Pelvic Anatomy for Distal Rectal Cancer Surgery

Worldwide, colorectal cancer is the third most common cancer and one of the leading causes of cancer-related deaths. Currently, total mesorectal excision (TME) is considered as the gold standard surgical procedure for rectal cancer. To achieve a good oncologic outcome and functional outcome after TME in distal rectal cancer, exact knowledge regarding the pelvic anatomy including pelvic fascia, pelvic floor, and the autonomic nerve is essential. Accurate TME along the embryologic plane not only reduces local recurrence rate but also preserves urinary and sexual function by minimizing nerve damage. In the past, pelvic floor muscles and autonomic nerves could not be visualized clearly, however, the development of imaging studies and improvements of minimally invasive surgical techniques such as laparoscopic and robotic surgery can clearly show the anatomy of the pelvic region. In this chapter, we will provide accurate anatomy of the rectum and the anal canal, pelvic fascia, and the pelvic autonomic nerve. This anatomical information will be an important indicator for performing an adequate operation for distal rectal cancer.


Introduction
Colorectal cancer is the third most common cancer and the fourth leading cause of cancer-related deaths worldwide [1]. Especially, rectal cancer accounts for 30-40% of colorectal cancer, and the treatment strategy is different and more complicated compared to colon cancer because of its anatomical features. Although the treatment outcome of rectal cancer has greatly improved with the development of multimodality treatment including neoadjuvant radiotherapy, cytotoxic chemotherapy, and target agents, surgery remains the mainstay of therapy. Since the concept of total mesorectal excision (TME) was first described by Richard Heald in 1979, this procedure became the gold standard technique for rectal cancer surgery until now [2]. The fundamental principle of TME is en bloc resection of the rectum with its surrounding fatty tissue complex which contains the blood vessels and lymphatics down to the pelvic floor. To achieve complete TME and sphincter preserving surgery in low-lying rectal cancer, knowledge for regarding the pelvic fascia (mesorectal, parietal) and autonomic nerves, a thorough understanding of the pelvic floor anatomy is essential.

Fascia structures around the rectum
Dissecting the correct anatomical plane can lead to good oncological outcomes and preserve the autonomic nerves to prevent postoperative urinary, sexual, and defecatory dysfunction. If pelvic dissection is performed along the exact embryologic fascial plane, the operation can be done without bleeding. To perform precise total mesorectal excision, a thorough understanding of the fascia around the rectum and pelvic cavity is essential. Figure 2 shows the anatomical relationship of the fascia around the rectum.

Fascia propria of the rectum and presacral fascia
The rectum and mesorectum are enveloped by the fascia propria of the rectum, also called as mesorectal fascia. The mesorectal fascia corresponds to the visceral fascia of the rectum. Caudally, it ends at the internal sphincter and laterally ends at the internal iliac artery, and is connected to the parietal pelvic fascia [7]. A magnetic resonance image scan (MRI) can clearly show the boundaries of these mesorectum and mesorectal fascia (Figure 3). During total mesorectal excision, it is important to completely excise this mesorectal fascia without damage to obtain optimal oncologic outcome [6,8,9].
The presacral fascia, also called as parietal pelvic fascia, covers the anterior surface of the sacrum and encloses the sacral vessels and nerves. It combines with the mesorectal fascia at the S4 level and became part of the anococcygeal ligament at the level of anorectal junction. The presacral venous plexus is formed by the two lateral sacral veins, the middle sacral vein, and the communicating veins, and it runs underneath the presacral fascia. If the dissection plane is too deep to damage the presacral fascia during the posterior dissection, life-threatening massive bleeding can occur and it often is difficult to control. Therefore, dissection should be done along with the space between the mesorectal fascia and the presacral fascia until the recto-sacral fascia is encountered [10,11].

Recto-sacral fascia (Waldeyer's fascia)
Recto-sacral fascia, also known as Waldeyer's fascia, is a dense connective tissue linking the presacral fascia to the mesorectal fascia at the S4 level. As the posterior dissection proceeds down along the plane between the mesorectal fascia and the presacral fascia, a dense, tough recto-sacral fascia is identified. To enter the retro-rectal space and reach the pelvic floor, this fascia must be incised and dissected further caudally. This fascia has a different thickness from individuals, it is not visible when it is too thin. Because the presacral artery and venous plexus and autonomic nerves pass behind this fascia, it is important to perform sharp division to avoid excessive bleeding due to presacral vein injury (Figure 4) [8,12].

Denonvilliers' fascia
During the anterior dissection of the rectum, a thin, dense connective tissue layer known as the Denonvilliers' fascia presents between the seminal vesicles and   rectum [13]. The rectum can be separated from the seminal vesicles and prostate by opening this membrane at the level of anterior peritoneal reflection. After incising the fascia and entering the embryologic plane between the rectum and the seminal vesicles, the dissection should be performed below the Denonvilliers' fascia [14]. It is because there were neurovascular bundles running from the pelvic plexus to the ventral side of the Denonvilliers' fascia, especially in the directions of 10 and 2 o'clock, and these neurovascular bundles were related to urogenital function ( Figure 5) [15]. However, if the deeply infiltrative tumor is located on the anterior wall of the rectum, the dissection should be performed in front of the Denonvilliers' fascia for curative resection. In females, there is a thin membranous structure that separates the rectum and vagina, which is called the rectovaginal septum. Although Denonvilliers reported that the Denonvilliers' fascia was not present in females, many researchers considered that the rectovaginal septum was consistent with the Denonvilliers' fascia in males ( Figure 6) [16][17][18][19]. During the anterior dissection of the rectum in female, care must be taken not to perforate the vagina since this septum is very thin.

Anal canal
The rectum enters the pelvic floor and becomes the anus. The anal canal is defined as from the dentate line to the anal verge by anatomists, but most surgeons consider the anal canal from the anorectal ring to the anal verge [20]. The anorectal ring is where the rectum enters the pelvic floor and is angled by the puborectalis muscle. This ring can be palpated by a meticulous digital rectal exam. The dentate line, which divides the upper two-thirds and lower third of the anal canal, is an anatomically important landmark of the anal canal, and there are 6-14 longitudinal folds on the dentate line known as columns of Morgagni (Figure 7). The upper and lower part of the anal canal differs in venous and lymphatic drainage, innervation, and the epithelial surface based on the dentate line. Above the dentate line, the blood drains into the portal venous system, and lymphatics drains to the superior rectal and iliac lymphatic chains. Below the dentate line, the blood drains into the caval system, and lymphatics drain into the inguinal lymph nodes.
There are two sphincter muscles surrounding the anus, the internal sphincter and the external sphincter. The internal sphincter is connected from the inner circular smooth muscle of the rectum and descends to 1-1.5 cm below the dentate line. Its length is about 2.5-4 cm and the mean thickness is about 0.5 cm. It is an involuntary smooth muscle and plays an important role in the maintenance of fecal incontinence because it contributes a majority of the resting pressure of the anal canal. The outer longitudinal muscle of the rectum conjoins the fibers from the puborectalis muscle and is located between the external and internal sphincter. The external sphincter muscle is a striated muscle surrounding the internal sphincter in the shape of a cylinder, and it extends slightly below the internal sphincter. The external sphincter consists of three separate parts: subcutaneous, superficial, and deep part. The subcutaneous external sphincter attaches to the perianal skin encircling the anus. The external anal sphincter is innervated by the rectal branch   [20,22,23]. The intersphincteric groove between the internal and external sphincter is an important landmark in surgery for patients with distal rectal cancer such as intersphincteric resection (ISR) [24].

Pelvic floor
The pelvic floor is a structure that forms the bottom of the pelvis, and plays an important role in supporting the pelvic organs. In the past, pelvic floor muscles could not be visualized clearly, however, the development of magnetic resonance imaging assessments and improvements in minimally invasive surgery techniques such as laparoscopy and robotic surgery can clearly show the anatomy of this region It is mainly composed of the levator ani muscle complex: pubococcygeus, iliococcygeus, and puborectalis muscle. The levator ani muscle received direct innervation from sacral nerve roots (S3-S5) and play an important role in cooperative action through coordinated contraction and relaxation during defecation [25]. The pubococcygeus is located in the most anterior portion of the levator ani muscles, and from both pubic bone to the coccyx. The iliococcygeus is the posterior part of the levator ani muscle and extends from the ischial spine to the anococcygeal raphe and coccyx. The puborectalis muscle, which is located below the pubococcygeus, forms a U-shaped ring around the rectum and makes an anorectal angle to prevent fecal incontinence. The coccygeus muscle, which is also a part of the pelvic floor, is located posterior portion of the levator ani muscle and reinforces the posterior pelvic floor (Figure 8) [20]. The pelvic floor has two hiatuses: the urogenital hiatus and the rectal hiatus. The rectal hiatus is located in the posterior of the pelvic floor through which the anal canal passes. The perineal body, a pyramidal fibromuscular mass, is located between the urogenital hiatus and the anal canal, strengthens the pelvic floor [26]. During distal rectal cancer surgery for sphincter preservation such as ISR, the intersphincteric space between the puborectalis muscle and the rectal wall should be identified, and the dissection continues down to the deep part of the anal canal through the intersphincteric space (Figure 9) [24]. On the other hand, during an abdominoperineal resection, the levator ani muscles must be cut [27].

Anococcygeal ligaments
The anococcygeal ligament is a fibrous membrane, which extends between the coccyx and the margin of the anal canal. In an anatomical study, the anococcygeal  ligament was divided into two layers. The ventral layer of the ligament was loose and rich in small and fragile vessels and extended from the presacral fascia to the conjoint longitudinal muscle layer of the anal canal. The dorsal layer of the ligament was thin and dense and extended between the coccyx and external anal sphincter (Figure 10) [28]. To fully mobilize the rectum from the pelvic floor at the final stage of total mesorectal excision, the anococcygeal ligament must be divided. If the anococcygeal ligament cannot be seen in the final step, it can be visualized after the mesorectum is completely mobilized from the pelvic floor.

Surgical plane for very low-lying rectal cancer
In case of very low-lying rectal cancer, several surgical options can be considered (Figure 11). If the tumor did not invade the anal sphincter complex, the ultra-low anterior resection with coloanal anastomosis could be considered. If the tumors are located close to the dentate line, the intersphincteric resection (ISR) could be considered. The ISR is the partial or complete resection of the internal anal sphincter along the intersphincteric plane. However, if the tumor invades the external sphincter complex, the abdominoperineal resection (APR) should be performed. For invasive low rectal cancer which invades the levator ani muscle, extralevator APR (ELAPE) should be considered to achieve adequate resection margin. The ELAPE is the cylindrical anorectal excision and removes more tissue around the tumor including levator ani muscle (Figure 12). This procedure has the advantage of reducing the risk of tumor perforation during operation and acquiring sufficient   safety resection margin, but there is still controversy about the long-term oncologic outcome [29]. In addition, the postoperative complications can be increased due to the wide resection range.

Pelvic autonomic nerve system
In terms of quality of life, the importance of not only oncological outcomes but also functional outcomes such as urinary function, sexual function, and defecatory function after rectal cancer surgery have been emphasized. Urinary dysfunction after rectal surgery occurs in approximately 27%, and it includes difficulty emptying the bladder and incontinence [30,31]. Sexual dysfunction for males consists of erectile dysfunction, absence of ejaculation, or retrograde ejaculation. For females, it causes sexual dysfunction such as impaired ability to achieve orgasm, decreased vaginal secretion, or dyspareunia [15]. The major cause of postoperative urogenital dysfunction is autonomic nerve damage that occurs during surgery. As minimally invasive surgery such as laparoscopy and robotic approach develops, meticulous nerve preserving surgery became possible with good visualization of the pelvic autonomic nerves [32][33][34]. To preserve the postoperative urogenital function, a thorough understanding of the anatomy of the pelvic autonomic nerve is crucial.

Superior hypogastric plexus and hypogastric nerves
The superior hypogastric plexus, which is a collection of sympathetic nerve bundles arising from T10-L3, forms a dense nerve plexus at the anterior area to the body of L5 and bifurcates into hypogastric nerves at the level of the sacral promontory (Figure 13). The superior hypogastric plexus runs around the inferior mesenteric artery. Therefore, this nerve can be damaged during dissection around the origin of the inferior mesenteric artery, and it results in retrograde ejaculation, urinary incontinence [35]. The hypogastric nerve crosses the left common iliac artery at the level of the first sacrum and descends to the pelvic cavity along the lateral pelvic wall.

Pelvic splanchnic nerves
The pelvic splanchnic nerves are considered to be parasympathetic nerves that arise from the second to fourth sacral spinal nerves. These nerves enter the pelvis through the sacral foramen, posterior to the parietal fascia that covers the piriformis  (Figure 14). These nerves regulate the emptying of the urinary bladder and influence erectile functions and motility of the rectum. Therefore, damage to these nerves causes erectile dysfunction and decreased blood flow to the vagina and vulva, which can reduce vaginal lubrication.

Inferior hypogastric (pelvic) plexus
The pelvic splanchnic nerves meet the hypogastric nerves and form the inferior hypogastric plexus at the lateral pelvic wall. It lies outside the fascia propria in the superficial layer of the parietal fascia. The inferior hypogastric plexus can be observed as a mesh-like structure at the posterolateral pelvic wall close to the prostate and seminal vesicles. Because the inferior hypogastric plexus consists of both sympathetic and parasympathetic efferent fibers, any damage to this plexus may cause severe disturbances in urogenital and sexual function including erection and ejaculation. It extends forward to form neurovascular bundles running down the seminal vesicle at 2 o'clock and 10 o'clock direction (Figure 15).  These neurovascular bundles run through the posterolateral border of the prostate and continue to the periprostatic plexus, which supplies to the prostate, seminal vesicles, corpi cavernosi, and the vas deferens [15,36]. Injury to the neurovascular bundles during anterior dissection may cause urinary and sexual dysfunction. Meticulous dissection is required because nerve damage may occur when surgery is performed along the wrong plane or excessive traction is performed.

Conclusion
The rectum is surrounded by a fatty tissue complex called the mesorectum, which contains abundant blood vessels, lymphatics, and lymph nodes. The rectum and mesorectum are enveloped by the mesorectal fascia. During total mesorectal excision, it is important to completely excise this mesorectal fascia without damage. The mesorectal fascia conjoins with the recto-sacral fascia, which extends forward from the presacral fascia at the level of S4, and descends to the pelvic floor. To enter the retro-rectal space and reach the pelvic floor, this fascia must be incised and sharp dissection should be performed to prevent severe bleeding due to injury to the presacral plexus. During the anterior dissection of the rectum, it is important to recognize Denonvillers' fascia located between the rectum and seminal vesicles, and dissection should be performed below the Denonvilliers' fascia. The pelvic floor is a structure that forms the bottom of the pelvis and is mainly composed of the levator ani muscle complex: pubococcygeus, iliococcygeus, and puborectalis muscle. The levator ani muscle received direct innervation from sacral nerve roots (S3-S5) and play an important role in cooperative action during defecation. To reach the deep part of the anal canal, the dissection should be performed between the puborectalis muscle and the rectal wall. During the whole process of TME, surgeons should take care to identify and preserve the autonomic nerve in order to avoid postoperative urogenital dysfunction. Care should be taken not to damage the superior hypogastric nerve during IMA ligation, and not to damage the pelvic plexus during posterolateral pelvic dissection. In addition, during anterior dissection of the rectum, it is important to perform meticulous dissection so as not to injure small numerous neurovascular bundles running in the 2 o'clock and 10 o'clock directions of the seminal vesicle. Based on a sufficient understanding of pelvic anatomy, precise surgical techniques using advanced surgical tools will give favorable oncologic and functional outcomes for rectal cancer patients.