直肠解剖

直肠解剖 Total laparoscopic mobilization of the rectum is dependent on an exact, detailed knowledge of the anatomy of the lesser pelvis. We will therefore begin with an anatomical review, followed by a description of the operative steps, with an emphasis on the particular points pertinent to the laparoscopic approach while bearing in mind that operative difficulties may modify the surgical strategy as well as the identification of anatomical structures. 1. Peritoneum 2. Fascia propria 3. Mesorectum The rectum is the straight and terminal portion of the digestive tract. The superior third of the rectum is covered by the peritoneum over its anterior and lateral surfaces. In addition, the rectum is surrounded by a fascia (fascia propria) which defines the limits of the mesorectum from behind. The rectum, mesorectum and fascia propria are surrounded by the pelvic fascia located anterior to the sacrum and the sacral nerves, medial to the ureters, large pelvic vessels and hypogastric nerve plexus, and posterior to the urinary/genital organs. These various fascial layers comprise circular zones in close proximity. Theoretically, therefore, a surgical plane exists between the fascia propria of the rectum and the parietal fasciae. This plane is used in the TME technique described by Heald, in which dissection is performed between the 2 fasciae inferiorly to the pelvic floor. The rectum has the shape of a cylindrical reservoir, from 12 cm to 15 cm long, and extends from the sacral promontory posteriorly against the anterior surface of the sacral concavity. Proximally and distally it is 3 to 4 cm in diameter, and its middle portion is from 6 to 8 cm in diameter, although it may be much wider. The rectum, and especially its posterior surface, is surrounded by a fatty tissue called the mesorectum. The mesorectum is an extension inferiorly from the mesosigmoid, becoming progressively more narrow and ending on the posterior surface of the rectum above the anal sphincters. It contains the terminal and proximal branches of the inferior mesenteric vessels and their lymphatic glands. The pelvic organs are covered with protecting fasciae. In the posterior pelvic space, the rectum is covered with different fasciae which determine the surgical dissection planes: - the pelvic visceral fascia or the fascia propria; - the right and left parietal fasciae; - the presacral fascia; - Denonvilliers’ fascia. Aside from the anterior superior and lateral third of the rectum which is covered by the peritoneum, the rectum is surrounded by a fascia (fascia propria) which defines the limits of the mesorectum from behind. This fascia is fragile, but easily identified (in laparoscopy notably) except on the upper third of the rectum where it is covered by the peritoneum. Its upper limit is situated at the level of the rectosigmoid junction. In laparoscopy, it appears to extend upwards, behind the superior rectal artery which it protects from the underlying nerve structures. The optimal dissection plane is found at this level. The presacral fascia is a membrane of various thickness attached to the anterior surface of the sacrum, covering the sacral nerves and vessels; the piriformis muscle at its base; and infero-laterally, the coccyx, the rectococcygeus muscle and the anococcygeal ligament (which splits to form Waldeyer's fascia) to reach the anorectal junction where it joins the fascia propria of the rectum on its posterior lateral surface. Its superior portion contains the superior hypogastric nerve plexuses which are followed by the inferior hypogastric plexuses. The pelvic organs are covered with protecting fasciae. In the posterior pelvic space, the rectum is covered with different fasciae which determine the surgical dissection planes: - the pelvic visceral fascia or the fascia propria; - the right and left parietal fasciae; - the presacral fascia; - Denonvilliers’ fascia. The parietal fascia is the anterior extension of the presacral fascia to the right and to the left. It is the posterior and medial portion of the fibrous sacro-recto-genito-pubic membrane. It protects the hypogastric vessels, the ureters and the hypogastric nerve plexuses. Nerve branches as well as vascular and lymphatic branches run through the parietal fascia laterally to medially. It is a fibrous membrane which extends between the levator ani muscles and the Douglas’ pouch. It posteriorly covers the prostate and the seminal vessels. This triangular-shaped fascia, its base on top, merges cranially and laterally with the parietal pelvic fascia and the outer edges of the seminal vessels, and medially with the peritoneum of the Douglas’ pouch. Caudally, it merges with the superior pelvic fascia. This fascia protects the urinary and genital nerves and vessels anteriorly. The various posterior pelvic fasciae, which are more or less approximated, form spaces. The success of a surgical act is dependant on the surgeon’s knowledge of these anatomical structures. It is situated anteriorly to the sacrum approximating the periosteum, the sacral nerves and vessels and the superior hypogastric nerve plexuses. This was the usual dissection plane for curative rectal cancer surgery before Heald demonstrated the advantages of the interfascial dissection plane. Inferiorly, the oncologic dissection plane is situated behind the rectosacral fascia. Dissection is performed between the presacral fascia and the fascia propria of the rectum anteriorly. This space contains few structures apart from a few nerve branches coming either directly from sacral foramina or from superior hypogastric nerve plexuses. This space can be easily separated, notably in laparoscopic surgery due to the pressure of CO2. Such separation is more difficult at the level of the fourth or fifth portions of the sacrum at the border between the horizontal and vertical portions of the rectum. The thickness of this portion, termed the rectosacral fascia, is variable. Situated anteriorly to the fascia propria of the rectum, this is the dissection plane for benign pathologies. It is situated anteriorly to the superior rectal vessels. The various posterior pelvic fasciae, which are more or less approximated, form spaces. The success of a surgical act is dependent on the surgeon’s knowledge of these anatomical structures. Dissection between the posterior aspect of Denonvilliers’ fascia and the anterior aspect of the rectum surrounded by its fascia propria is the dissection plane for benign pathologies of the rectum. Dissection behind Denonvilliers’ fascia decreases the risk of damage to the genitourinary and sexual nerves situated anteriorly to the aponeurosis. This is the dissection plane described by Heald in Total Mesorectal Excision. It requires dissection close to the prostate and the seminal vesicles which presents a risk for the genital nerve branches. The various posterior pelvic fasciae, which are more or less approximated, form spaces. The success of a surgical act is dependant on the surgeon’s knowledge of these anatomical structures. Described as a thickening of the lateral connective tissues of the rectum, the lateral attachments demarcate, along with the rectum, an anterior and a posterior portion of the posterior pelvic space. They form a triangle whose base is on the lateral pelvic wall and whose apex joins the external surface of the rectum. It extends from top to bottom from the third to the fifth sacral vertebral segments. These ligaments contain the middle rectal vessels and the rectal branches of the superior hypogastric plexuses. The middle rectal vessels are infrequently present and unilateral in 25% of cases, and are most often situated anterolaterally in the distal third of the rectum, ie, in the lower part of the lateral attachments. The size of these vessels is variable, but is most often they are inversely proportional to the size of the superior rectal vessels (Ayoub, 1978). This is the dissection plane described by Japanese authors in curative rectal cancer surgery. Dissection is performed close to ureteral, vasculo-nervous and hypogastric ganglial structures. The blood supply of the rectum emanates from the superior, middle and inferior branches of the inferior hypogastric mesenteric vessels. The inferior mesenteric artery (IMA) arises from the aorta, 1 cm to 2 cm below the third portion of the duodenum. This is the artery of the left colon and the rectum. It divides more or less rapidly into the left colic artery, the sigmoid artery and the superior rectal artery. The sigmoid trunk can arise from the left colic branch. The inferior mesenteric vein is situated to the left of the IMA and the left sympathetic plexus trunk. It drains the sigmoid, rectal and left colic areas. It joins the splenic vein, posterior to the pancreas, to form the small splenaraic vein. The blood supply of the rectum emanates from the superior, middle and inferior branches of the inferior hypogastric mesenteric vessels. The SRA is the terminal branch of the inferior mesenteric artery. Situated just anterior to the fascia propria of the mesorectum, at the level of the rectosigmoid junction, it progressively approaches the rectal wall distally, dividing into a right and left branch (or into more) which irrigate the inferior two-thirds of the rectum. The more voluminous right branch is often a continuation of the IMA. All of these arterial branches remain in the mesorectum along with the rectal veins which drain into the superior rectal vein and subsequently into the inferior mesenteric vein with the lymphatic vessels. The middle rectal arteries are branches of the internal iliac arteries. In our experience, they are not always present and are rarely bilateral. They reach the inferior third of the rectum on its anterior lateral surfaces. The functional importance and the size of the middle rectal arteries are inversely proportional to that of the SRA (Ayoub, 1978). Extrarectal communications do not occur with the other rectal arteries. The anastomotic, intramural network is more developed, but is limited primarily in the supra-anal region. The inferior rectal vessels are also branches of the internal iliac vessels. Situated below the pelvic floor, they run through it outside of the anal canal, and join the intramural branches of the superior rectal vessels. The rectum is drained by the lymphatic vessels of the mesorectum which drain into the inferior mesenteric vessels. When the lymphatic vessels are invaded, drainage is directed towards the hypogastric ganglia via the middle rectal vessels. This explains the theory behind the operative technique described by Heald. The somatic nerves of the pelvis are composed of the sacral and the pudendal plexuses. They are interlinked with the autonomic nervous system, from the superior and inferior hypogastric plexuses. It is in close contact with the rectum, accounting for the nerve damage risks involved in mobilization of the rectum. The pelvic autonomic nervous system is comprised of sympathetic and parasympathetic fibers originating from the lumbar and sacral plexuses. This system notably includes the superior hypogastric plexus with its right and left branches, the left and right inferior hypogastric plexus with its efferent branches. It is made up of pre- and post-ganglionic fibers. It extends from the pre- and lateral aortic sympathetic trunks which also have proximal branches leading towards the inferior mesenteric plexus. Situated close to the aortic bifurcation, the SHP gives rise to the right and left superior hypogastric nerves. Initially situated 1 cm to 2 cm one another, medial to the ureters, the superior hypogastric nerves (SHN) then run laterally along the pelvic wall behind the parietal fascia. The left branch is situated behind the superior rectal artery from which it is separated by a fibrous tissue which is the caudal extension of the fascia propria of the rectum. Inferiorly, the SHN trunks, including sacro-rectal-genito-pubic branches, extend towards the postero-lateral edge of the seminal vesicles where anterior afferent branches supply the anterior pelvic organs. On their medial aspect, the SHN also give branches directly to the rectum, notably at the level of the lateral attachments. These nerve trunks may be fixed laterally to the pelvic wall below the parietal fascia or they may be very mobile and therefore at risk of being divided in case of traction during dissection of the lateral attachments. The inferior hypogastric plexuses (IHP) make up the totality of the afferent and efferent fibers leading into the pelvic-perineal organs. They lie on the lateral surfaces of the pelvis and near the lateral attachments of the levator ani muscles, and are therefore under the lateral ligaments and outside of the fascia propria of the rectum. They receive the afferent branches coming from the superior hypogastric plexuses and from the S2 and S3 sacral sympathetic fibers. The deepest sympathetic branches of the pelvic plexuses combine with parasympathetic elements from the spinal cord, cauda equina, then sacral nerve roots emanating from the S3, S4, and even S5 sacral foramina. These branches also supply the pelvic viscera and represent the erectile nerves, among others. All of these nerve branches run deep to the pelvic fascia and travel towards the anterior portion of Denonvilliers' fascia. Avoiding dissection anterior to Denonvilliers' fascia therefore protects these nerve branches.