Atsushi Takenaka md, Division of Urology, Department of Surgery related, Kobe University Graduate School of Medicine, 7-5-1, Kusunoki-cho, Chuou-ku, Kobe, Japan. Email: email@example.com
Objectives: To clarify the topographical relationship between the urethral rhabdosphincter and the rectourethralis muscle as these structures lying dorsally to membranous urethra are important factors to post-prostatectomy urinary continence.
Methods: Pelvic floor specimens including prostate, bulbus penis, and anorectum, obtained from 15 male cadavers (ages at death 66 to 80 years), were examined with standard histologic and immunohistochemical techniques using semiserial sagittal and transverse sections.
Results: The rectourethralis muscle was defined. It was found to be located at the interface between the levator ani muscle and rectum. It was not possible to histologically identify the fibromuscular node known as the perineal body. The urethral rhabdosphincter was found to be inserted into the rectourethralis muscle, which is composed of the smooth muscle fibers. Abundant nerves passed between the rectourethralis muscle and the levator ani, or through the rectourethralis muscle. The urethral rhabdosphincter was closely attached to the apical or ventral portion of the rectourethralis muscle. Morphologically, the membranous urethra was fixed to the rectourethralis muscle through the urethral rhabdosphincter.
Conclusions: The rectourethralis muscle influences the stabilization of membranous urethra. The posterior stitches for the reconstruction of the dorsal musculofascial plate might injure the nerve fibers running along and through the rectourethralis muscle.
Radical prostatectomy (RP) is widely carried out as one of the conventional and optimal definitive treatments for localized prostate cancer because it provides effective cancer control. Postoperative urinary incontinence is one of the considerable drawbacks after RP. Despite progressive improvement over time, postoperative urinary incontinence persists beyond 1 year in 2–5% of patients, representing a major impairment of quality of life.1 To obtain urinary continence or shorten time to continence after RP, several methods have been demonstrated, such as preserving or reinforcing the bladder neck and sparing the puboprostatic ligaments.2,3 According to Lee et al.,4 more attention should be paid to the shape of apical prostate to keep the functional urethral length, which is positively correlated with postoperative urinary continence. A meticulous dissection at the apical prostate is one of the most important surgical factors for recovery of urinary continence after RP.5
Laparoscopic radical prostatectomy (LRP) and robotic assisted laparoscopic radical prostatectomy (RoLRP) provide the fine visualization during their antegrade procedure. Especially during RoLRP, the articulated scissors with 3-D visions assist meticulous dissection not only of the ventral and lateral parts, but also of the dorsal part of the apical prostate.6–8 The structures dorsal to the membranous urethra have been recognized as the aggregation of Denonvilliers' fascia (DF), the perineal body, the rectourethralis muscle (RUM), and the urethral rhabdosphincter (RS). Disregarding the better visualization during RoLRP, the interface between the RS and the other portions of the structures dorsal to the membranous urethra is not clearly identified.
The perineal body is regarded as an important structure of an insertion for DF and the RS.9,10 The relationship between the perineal body and the RUM remains controversial.11,12 Furthermore, to our best knowledge, little is available on the histologic relationship between the RS and the RUM. Thus, the structures dorsal to the membranous urethra are still the subject of controversy. The aim of this study was to clarify the topographical relationship between the RS and the RUM.
Pelvic floor specimens, including the prostate, bulbus penis, and anorectum, were obtained from 15 Japanese male cadavers (ages at death 66 to 80 years). The cadavers, donated to Sapporo Medical University and the Health Science University of Hokkaido School of Dentistry for medical and dental education and research, had been fixed with arterial injection of 10 L of 10% (v/v) formalin solution in water. After routine procedures for preparing paraffin-embedded histologic sectioning, semiserial sagittal sections (seven specimens) and transverse sections (eight specimens) of 10 to 20 µm in thickness were cut at 0.5 to 1.0 mm intervals. Most sections were stained with haematoxylin and eosin, while others were stained immunohistochemically with α-smooth muscle actin (Dako-Smooth muscle actin; for details see Matsubara etal.)13 The sagittal sections included all of the levator hiatus between the right and left sides of the levator ani sling, whereas the transverse sections covered an area extending from the urethra to the anterior rectal mucosa.
The present study was conducted with written permission received from the donors.
Histological identification of the structures in deep perineum
The RS occupied a columnar space along the membranous urethra at a level between the prostate and the bulbus penis ( Figs 1,2). The RS protruded inferiorly as demonstrated by Kokua etal.14 (Fig. 1b). Cowper's gland (CG) was identified between the RS and the bulbus penis (Figs 1,2). The levator ani was easily identified near the prostate and rectum (Fig. 2). Similarly, the bulbospongiosus muscle was identified as a sheet-like muscle surrounding the bulbus penis (Fig. 1). The external anal sphincter was a well-defined muscle immediately ventral or lateral to the rectum (Figs 1,2d). In 2 of 15 specimens, deep transverse perineal muscle (DTP) was large and had a sheet-like structure, extending mediolaterally. The medial end of this sheet-like DTP was attached to CG, the rectal longitudinal muscle, and a rich smooth muscle (Fig. 2). In 13 specimens, DTP was small and had a short, bundle-like shape (Fig. 1). Disregarding the type of DTP, we did not find the connective tissues between the two sides of DTP.
Smooth muscle structure at interfaces between pelvic floor structures
The smooth muscular structure was clearly identified in the area ventral to the rectum in all specimens. The levator hiatus between the two sides of the levator ani sling contained extensions of this smooth muscle, which further extended to the deep perineal area (Fig. 2). We identified those smooth muscles as the RUM. We divided the RUM into two groups, well-developed RUM growing cephalad beyond the levator ani (three specimens; 20%, Figs 1–3b) and poorly-developed RUM limited to the area between the levator ani slings (12 specimens; 80%, Fig. 3a). Figure 3b displays the largest RUM extending dorsally to the apical prostate and dorsolaterally along the external anal sphincter. Abundant nerves passed between the RUM and adjacent structures such as the levator ani and CG (Fig. 3a), or through the RUM (Fig. 3b). Another smooth muscle structure, a rectal longitudinal muscle, was seen as a particularly thick bundle extending inferiorly toward the bulbus penis (Fig. 2b,c). This bundle was essentially embedded in the RUM. The interface between the rectal longitudinal muscle and the RUM was not clearly identified (Figs 1,2).
Correlation between the rectourethralis muscle and other structures in the perineum
In all specimens, the RS was not attached to connective tissues known as the perineal body, but closely attached to the apical or the ventral portion of the RUM. Furthermore no mass-like fibromuscular structure was present in the center of the perineum. In sagittal sections, DF existed between the dorsal aspect of prostate, the RUM, and the RS. Further, DF ended at both the apical portion of the RUM and the RS (Figs 1,3).
A smooth muscular structure encircled by the male urethra, the RS, external rectal muscularis propria, and both sides of the levator ani slings is termed the RUM.11,13,15,16 Clinically, the location of the RUM is important for urinary continence and erectile function, given its proximity to the pudendal nerves and cavernosal nerves, respectively.10,17–20 Brooks etal. demonstrated the RUM as a Y-shaped muscular structure using 3-D computer reconstruction and proposed the RUM to be essentially distinct from the perineal body.11 Sebe etal. demonstrated that the RUM corresponded to an apical portion of the perineal body.12 On the other hand, Strasser etal. documented that the perineal body into which the RS is inserted was composed of connective tissues and smooth muscles.10,21 Thus, the topographical relationship between the RUM and the perineal body remains controversial. In the present study, we identified a smooth muscular structure dorsal to the membranous urethra as the RUM, whereas we did not histologically identify the fibromuscular node termed the perineal body.
The present study showed that the RS was attached to the apical and/or ventral portion of the RUM, and DF ended at the RS and the apical portion of the RUM. Based on this histologic study, the membranous urethra was fixed to the RUM through the RS. Steiner documented that the pubourethral ligament suspended the membranous urethra and distal external urethral sphincter ventrally from the subpubic arch.22 Consequently, we assume that the RS and pubourethral ligament influence the stabilization of the membranous urethra (Fig. 4). We divided the RUMs into two groups; those that were well-developed and those that were poorly-developed. The correlation between the variation of the RUM and urinary continence remained obscure in the present study since it was difficult to clarify the function of the RUM in morphological investigation. However, the present study showed that the RUM influenced the stabilization of the membranous urethra with the RS. Thus, we assume that the RUM is one of the important structures forming a urethral sphincteric complex.
Recently, Rocco et al. described the efficacy of reconstruction of the dorsal musculofacial plate and suspension of the urethral sphincteric complex from the bladder with the posterior stitches for early recovery of urinary continence after RP.23 They reconstructed the dorsal musculofacial plate by suturing the RS and the median fibrous raphe to the remaining DF.24 Myers et al. also reported that there was an apparent midline connection that must be interrupted in the perineal, but not the retropubic radical prostatectomy (RRP). Further, they surmised that the rectourethralis muscle was not encountered in radical retropubic prostatectomy.25 The result from our histologic study was in agreement with their surmise in most of the cases since 80% of the RUM belonged to the poorly-developed group. However, the interface between the RUM and the RS may be obscure in RRP. Consequently, in a well-developed RUM case, one might encounter the RUM during RRP. Our histologic study also indicated that the median fibrous raphe which Rocco et al. handled in their procedure corresponded to the portion of the RUM. The reconstruction of this dorsal musculofascial plate might contribute to preserve the urethral functional length. However, we previously reported that there were nerves running along or through the RUM to the RS and cavernous tissue.18 We assumed that these nerves might be injured by their posterior stitches.
This study revealed the topographical relationship between the RUM and the RS. We also demonstrated that the RUM influenced the stabilization of the membranous urethra. However, a definitive explanation of the role of the RUM for voiding remains obscure in the present study so that further study is needed to clarify the function of the RUM.
In conclusion, the structure dorsal to the membranous urethra was composed of the RS, DF, rectal longitudinal muscle, and the RUM. The fibromuscular node known as the perineal body was not histologically identified in the present study. The RS was closely attached to the RUM. Although the present study did not clarify the function of the RUM, the topographical relationships between the RUM, the RS, and the membranous urethra was disclosed. We assume that the RUM is an important structure for the urethral sphincteric complex. The nerve fibers running along or through the RUM to the RS and cavernous tissue might be injured by the posterior stitches.
We are grateful to Dr Seiji Ohtani and colleagues (Biomedical Center in Sapporo Medical University) for their excellent assistance with immunohistochemistry.