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Anatomy - Resection rectopexy, laparoscopic
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Viewed functionally, by acting as a reservoir for feces the rectum prevents continuous fecal expulsion. The rectum is continuous with the sigmoid colon and its S-shaped a/p and lateral curvature (sacral, anorectal and lateral flexure respectively) resembles that of the sigmoid. Usually, the rectum is divided into three sections:
1. Rectal ampulla (main reservoir, anterior to the sacrum)
2. Anal canal (see below, closure region between the anorectal flexure and cutaneous anal verge)
3. Between 1. and 2. an unnamed section emerging at right angles from 1. and transitioning into 2, also at right angles. Here, the mucous membrane displays characteristic folds (valves of Houston).
The gross relations of the rectum are: Posteriorly sacrum and coccyx, laterally the internal iliac artery and vein and the regional lymph nodes, the sacral plexus, and parts of the autonomic nervous system as well as the ureters and uterine appendages. In women the rectum is related anteriorly to the uterus and vagina, and in men to the bladder and prostate gland/seminal vesicles. It is delimited inferiorly by the pelvic floor.
Three muscles in the wall of the lower rectum acting together close off the rectum from the anal verge:
1. The internal anal sphincter is the continuation of the circular smooth muscle fibers of the rectum, ending in a thickened edge and being innervated by the sympathetic nervous system.
2. The levator ani is a striated muscle innervated by the sacral plexus; it also comprises the puborectalis attached to the pubis. Its fibers arising from the symphysis pubis form a large sling around the anal canal and its anteriorly directed pull creates the anorectal angle.
3. The external anal sphincter is also a striated muscle and attaches to the perineal body (centrum perinei) and coccyx. Its voluntary innervation stems from the pudendal nerve. Its contraction completely closes off the distal end of the anal canal.
The different innervation of the three closing muscles provides additional protection against failure with its subsequent incontinence.
The mucosa of the anal canal contains numerous longitudinal folds (columnae anales) with dense arterial (!) plexus and venous drainage. During sphincteric contraction these plexuses fill up quickly with blood, thereby engorging and apposing the mucosa. Hemorrhoids and venous thrombosis are common vascular complications in this region.
Defecation takes place by relaxation of the anal sphincters and puborectalis (initiated by the striated musculature, emptying of the cavernous bodies) and at the same time by rectal motility and actively increased intraabdominal pressure.
Three main arterial trunks provide blood supply to the rectum:
- Superior rectal artery (from the inferior mesenteric artery) via the mesosigmoid (Caution: Dividing this artery at the level of the sigmoid will result in ischemia of the superior third of the rectum!)
- Medial rectal artery (from the internal iliac artery), superior to the levator ani
- Inferior rectal artery (from the internal pudendal artery), inferior to the levator ani
The veins run as rectal venous plexus underneath the rectal mucosa. Through the superior rectal vein, they drain the blood of the superior rectum via the inferior mesenteric vein to the liver, while the blood from the middle and inferior rectum is returned to the inferior vena cava via the medial and inferior rectal veins and internal iliac vein (portocaval anastomosis).
Lymphatic drainage of the rectum follows the vascular supply: At the superior rectum the large group of inferior mesenteric lymph nodes forms its own group (nodi lymphatici rectales superior), while the internal iliac lymph nodes receive lymph from the middle rectum (from the pararectal lymph nodes) and the superficial inguinal lymph nodes lymph from the inferior anal region, anus and perineal skin.
The perineum comprises the region caudad to the pelvic diaphragm (urogenital and anal region) and is delimited:
- Superiorly by the inferior fascia of the pelvic diaphragm
- Anteriorly by the pubic symphysis
- Laterally by the ischium
- Posteriorly by the lower margin of the gluteus maximus
The posterior perineum corresponds to the anal region and in anatomical terminology is called ischioanal fossa ; the anterior perineum corresponds to the pubic region and in turn can be subdivided into three superimposed sections: the most superficial being a subcutaneous perineal space (between the stratum membranosum telae subcutaneae perinei and the perineal fascia = Colles fascia), a superficial perineal space between the perineal fascia and perineal membrane, and a profound perineal space (open to the pelvis) superior to the perineal membrane.
The posteriorly located ischioanal fossa below the skin mainly comprises fatty tissue and numerous vessels/nerves (branches of the inferior rectal artery and internal pudendal artery as well as the pudendal nerve to the anal region). At the transition to the sacral region, an epithelium lined duct between the tip of the coccyx and the anal verge can develop in the gluteal cleft. This may be encapsulated like a cyst (pilonidal/dermoid cyst) or may have an external orifice (pilonidal sinus).
The anterior superficial perineum (spatium superficiale perinei) comprises the superficial perineal musculature and the pathways to the genitalia (perineal and bulbi vestibuli artery as well as branches of the pudendal nerve to the labia/clitoris or scrotum).
The anterior profound perineal space (spatium profundum perinei) is less clearly delimited: Posteriorly it continues into the ischioanal fossa and comprises the profound perineal musculature and other profound pathways.