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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 [16]. 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 [16]. 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 [16].
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 [3], 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 [16] 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 [9]. Gosling et al. [6] 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 [6]; 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 [9]. We
therefore believe that the rhabdosphincter is the main
structure pertinent to urinary continence following
surgery of the prostate and that injury to either the
rhabdosphincter or the pudendal nerve will impair
the sphincter mechanism and result in postoperative
urinary incontinence. Further studies will be carried
out to show the function and the age-related changes
of the rhabdosphincter, by means of modern imaging
techniques, as well as additional morphometric
studies.
Rhabdosphincter of the Male Urethra
31
~
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