to the superior rectal lymphatic and inferior mesenteric nodes, laterally to the middle rectal nodes, and inferiorly to the internal iliac nodes. Below the dentate line, they usually drain to the inguinal nodes ( Figure 28-1-4B ).

Innervation