The Prostate 2824-3 I ( I 996) Anatomy and Innervation of the Rhabdosphincter of the Male Urethra H. Strasser, G. Klima, S. Poisel, W. Horninger, and G. Bartsch Department of Urology (H.S., W.H., G.B.), Institute of Histology (G.K.), Institute of Anatomy (S.P.), University of Innsbruck, Austria ABSTRACT: The striated sphincter of the male urethra and its innervation are still a subject of controversy. Essentially, two concepts of its anatomy can be found in the literature. Some authors describe the rhabdosphincter as part of the urogenital diaphragm caudal to the prostate, others as a striated muscle which extends from the base of the bladder to the ”urogenital diaphragm.” In a combined anatomic-histologic study the striated sphincter and the pudendal nerve were examined by means of anatomical dissections and serial anatomical as well as histological sections of 12 male pelves. Furthermore, radical prostatectomy was performed in a cadaver specimen; subsequently, the so-called “urogenital diaphragm” was excised and then examined histologically. The varying number of striated muscle fibers caudal to the prostate is of particular interest. In fetuses, there are abundant striated muscle fibers dorsal to the membranous urethra, where they are arranged as a circular collar around the urethra. In the adult male, hardly any striated muscle fibers can be found dorsal to the urethra; in a majority of cases this region is devoid of striated muscle fibers. Inserting dorsally in the perineal body, the fibers form an omega-shaped loop around the anterior and lateral aspects of the membranous urethra. The existence of a “urogenital diaphragm” and a strong, circular, striated “external sphincter urethrae” completely encircling the urethra caudal to the apex of the prostate could not be confirmed by our anatomical and histological investigations. Our study shows that the striated muscle fibers run in a cranial direction from the bulb of the penis to the base of the bladder along the anterior and lateral aspects of the prostate and the membranous urethra. Further dissection studies revealed that the rhabdosphincter is supplied by branches of the pudendal nerve after leaving the pudendal canal. 0 1996 Wiley-Liss, Inc. KEY WORDS: rhabdosphincter of the male urethra, external striated urethral sphincter, male urinary continence INTRODUCTION The striated sphincter of the male urethra, the socalled rhabdosphincter, is discussed controversially in the literature. As this muscle contributes significantly to urethral closure pressure, urologists have become more and more interested in its morphology and function. Essentially, two standard concepts of its anatomy existed so far: while most authors described this structure as part of the “urogenital diaphragm” caudal to the prostate [7,8,10,24], others defined it as a striated muscle which extends from the base of the bladder to the region of the ”urogenital diaphragm” [16,22]; some authors even questioned 0 1996 Wiley-Liss, Inc. the existence of a classical ”urogenital diaphragm” extending between the pubic rami . In this article, we want to present the combined anatomical-histologcal study which we have undertaken to investigate the shape and the innervation of the rhabdosphincter of the male urethra. Received for publication October 17,1994; accepted February 15, 1995. Address reprint requests to Dr. Hannes Strasser, Department of Urology, University of Innsbruck, Anichstrasse 35, A-6020 Innsbruck, Austria. Rhabdosphincter of the Male Urethra MATERIALS AND METHODS The rhabdosphincter was examined by means of anatomical dissections and serial anatomical as well as histological sections of 12 male human pelves. Six complete pelves and six tissue blocks of prostate, rectum, membranous urethra, and the region of the “urogenital diaphragm” were investigated. Age of the cadavers used for our studies ranged from 52 to 78 years, including one fetal specimen (5 months) whose complete pelvis was studied by means of series of sagittal sections. We used normal haematoxylin-eosin-staining, azan-staining, and a variation of Masson-Goldner-staining. Sections were made in sagittal, horizontal, and coronal planes. Furthermore, radical prostatectomy was performed in a cadaver specimen; the region of the “urogenital diaphragm” was excised and then examined histologically. The cavernous nerves as well as the pudendal nerve and its branches supplying the rhabdosphincter were dissected along their complete course. For this reason the pubic symphysis was severed; subsequently the right hip bone was removed, while the pelvic organs were left intact; thus we were able to achieve complete exposure of the rhabdosphincter and the pudendal nerve. Magnifymg lenses and a dissecting microscope were employed for the dissection of the small nerve fibers supplying the rhabdosphincter. All different steps of this extensive study were documented by means of photographs and paintings. 25 omega-shaped loop around the urethra. Furthermore, the rhabdosphincter is separated from the ventral portion of the levator ani muscle by a sheet of connective tissue. Fetal Histology In the fetus (5 months) the striated fibers of the rhabdosphincter constitute a muscular sheath between the bulb of the penis and the bladder neck. Its morphologically dominating part forms a collar of striated muscle fibers enclosing the membranous urethra on the ventral, lateral, and dorsal aspects. More cranially, this cylinder-shaped muscular sheath is incomplete; on the dorsal aspect of the prostate no striated fibers can be found (Fig. 4). On histological examination the smooth muscle fibers of the membranous urethra and the striated fibers of the rhabdosphincter are easily distinguishable. In the fetus, the rhabdosphincter is very strong and represents the dominant muscular structure in the small pelvis. A horizontal muscular pIate corresponding to the ”deep transverse perineal muscle” could not be found (Fig. 4). The lateral and distal margins of the rhabdosphincter radiate laterally and dorsally. These fibers-lying approximately in a horizontal plane-correspond to the ”deep transverse perineal muscle” of the adult male described in most anatomical and urological textbooks and publications of pelvic surgery. Histology of the Adult Male The histological findings in adult males confirmed the data gained by gross anatomical examination (Fig. RESULTS Macroscopic Anatomy Neither in the anatomical dissections nor in the serial sections of adult males did we find any evidence of the classical ”urogenital diaphragm.” Conversely, the muscle fibers in this region were found to be arranged in a loop-shaped fashion on the ventral and lateral aspects of the membranous urethra (Figs. 1,2). On gross anatomical examination comparatively strong smooth muscular and connective tissue was noted dorsal to the membranous urethra, i.e., in the region of the perineal body. Both ends of the omegashaped sphincter insert at the perineal body. The sphincter-loop is continuous with muscle bundles which run along the anterior and lateral aspects of the prostate and extend cranially to the bladder neck (Fig. 3). Thus, in the adult male the rhabdosphincter of the urethra does not form a complete collar around the membranous urethra. It should rather be described as a muscuIar coat ventral and lateral to the membranous urethra and prostate, the core of which is the Fig. I. Anatomical specimen (adult rhabdosphincter. cranial view): I = rhabdosphincter, 2 = urethra, 3 = perineal body, 4 = dorsal vein complex. Figures 1-5 appear on page 26. Fig. 2. Corresponding drawing t o Figure I (cranial view): I = dorsal vein complex, 2 = rhabdosphincter. 3 = urethra, 4 = perineal body, 5 = rectum, 6 = right and left levator ani muscle, 7 = right and left hip bone (pubic symphysis has been removed). Fig. 3. Anatomical specimen (adult, lateral view, sagittal section): I = urethra, 2 = prostate, 3 = rectum, 4 = bulb of the penis, 5 = rhabdosphincter, 6 = perineal body, 7 = right bulbo-urethral gland. Fig. 4. Histological specimen (fetal, sagittal section): I = bladder, 2 = pubic symphysis, 3 = rectum, 4 = rhabdosphincter, 5 = bulb of the penis, 6 = urethra. Fig. 5. Histological specimen (adult, sagittal section): I = urethra, 2 = apex of the prostate, 3 = rectum, 4 = habdosphincter, 5 = perineal body, 6 = bulb of the penis. Figs. 1-5 Figs. 6-9 28 Strasser et al. 5). The rhabdosphincter was found to be a vertical muscular coat ventral and lateral to the prostate and membranous urethra extending from the bulb of the penis towards the region of the bladder neck. In all investigated specimens, striated muscle fibers corresponding to the ”deep transverse perineal muscle” could not be identified. In contrast to the findings obtained in the fetus, striated muscle fibers could not be detected dorsal to the membranous urethra on histological examination. The caudal fibers of the rhabdosphincter are arranged in an omega-shaped loop ventral and lateral to the urethra, which is in contradiction to the standard descriptions of the “external urethral sphincter.” The proportions of the different components of this mechanism seem to change with advancing age; the amount of connective tissue and non-striated muscles is, by far, higher in the adult male than in the fetus. Dorsal to the urethra, i.e., in the region of the perineal body-where the two insertions of the rhabdosphincter are located-only irregularly arranged connective tissue and smooth muscle cells were found (Fig. 5). The rhabdosphincter represents an independent muscle unit that is not in direct contact with the fibers of the levator ani muscle. Nerve Supply Following morphological assessment, the nerve supply of the rhabdosphincter was studied. Theoretically, it could be supplied either by the pudendal nerve [5,6,18] or by the pelvic plexus [3,12,15]. The parasympathetic fibers coming from the sec- ond to the fourth sacral spinal segments first run to the right and left pelvic plexus which give off nerve fibers to both prostatic plexus. The right and the left prostatic plexus are directly adjacent to the seminal vesicles. The cavernous nerves then course caudally as part of the “neurovascular bundle” which lies in the triangle of connective tissue between the levator ani, the prostate, and the rectum. Lateral to the membranous urethra, these nerve fibers pass the urogenital hiatus, finally reaching the corpora cavernosa [1,2,17,20,21]. The pudendal nerve derives from the second, third, and fourth sacral spinal nerves. It takes a completely different course than the autonomic fibers of the pelvic plexus. By removing the complete right hip bone, we were able to expose the complete nerve, leaving the pelvic organs, the pelvic plexus, the rhabdosphincter, and the region of the ”urogenital diaphragm” intact. The pudendal nerve was dissected along its entire course; its branches were prepared with the help of magnifymg lenses and a dissecting microscope (Figs. 6,7). Thus, we were able to demonstrate that the rhabdosphincter of the male urethra is supplied by fine branches of the pudendal nerve. These branches are given off lateral to the rhabdosphincter and reach the muscle at its dorsolateral aspects; the mean distance from the membranous urethra to the point of entry of these fibers into the rhabdosphincter is 0.7 to 1.3 cm (Fig. 8). In all specimens we were not able to dissect anastomoses between the pudendal nerve and the cavernous nerves. Furthermore, no branches of the cavernous nerves reaching the rhabdosphincter could be found. Applied Anatomy of the Rhabdosphincter in Radical Prostatectomy Fig. 6. Anatomical specimen (adult, lateral view; the right hip bone has been completely removed): I = levator ani muscle, 2 = pudendal nerve, 3 = rhabdosphincter, 4 = pubic symphysis. 5 = prostate, 6 = nerve fibers innervating the rhabdosphinner. 7 = right crus penis. Figures 6-9 appear on page 27. Fig. 7. Corresponding drawing t o Figure 6 I = levator ani muscle, 2 = pudendal nerve, 3 = rhabdosphincter. 4 = pubic symphysis, 5 = prostate, 6 = nerve fibers innervating the rhabdosphincter. 7 = right crus penis. Fig. 8. Anatomical specimen (adult, caudal view; the bulb of the penis has been removed): I = rhabdosphincter. 2 = urethra, 3 = perineal body (transected), 4 = pudendal nerve, 5 = fibers innervating the rhabdosphincter. Fig. 9. Radical prosratectomy (the membranous urethra and the rhabdosphincter have just been divided): I = dorsal vein complex, 2 = rhabdosphincter. 3 = membranous urethra. 4 = perineal WY. Radical prostatectomy was performed in a specially fixed (alcohol-glycerine) cadaver specimen to study the direct clinical application of this anatomicalhistological study. Following division of the dorsal venous plexus, the anterolateral aspects of the external rhabdosphincter can be identified. The next step involves division of the rhabdosphincter and the membranous urethra (Fig. 9). This apical dissection should be done as far proximally and medially as feasible without including the apex of the prostate to leave an effective amount of sphincter tissue intact and to preseve the fibers of the pudendal nerves which innervate the rhabdosphincter (Figs. 6,7). The first two anastomotic sutures, passed forward through the membranous urethra and rhabdosphincter, are used to elevate the anterior portion of the sphincter and the urethra. After division of the membranous urethra, the Rhabdosphincter of the Male Urethra Fig. 10. Radical, prostatectomy (the posterior prostatic faxia has been severed from its attachment t o the perineal body): I = ligated dorsal vein complex, 2 = rhabdosphincter, 3 = membranous urethra, 4 = perineal body, 5 = ventral wall of the rectum. Fig. 11. Histological specimen (coronal section, dorsal view): I = prostate, 2 = membranous urethra, 3 = left levator mi mus- posterior adventitial “fascia” of the prostate (“Denonvilliers’ fascia”) and the rectourethral septum are severed from the perineal body to open the rectogenital space (Fig. 10). The rectal fascia is then identified at the posterior aspect of the rectogenital space. The loop-shaped rhabdosphincter can be clearly seen during the operation. This fact is of great importance since the anatomic relations between this muscle, the levator ani muscle, the membranous urethra, and the pudendal nerves play a crucial role in radical 29 cle, 4 = right levator ani muscle, 5 = rhabdosphincter, 6 = left crus of penis, 7 = right crus of penis, 8 = corpus spongiosum penis. Fig. 12. Schematic drawing of the rhabdosphincter of the male urethra: BL = urinary bladder, PR = prostate. U = urethra, SUS = striated urinary sphincter = rhabdosphincter. prostatectomy, particularly with regard to postoperative urinary continence (Fig. 11). DISCUSSION In 1873, Henle stated that the “M. sphincter vesicae externus” surrounds and compresses the membranous urethra [101. Furthermore, he described two “aponeuroses,” an upper and a lower, which contain the “M. transversus perinei profundus.” The concept of a “urogenital diaphragm” and a circular external 30 Strasser et al. sphincter which is more or less in direct contact with the levator ani muscle and the “deep transverse perheal muscle” dates from these anatomical descriptions. This model which is still the standard concept in most textbooks and scientific publications implies that the urethral sphincter is a horizontal plane of muscle fibers extending between the two ischiopubic rami and surrounding the membranous urethra [7, 8,241. In 1980, Oelrich showed that the rhabdosphincter is not a transverse muscular plate but a vertical muscle . By studying the muscular structures around the urethra in tissue blocks obtained from fetuses and adult males, he stated that the rhabdosphincter extends from the bulb of the penis to the regon of the bladder neck . With the increase in interest in radical prostatectomy, urologic surgeons have become more concerned about the anatomical basis of pelvic surgery to maintain functional integrity after operation. Most authors described the urethral sphincter as a circular muscle that completely encloses the membranous urethra [13,16,22,23]; but in the last few years some authors stated that the urethral sphincter is not a complete ring around the urethra [4,14,19]. The purpose of the present study has been to analyze the morphology of the rhabdosphincter of the male urethra in detail, in order to provide basic anatomical data. Our results disagree with the standard concepts of the striated sphincter of the male urethra. The rhabdosphincter is not a horizontal muscular plate corresponding to the “deep transverse perineal muscle” and the “external urethral sphincter” but a vertical muscle extending from the bulb of the penis to the region of the bladder neck (Fig. 11). Neither in the anatomical dissections nor in the histological sections did we find any evidence of a ”urogenital diaphragm.’’ The existence of a horizontal muscular plate caudal to the apex of the prostate corresponding to the “deep transverse perineal muscle” could not be confirmed (Fig. 11). The striated fibers which course ventral and lateral to the urethra form an omega-shaped sphincter both ends of which insert at the perineal body in all our adult specimens (Fig. 12). Oelrich and Walsh et al. correctly described the rhabdosphincter as a vertical muscle between the bulb of the penis and the urinary bladder [16,22,23]. But contrary to their observations, it does not form a complete collar around the membranous urethra in the adult male. Our anatomicalhistological data confirm the concept that the rhabdosphincter has an omega-shaped configuration in the adult male [4,14,19]. The exact nerve supply of the rhabdosphincter has been a subject of controversy. Most authors maintain that the rhabdosphincter is innervated by the pudendal nerve [5,6,18,19], others believe that it is under the control of the autonomic pelvic plexus [3,12,15]. By using magnifying lenses we were able to demonstrate the tiny branches of the pudendal nerve which supply the rhabdospincter (Figs. 6-8). These nerve fibers and the remaining rhabdosphincter-loop have to be saved in the course of radical prostatectomy to obtain good postoperative continence results. Contrary to other anatomical descriptions , we were not able to dissect any nerve fibers of the cavernous nerves or pelvic plexus supplying the rhabdosphincter. The proportions of striated and smooth muscle fibers change during life. In the fetus, the number of striated fibers of the rhabdosphincter, which at this stage of development forms a collar around the urethra, is comparatively high (Fig. 4); in the adult male, on the contrary, the rhabdosphincter seems to be more and more replaced by smooth muscle fibers and connective tissue (Fig. 5). These findings confirm the data of Oelrich  and Kokoua et al. (111 regarding the morphologic changes of the rhabdosphincter with advancing age. These changes could be the reason for the increasing incidence of postoperative urinary incontinence in men older than 70 years. During contraction, the rhabdosphincter pulls the urethra towards the perineal body to compress the membranous urethra . Gosling et al.  showed that significant morphological differences exist between the muscle fibers of the rhabdosphincter and the levator ani muscle (Fig. 11).The rhabdosphincter consists of fibers which are functionally capable of maintaining tone over prolonged time periods without fatigue ; therefore this loop-shaped sphincter caudal to the apex of the prostate is capable of producing steady tonic compression of the membranous urethra. As the rhabdosphincter is, by far, the most prominent muscular structure caudal to the apex of the prostate, this muscle appears to be the core of the mechanism responsible for urinary continence in the adult male after radical prostatectomy. Furthermore, transurethral sonographic evaluation of the rhabdosphincter reveals that injuries to this muscle inevitably lead to urinary stress incontinence . 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