**2. Pelvic anatomy**

### **2.1 Basic anatomy of the rectum and mesorectum**

The rectum is the most distal part of the large intestine that exists from the sacral promontory level to the anorectal ring. The anterior and lateral portion of the upper one-third of the rectum is covered with peritoneum, and the middle onethird of the rectum is covered with peritoneum on its anterior portion. The lower one-third cannot be observed in the intraperitoneal space because it is located in the extraperitoneal space. The taenia coli disappears in the rectum, forming one longitudinal muscle layer surrounding the rectum. The length of the rectum is approximately 12-15 cm and has three curvatures, which is related to Houston's valves. The upper and lower part are convex to the right, and the middle portion is convex to the left. The middle valve is the most prominent and is located approximately equal to the level of peritoneal reflection [3].

The rectum is surrounded by a fatty tissue complex called the mesorectum, which corresponds to the mesentery of the rectum. Mesorectum contains abundant blood vessels, lymphatics, and lymph nodes, and it is enveloped by thin visceral pelvic fascia [4]. It is developed thickest in the posterolateral side and the anterior part is formed relatively thin. In addition, the volume of the mesorectum decreases as it approaches the pelvic floor, and disappears approximately 2 cm above the levator ani muscle (**Figure 1**). A number of studies have revealed that the mesorectum is an important structure for tumor spreading, and en bloc resection through sharp dissection of mesorectum is very important in improving treatment outcomes [2, 5, 6].

#### **2.2 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.

#### **Figure 1.**

*Anatomy of the rectum and mesorectum. (a) Structures around the rectum. The rectum is surrounded by mesorectum, and the rectum and mesorectum are enveloped by the fascia propria of the rectum. (b) Total mesorectal excision (TME). En bloc resection of mesorectum is important.*

*Pelvic Anatomy for Distal Rectal Cancer Surgery DOI: http://dx.doi.org/10.5772/intechopen.99120*

#### **Figure 2.**

*Anatomy of fascia around the rectum. The fascia propria of the rectum covers the rectum and mesorectum. The presacral fascia covers the anterior surface of the sacrum. It combines with the fascia propria of the rectum at the S4 level (recto-sacral fascia = Waldeyer's fascia). Denonvilliers' fascia is a dense membrane between the rectum and seminal vesicles.*

#### *2.2.1 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].

#### *2.2.2 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

#### **Figure 3.**

*Magnetic resonance image scan. Magnetic resonance image scan (MRI) can clearly show the boundaries of these mesorectum and mesorectal fascia. (a) T2 weighted image on MRI. Axial view. The rectum and mesorectum are enveloped by the fascia propria of the rectum (mesorectal fascia). (b) T2 weighted image on MRI. Coronal view. Mesorectum, mesorectal fascia, and puborectalis muscle.*

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].

### *2.2.3 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

#### **Figure 4.**

*Recto-sacral fascia (Waldeyer's fascia). Recto-sacral fascia (Waldeyer's fascia) is a dense connective tissue linking the presacral fascia to the mesorectal fascia at the S4 level. It is important to perform sharp dissection [11].*

#### **Figure 5.**

*Denonvilliers' fascia. During anterior dissection of the rectum. The dense connective tissue between rectum and seminal vesicles can be seen. The dissection should be performed below the Denonvilliers' fascia.*

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**) [16–19]. During the anterior dissection of the rectum in female, care must be taken not to perforate the vagina since this septum is very thin.
