The characteristics of the urogenital fascia in the retrorectal space based on male cadaveric dissection and its clinical application

Our study provides a comprehensive description of the UGF and its features. The UGF, originating from the renal fascia, comprises the visceral and parietal layers and extends downward to the pelvic floor along the sacrum. The UGF envelops the bilateral HGNs and ureters and extends to the pelvis. At the level of the fourth sacral vertebra, the visceral fascia inverts ventrally and forms the rectosacral fascia. However, the parietal layer continues along the sacrum and ends at the levator ani muscle. The parietal pelvic fascia is located on the surface of the sacrum.

The UGF is known to originate from the renal fascia and envelop the ureters and HGNs. However, there are distinct discrepancies in opinions on the composition of the UGF. In a previous study, the ureters and genital vessels were found to be located in the two layers of the UGF, whereas the HGNs were not mentioned [17]. Nevertheless, in another study, the researchers noted that the HGNs and the ureters are located in the two layers of the UGF [7]. In our previous anatomical study, we stated that the double layers of the UGF ensheath the ureters, vas deferens, genital vessels, and superior hypogastric plexus and extend along the sacrum toward the pelvic floor [13]. Notably, the bilateral HGNs and ureters are covered by the visceral layer. Because the UGF envelops the HGNs, it has been termed a “hypogastric sheath” [18]. In another study of 12 formalin-fixed and 12 fresh cadavers, the researchers concluded that the urogenital-hypogastric sheath consists of multiple layers of renal fascia ensheathing the HGNs, ureters, and genital vessels downward to their terminations in the pelvis [12].

Understanding the UGF may be instrumental in comprehensively understanding the terminology used for different anatomical regions located posterior to the rectum, as it comprises visceral and parietal layers. For example, different designations exist for the fascia covering the HGNs [19‐21]. These concepts were discussed in a previous study [6]. Because the fasciae in the posterior rectum are complex, it is necessary to avoid challenges in differentiating the fasciae. The UGF refers to the fasciae from a holistic and continuous view, which is beneficial for us to further comprehend the complex terminology used for the fascia. A strength of a previous study involving 21 cadavers was that the researchers demonstrated that the prerenal fascia is continuous with the presacral fascia [11]. Nevertheless, they did not describe it as double-layered. Similarly, in another previous study involving histologic identification, the researchers referred to the fascia covering the HGN as the pre-HGN fascia [6]. Although we did not conduct histological examinations in this study, we utilized the cadavers for careful anatomy and analysis to better illustrate the results. Remarkably, the pre-HGN fascia is the visceral layer of the UGF. The parietal layer of the UGF may be referred to as the post-HGN fascia, as it is dorsal to the HGNs and extends along the sacrum. Therefore, the presacral fascia and the pre-HGN fascia are part of the UGF. The double layers of the UGF enveloping the HGNs are the basis for differentiating related fasciae.

In other studies, researchers have shown multiple layers of fasciae in the posterior rectum. Based on macroscopic dissection and histology, a previous study detailed that the parietal pelvic fascia comprises 2 lamellae ensheathing the autonomic pelvic nerves [22]. Notably, we refer to the parietal pelvic fascia here as the UGF. However, the inner lamellae that are inverted ventrally to the rectum and form the rectosacral fascia have not been described. The UGF is more detailed, with the visceral layer of the UGF reversing ventrally to form the rectosacral fascia at the fourth sacral vertebra. This description was consistent with a previous description [22]. In a combined retrospective and prospective study, researchers reported that the rectosacral fascia terminates at the pre-HGN fascia at the level of the fourth sacral vertebra and comprises two layers (upper and lower) [23]. Based on our results, we concluded that the upper and lower segments of the rectosacral fascia they referred to are the visceral and parietal layers of the UGF. Notably, we used the parietal pelvic fascia instead of the piriformis fascia [11] and the presacral fascia [22] referred to in other studies, as they were both a continuation of the transversalis fascia into the pelvis [12]. By combining our results with those of other studies, we concluded that the fasciae between the rectum and the sacrum are the fascia propria of the rectum, the double layers of the UGF, and the parietal pelvic fascia. This conclusion may eliminate confusion regarding fascial terminology to help us better understand key anatomical features.

The mesentery-like structures formed by the UGF and the pelvic plexus in the lateral wall help us to comprehend the lateral rectal ligament (Fig. 4). Previous studies presented different views on the lateral rectal ligament [3, 9, 24, 25]. Similar to our study, another study also observed fascia covering branches of the pelvic plexus, which anchor the rectum to the lateral pelvic wall [12]. In addition, by analyzing and discussing rectal cancer cases, a previous study stated that the lateral ligament is a pathway of blood vessels and nerve fibers extending toward the rectum and lymphatic vessels from the lower rectum toward the iliac lymph nodes [9]. Therefore, instead of the lateral rectal ligament, we propose the symmetrical lateral rectal mesentery to better understand the lateral rectal nerves, blood vessels, and lymphatic metastases. During the laparoscopic TME, the separation of the lateral rectal mesentery (the lateral rectal ligament) is guided by the visceral layer of UGF to protect the HGNs and pelvic splanchnic nerves, as shown in Fig. 4 (b).

According to the UGF’s characteristics, we propose that the retrorectal space is divided into upper and lower segments by the visceral layer of the UGF. On the cranial side of the fourth sacral vertebra, the space between the fascia propria of the rectum and the visceral layer may be called the upper segment of the retrorectal space. Correspondingly, the lower segment of the retrorectal space lies between the visceral and parietal layers on the caudal side of the fourth sacral vertebra. The presacral space is located between the parietal layer of the UGF and the piriformis fascia [22]. Based on this, our team drew a sagittal pattern of the UGF posterior to the rectum (Fig. 5). No nerves or blood vessels are present in the upper segment of the retrorectal space, and the lower segment contains branches of the pelvic plexus (Fig. 6). The parietal layer extends toward the pelvic floor, possibly terminating in the levator ani muscle, which may need to be further confirmed by cadaver studies and in clinical practice.

Based on the above perspectives, the optimal operation plane of the retrorectal space is the upper segment of the retrorectal space between the fascia propria of the rectum and the visceral layer. This plane corresponds to the “holy plane” of TME. Notably, after the operation reaches or crosses the rectosacral fascia, separation is performed in the lower segment of the retrorectal space (Fig. 7). However, the surgical plane always lies between the two layers of the UGF, inevitably causing damage to the related nerves. Dissection separates the parietal layer of the UGF and allows entry into the presacral space. As a result, the sacral nerves and presacral venous may be exposed, and an inappropriate operation leads to bleeding.

This study is the first to propose the double layers UGF as a theoretical guide in TME and expound on the relevant noteworthy details. Comprehending the characteristics of UGF and discerning the relevant, distinct fasciae are fundamental for colorectal surgeons to distinguish the surgical-related plane better, avoid nerve injury or bleeding, and conduct TME faster at the criteria plane. Establishing the retrorectal space based on the characteristics of UGF also provides a guide for separating the lateral rectal mesentery (namely, lateral rectal ligament) and discerning Denonvilliers’ fascia adjacent to the UGF [26]. Moreover, a complete comprehension of the distribution characteristics of the two-layered structure of UGF enhances the understanding of the lymphatic metastasis pattern of the low rectum and the pathway of retrorectal space infection spreading to the cranial side.