Recto-anal cancer is a cancer of the rectum far from the sigmoid junction and is the second most common malignant tumor of the gastrointestinal tract in the United States. The location of rectal cancer is low, so it can be easily diagnosed by rectal diagnosis and sigmoidoscopy. However, because of its location deep into the pelvic cavity and complex anatomical relationship, surgery is not easy to complete and the recurrence rate is high after surgery. The lower and middle rectal cancer is close to the anal sphincter, so it is difficult to operate. Preservation of the anus and its function is a difficult problem in surgery, and it is also one of the most debated diseases in terms of surgical methods. The incidence in the United States begins to increase at the age of 40-45 years, with a peak at 75 years. The median age of onset of rectal cancer in China is around 45 years. There is a trend of increasing incidence in young people.
Anatomical and physiological dissection and physiology of rectum and anus.
1, rectum rectum is the end of the large intestine, the continuation of the colon, similar to the colon. The rectum is connected to the sigmoid colon and the lower part is connected to the anal canal, which is about 12-15 cm long. clinically it is divided into upper, middle and lower three, high: >8 cm, middle and low: <8 cm, low: within 3-6 cm of the anal verge, ultra-low resection: within 2 cm of the anastomosis on the dentate line when the length of the anal canal is resected.
The lower rectum is enlarged into a potbelly with temporary storage of feces. The upper 1/3 is covered with peritoneum anteriorly and on both sides; the middle 1/3 is covered with peritoneum anteriorly and reflexed into a depression; the lower l/3 is extraperitoneal, and the rectum is half inside and half outside the peritoneal cavity. The muscle layer is divided into two layers: the outer longitudinal muscle and the inner circular muscle. The annulus muscle is thickened below the lower end of the rectum and constitutes the internal sphincter of the anal canal. The lower end of the longitudinal muscle is connected to the anal levator and the internal and external sphincter, which acts as a sphincter with the sphincter. The rectal mucosa is smooth against the intestinal wall, and there are three semilunar folds – transverse rectal folds – in the jugular abdomen. The lower rectum is connected to the anal canal, and there are 8-10 raised anal columns in the mucosa. There are anal flaps between the bases of two adjacent anal columns. The anal sinus (or crypt) is located between the anal flaps and the anal columns. The opening of the sinus is upward, with an opening for the anal glands at the base. It is susceptible to injury and infection. There is an anal papilla at the junction of the anal canal and the anal column. There is a dentate line at the junction of the rectum and the anal canal.
The upper part of the anal canal is migratory epithelium and the lower part is squamous epithelium. The anal canal is surrounded by the internal and external anal canal sphincters. The internal sphincter is a non-random muscle, which is actually a thickened circular muscle extending from the lower rectum, surrounding the upper 2/3 of the anal canal. the external sphincter is a random muscle, which is divided into subcutaneous, superficial and deep parts by the longitudinal rectal muscle and anal levator fibers. The superficial sulcus-white line between the internal and external sphincters can be palpated on finger examination, which is comparable to the junction of the middle and lower 1/3 of the anal canal. The superficial muscle bundle starts from the caudal bone and divides into two bundles forward, surrounding the anal canal and ending at the perineum; the part connected with the caudal bone forms the caudal ligament. The deep part is a circular muscle bundle that merges posteriorly with the fibers of the puborectalis muscle. The deep part of the external sphincter, puborectalis, internal sphincter and rectus longitudinal muscle fibers form a muscle ring for the rectal ring of the anal canal. The deep part of the external sphincter consists of three muscle rings: the deep part is the superior ring, which merges with the puborectalis muscle, attaches to the puborectalis union, and lifts forward and upward at the same time when contracting; the superficial part is the middle ring, which connects with the coccyx and pulls backward at the same time when contracting; the subcutaneous part is the inferior ring, which connects with the anterior subcutaneous of the anus and pulls forward and downward at the same time when contracting. When the sphincter contracts, the three rings contract and pull in different directions at the same time to strengthen the function of the anal sphincter. When the external anal sphincter contracts, the upper and lower rings pull forward on the posterior wall of the anal canal, and the middle ring pulls backward on the anterior wall of the anal canal, causing it to close tightly. The upper ring is the most important and causes incontinence when severed; the lower ring does not cause incontinence when severed.
The anal raphe is a wide and thin layer of muscle around the rectum forming the pelvic floor, and its puborectalis muscle part merges with the posterior part of the external anal canal sphincter, which plays the function of the anal canal sphincter together.
3.Perirectal anal canal gap
(1) pelvic rectal gap, one on each side of the rectum, above the anal raphe and below the pelvic peritoneum.
(2) posterior rectal hiatus: in the rectum and sacral he, also above the anal raphe, which can be connected to the pelvic rectal hiatus on both sides.
Under the anal raphe are.
(1) the sciatic anal canal hiatus (also called the sciatic rectal hiatus), on both sides of the anal canal, under the anal raphe, on the transverse septum of the sciatic anal canal, one on each side, communicating with each other via the posterior anal canal (also called here the deep posterior anal canal hiatus).
(2) The perianal hiatus, between the transverse septum of the sciatic anal canal and the perianal skin, on the left and right sides, also communicating with each other posteriorly through the anal canal (also referred to here as the superficial posterior anal canal hiatus.
4.Arteries of the rectoanal canal come from the superior rectal artery, inferior rectal artery, anal canal artery and middle sacral artery. The superior rectal artery is the most important and comes from the inferior mesenteric artery, which is divided into two branches at the back of the upper rectum, and travels down both sides of the rectum, penetrating into the muscular layer and reaching the submucosa above the dentate line, which is the main supply vessel for internal hemorrhoids, and its branches are located on the left side, right front and right back. These three areas were previously known as the good sites for hemorrhoids. The inferior rectal artery, which branches from the internal iliac arteries on both sides, is the main artery of the lower rectum and anastomoses with the superior rectal artery above and below the dentate line. The anal canal artery is derived from the internal pubic artery supplying the anal canal and anastomoses with the superior and inferior rectal arteries. The middle sacral artery is a small branch from the aorta, which runs along the front of the sacrum and is not important.
5.There are two venous plexuses in the rectoanal canal: the superior rectal plexus is located in the submucosa above the dentate line, which converges into several small veins and crosses the rectal muscular layer to become the superior rectal vein, via the inferior mesenteric vein. The inferior rectal plexus is located below the dentate line, which collects the veins in and around the anal canal and forms the anal vein and inferior rectal vein through the external side of the anal canal rectum, and returns to the inferior vena cava through the internal pubic vein and internal iliac vein respectively.
6, the lymphatic drainage of the rectoanal canal is divided into three groups: the upper group drains the part of the rectum above the attachment of the puborectal muscle (the pot belly and above). Most of them go through the pararectal lymph nodes, and part of them directly along the superior rectal artery and inject into the lymph nodes at the beginning of the superior rectal artery within the rectal mesentery. This is the main route of rectal cancer metastasis. The middle group drains the anal canal below the dentate line, mainly through the perineum and inner thigh subcutaneously into the inguinal lymph nodes, and then upward through the extra-iliac parietal iliac lymph nodes; there are also lymph nodes through the occluding artery parietal to the parietal iliac nodes. Where low rectal cancer (below peritoneal reflex) drains to the middle group, or even to the lower group as well. In addition to the upper group lymph nodes, the middle and lower group lymph nodes should also be cleared during surgery, generally speaking, the lymph does not drain downward.
7, rectal anal canal nerves around the anal canal are mainly innervated by the branches of the pubic nerve and the anal caudal nerve and the perineal branch of the 4th sacral nerve. The rectal nerves have sympathetic and parasympathetic nerves. The sympathetic nerves arise from the presacral plexus. This plexus divides into two branches, each downward to meet the sacral parasympathetic nerve, forming the pelvic plexus on either side of the lateral rectal ligament. Injury to the presacral nerve can cause the seminal vesicles and prostate to lose their contractile function and prevent ejaculation. The sacral parasympathetic nerve is divided by the 2nd to 4th sacral nerve, which innervates urination and penile erection.
8.Dentate line anatomy clinical significance.
(1) Above the dentate line is the mucosa and below is the skin.
(2) above the dentate line is the superior rectal venous plexus, which returns to the portal vein; below the dentate line is the inferior rectal venous plexus, which returns to the inferior vena cava; therefore, the vicinity of the dentate line is the anastomosis of the lateral branches of the portal and corporal veins.
(3) Above the dentate line is supplied by the superior and inferior rectal arteries, and below is the anal canal artery supply.
(4) Lymphatic drainage above the dentate line is mainly into the periaortic or internal iliac lymph nodes, while lymphatic drainage below is mainly into the inguinal lymph nodes and external iliac lymph nodes.
(5) The rectal mucosa above the dentate line is innervated by the vegetative nervous system and is painless, while the skin of the anal canal below the dentate line is innervated by the internal pubic nerves and is obviously painful.
9, the physiological function of the rectum-anal tube is mainly defecation. The rectum can absorb a small amount of water, salt, glucose and part of the drug, but also can secrete mucus to facilitate defecation.