Structural
and Functional Anatomy of Female Urogenital Organs and Pelvic Floor
Introduction
1. Urogenital
female organs & back passage (anorectum)
A- Urinary bladder
B- Urethra
C- Pelvic ureter
D-
Vagina
E- Rectum and anal sphincters
2. Female pelvic floor
A- Pelvic support
B- Levator ani muscle
C-
Pelvic ligaments and fascia
D- Connective tissues
E- Perineal
body
F- Urogenital diaphragm
Introduction
The
anatomy of the pelvic floor includes structures responsible for active and
passive support of the urethrovesical junction, vagina, and anorectum.
Intrinsic and extrinsic properties of the urethrovesical neck and anorectum
allow maintenance of urinary and anal continence at rest and with activity.
Damage to these structures may lead to loss of support and loss of normal
function of the urethra, bladder, and anorectum. Over time, this damage can
result in isolated or combined POP and urinary incontinence (Strohbehn,
1998). Stress continence control
system can be divided into two parts: the system responsible for BN support,
and the system responsible for sphincteric closure (Ashton-Miller, et al,
2001).
1-Urogenital female organs and back
passage
A-Urinary
bladder
It is a hollow, muscular
organ that is the reservoir for the urinary system. The bladder is flat when
empty and globular when distended. The superior surface and upper 1 or 2 cm of
the posterior aspect of the bladder is covered by peritoneum, which sweeps off
the bladder into the vesico-uterine pouch. The anterior bladder is extra peritoneal
and adjacent to the retropubic space. Between the bladder and pubic bones lie
adipose tissue, pubovesical ligaments and muscle, and a prominent venous
plexus. Inferiorly, the bladder rests on the anterior vagina and lower uterine
segment, separated by an envelope of adventitia (endopelvic fascia).
The bladder wall
musculature is often described as having three layers: inner longitudinal,
middle circular and outer longitudinal. However, this layering occurs only at
the BN, the remainder of the bladder musculature is composed of fibers that run
in many directions, both within and between layers. This plexiform arrangement
of detrusor muscle bundles is ideally suited to reduce all dimensions of the
bladder lumen on contraction. The inner longitudinal layer has widely separated
muscle fibers that course multidirectional. Near the BN, these muscle
fibers assume a longitudinal pattern that is contiguous through the trigone
into the inner longitudinal muscular layer of the urethra. The middle circular
layer is prominent at the BN, where it fuses with the deep trigonal muscle,
forming a muscular ring. This layer does not continue into the urethra. The
outer longitudinal layer forms a sheet of muscle bundles around the bladder
wall above the of BN. Anteriorly, these fibers continue past the vesical neck
as the pubovesical muscles and insert into the tissues on the posterior surface
of the pubic symphysis. The pubovesical muscles may facilitate BN opening
during voiding. Posteriorly, the longitudinal fibers fuse with the deep surface
of the trigonal apex and communicate with several detrusor muscle loops at the
bladder base; these loops probably aid in BN closure (Walters and Weber,
1999).
The trigone has two
muscular layers: superficial and deep. The superficial layer is directly
continuous with longitudinal fibers of the distal ureter and is also continuous
posteriorly with smooth muscle of the proximal urethra. The deep muscular layer
of the trigone forms a dense and compact layer that fuses somewhat with
detrusor muscle fibers. The deep layer is in direct communication with a
fibromuscular sheath, Waldeyer’s sheath, in the intravesical portion of the
ureter (Figure 1). The deep trigonal muscle has autonomic
innervation identical to that of the detrusor, being rich in cholinergic
(parasympathetic) nerves and sparse in noradrenergic (sympathetic) nerves. In
contrast, the superficial trigonal muscle has few cholinergic nerves, but a
greater number of noradrenergic nerves (Tanagho, 1986).
B- Female
urethra
The female urethra is a 4cm
long, narrow, membranous canal that extends from the bladder to the external
orifice on the vulvar vestibule (Sogor, 1999). Anatomically, the
urethra can be viewed as follows: the intramural or urethrovesical
junction portion has no defined support surrounding it and it rests entirely on
the anterior vaginal wall and endopelvic fascia. In the 20th to 60th
percentile of the urethra are found striated muscle, the urethral attachments
to the levator muscles, and the pubovesical muscles or ligaments. Moving
distally, the next 20th percentile of the urethra passes through the
urogenital diaphragm and has the compressor urethral muscle and urethral
sphincter muscle. The last 20th percentile is surrounded by the
bulbocavernosus muscle. Female urethra is supplied by inferior vesical and long
vaginal vessels and innervated by pudendal and pelvic nerves (Cruikshank
and Kovac, 1997).
Several theories exist as
to the role of the urethra and adjoining supportive structures in maintaining continence.
Most of these delineate the structural support providing the urethra by the
pubourethral muscles and ligaments, the anterior vaginal wall, and the
endopelvic fascia attached to the arcus tendineus fascia pelvis. Moreover, most
ideas stress one specific anatomic site as a primary factor assisting the
continence mechanism (Cruikshank and Kovac, 1997).
Integral theory is a universal theory of function and
dysfunction in the female pelvis. During closure, twin forward acting muscle
forces (anterior ligaments); stretch the distal vagina to close the urethra
from behind. Backward/downward muscle forces stretch the proximal vagina (and
bladder base) backwards, elongating and “kinking” the proximal urethra against
the anterior ligaments. During micturition, the forward forces relax. The
backward/downward forces then stretch open the urethra and bladder base. This
expands the outflow tract between midurethra and bladder base, vastly reducing
resistance to flow (Petros and Ulmsten, 1990 and 1993). Continence in women is thought to be maintained by
external sphincter mechanism. However, from functional point of view, it
is suggested that posterior vaginal wall and levator ani muscles also contribute
to the support of proximal urethra and closure of distal urethra during cough
is achieved by the striated compressor urethrae and the urethrovaginal
sphincter (Zivkovic et al, 1998) (Figure 2).
C-Pelvic ureter
The ureter enters the pelvis by crossing over the iliac vessels where
the common iliac artery divides into external iliac and hypogastric vessels. At
this point, the ureter lies medial to the branches of the anterior division of
the hypogastric artery and lateral to the peritoneum of the cul-de-sac. As it
precedes more distally, the ureter courses along the lateral side of the
uterosacral ligament and enters the cardinal ligament. The ureter passes
beneath the uterine artery approximately 1.5cm lateral to the cervix. The
distal ureter then moves medially over the lateral vaginal fornix to enter the
trigone of the bladder. Intravesical ureter is about 1.5cm long and is divided
into an intramural segment, totally surrounded by the bladder wall, and a
submucosal segment, directly under the bladder mucosa (Walters and Weber, 1999).
D-VAGINA
The
normal vaginal wall is from 2 to 3 mm thick and consists of an inner mucous
coat, and outer fibrous sheath, and a muscular layer in between. The inner
epithelial layer is stratified squamous epithelium without glands and a fibro elastic
tunica propria.
Figure
1: Normal
ureterovesicotrigonal complex. A: Side view with Waldeyer’s muscular
sheath surrounding vestige of the intravesical ureter and continuing downward
as the deep trigone, which extends to the BN. The ureteral musculature becomes
the superficial trigone, which extends to just short of the external meatus in
the female. B: Waldeyer’s sheath connected by a few fibers to the
detrusor muscle in the ureteral hiatus (quoted from Tanagho, 1986).
Figure 2: The component parts of the urethral support and sphincteric
mechanisms. Anterior vaginal wall and its musculo fascial attachments to the
pelvic diaphragm support the proximal urethra and BN. Inset, Contraction of the
levator ani muscles elevates the anterior vagina and overlying BN and proximal
urethra, contributing to BN closure. The sphincter urethrae, urethrovaginal
sphincter, and compressor urethrae are all parts of the striated urogenital
sphincter (quoted from Waters and Weber, 1999).
Figure 3: Anatomy of vaginal support. The bladder has been
removed at the vesical neck (Quoted from DeLancey, 1992).
This inner mucous
membrane is surrounded by a highly developed venous plexus under autonomic and
hormonal control, accounting for transudation of fluid during sexual arousal. Muscular
coat adjacent to the inner epithelial layer is made up of smooth muscle bundles
orientation allows tremendous vaginal distension, without tearing, such as
occurs during parturition. The outer fibrous coat of the vagina is a dense
sheath of collagen and elastic fibers. This CT sheath merges into the areolar CT,
which joins the vagina to surrounding endopelvic fascia. Vaginal depth and axis
are maintained as a result of multiple but varying muscular and ligamentous
supports along the length of the vaginal wall. Laterally, fibers of vaginal
fibrous layer join in strong bands of CT, which suspend the vagina and maintain
its orientation within pelvis.
Although the lateral
fascial support of the vagina within the pelvis is continuous and
interdependent, it can be anatomically subdivided based on the segment of the
vagina, from proximal to distal, supported. The upper level supports the apex
and proximal vagina and consists of relatively long, fibrous bands arising from
the greater sciatic foramen over the piriformis muscle, the pelvic bones at the
sacral iliac articulation, and from the lateral sacrum. Its fascial support as
well as the levator base-plate inferior supports in almost horizontal plane
orients the upper vagina. This horizontal orientation allows the displacement
of intra abdominal pressure and the downward pressure of the uterus and cervix
towards the posterior vaginal wall and below it, the base plate, rather than
pushing the vagina out the introitus. When this biomechanical orientation is
altered postpartum or iatrogenically, such as following hysterectomy, vaginal
apical eversion can occur (Figure 3).
The middle level of
vaginal support is located at the bladder base and attaches the vagina
laterally and more directly to the pelvic walls in the region of the vagina
between bladder and rectum. These supporting bands are much shorter than those
more proximal, near the vaginal apex. Mid vaginal bands join tendineus arc
laterally on either side. Anteriorly, this fascia located between bladder and
vagina corresponds to the pubocervical fascia, which is imbricate for anterior
repair of a cystocele. The lower third of the vagina is in close proximity to
the urethra. Here the vaginal wall attaches directly to the surrounding
structures within the urogenital diaphragm. As it passes through the urogenital
diaphragm, the lower third of the vagina rises almost vertically from the
introitus (Karram and Walters,
1993).
Perineal ultrasound
findings showed that in all women the vagina was an angulated organ. The mean
angle between the upper and lower vaginal portions was 108°, in both supine and
standing positions (Virtanen et al, 1996).
E-Rectum and
anal sphincters
The rectum extends from
its junction with the sigmoid colon to the anal orifice. The distribution of
smooth muscle is typical for the intestinal tract, with inner circular and
outer longitudinal layers of muscle. At perineal flexure of the rectum, the inner
circular layer increases in thickness to form the internal sphincter. The
internal anal sphincter is under autonomic control (sympathetic and
parasympathetic) and is responsible for 85% of the resting anal pressure. The
outer longitudinal layer of smooth muscle becomes concentrated on the anterior
and posterior walls of the rectum, with connection to the perineal body and
coccyx, and then passes inferiorly on both sides of the external anal
sphincter. The external anal sphincter is composed of striated muscle that is
tonically contracted most of the time and can also be voluntarily contracted.
Various divisions of the external anal sphincter have been described, and
although there is no consensus, recent descriptions favor superficial
(combining the previous superficial and subcutaneous components) and deep
components. The external anal sphincter functions as a unit with the
puborectalis portion of the levator ani muscle group. The anal sphincter
mechanism comprises the internal anal sphincter, the external anal sphincter,
and puborectalis muscle. As with the BN and urethra, a spinal reflex causes the
striated sphincter to contract during sudden increases in intra abdominal
pressure, such as coughing. The anal-rectal angle is produced by the anterior
pull of the puborectalis muscles. These muscles form a sling posteriorly around
the anorectal junction (Madoff et al, 1992).
2-Female pelvic floor
A-Pelvic support
The obturator internus
muscle, similar to the bony pelvis, provides a framework for attachment of the
pelvic floor to the pelvic bone. The obturator internus fascia, referred to as
the arcus tendineus or tendineus arc, is a tense fibrous band of fascia that
traverses the medial aspect of the muscle between pubic bone and ischial spine
bilaterally (Klutke and Siegel,
1995). The pelvic floor muscles functioning as a rigid structure
and provide dynamic support through constant activity, functioning more like a
self regulating trampoline that continually adjusts its tension in response to
changing circumstances (Zacharin, 1980). There are
three supporting layers comprising the pelvic floor: the endopelvic fascia, the
pelvic diaphragm and the urogenital diaphragm (Walters and Weber, 1999).
B- Levator ani
muscle
Although
variation in nomenclature has often confused structural pelvic anatomy, it is
usually agreed that levator ani muscle and levator fascia provides support of
bladder and urethra almost entirely (Raz et al, 1992) (Figure
4). Levator ani
functions as a unit but is described in two main parts: the diaphragmatic part
(coccygeus and iliococcygeus muscles) and the pubovisceral part (pubococcygeus
and puborectalis). Innervation is provided primarily through the anterior
sacral roots of S2, S3, and S4; additional innervation may be provided to pubovisceral
components through branches of pudendal nerve, although this is controversial (Walters
and Weber, 1999).
C- Pelvic
ligaments and fascia
Pelvic
ligaments serve mainly to keep structures in positions where they can be
supported by the muscular activity rather than as weight bearing structures
themselves. The loss of normal muscular
support leads to sagging and widening of the urogenital hiatus and predisposes
patients to the development of POP. Pelvic ligaments and endopelvic fascia
attach the uterus and vagina to the pelvic side walls so these structures can
be supported by the muscles of the pelvic floor. The entire complex then rests
on the levator plate, where it can be closed by increases in intra abdominal
pressure by a "flap-valve" effect (DeLancey, 1993).
Levator
fascia provides support to pelvic structures across its entire surface.
However, 4 specialized fascial condensations provide principal support for the
anterior vaginal wall, specially the pubourethral, urethropelvic, vesicopelvic
and cardinal ligaments (Safir et al, 1999) (Figure
5).
a- Pubourethral ligaments are bilateral
structures; they originate on the pubic bone and the arcus tendineus fascia
pelvis on the point where the arcus joins the anterior levator arch. They
attach superiorly and laterally along the urethra (Cruikshank and Kovac,
1997).
The
pubourethral ligament is the female analogue of
the puboprostatic ligament. Functionally, the pubourethral ligaments protect
against rotational descent of the mid urethra during increases in intra abdominal
pressure and provide passive support to maintain the urethra in a normal retro
pubic position (Safir et al, 1999).
b- Urethropelvic ligaments describe all structures that provide lateral support
of the urethra to the pelvic wall. Urethropelvic ligaments may undergo avulsion
and stretch consequent to vaginal delivery and aging, resulting in
deterioration of the lateral support for the proximal urethra (Safir et
al, 1999).
c- Vesicopelvic ligaments are levator fascia, originating at the tendineus arc
of the obturator and, after splitting upon the approach to the bladder, it is
renamed perivesical fascia on its vaginal and endopelvic fascia on its
abdominal surface (Safir et al, 1999).
d- Cardinal ligaments
Anatomically, the cardinal ligaments are posterior
extensions of the vesicopelvic ligaments. Because of the proximity of the
bladder base to the cervix, deterioration of cardinal ligaments may in tandem
jeopardize support of the bladder base and cervix, leading to cystocele and
uterine descents. At hysterectomy failure to re-approximate the cardinal
ligaments properly during culdoplasty may facilitate future development of the
central cystocele defect (Safir et al, 1999).
Uterosacral
ligaments are a more medial segment of the endopelvic fascia, at the level of
the cervix and upper vagina, and serve to stabilize the visceral structures
posteriorly toward the sacrum (Walters and Weber, 1999).
Figure 4: Sagittal section of pelvis, showing horizontal support
of pelvic viscera by levator ani muscle and fascia (Quoted from Raz et
al, 1992).
Figure 5: Levator fascia from abdominal side of woman. All gray
areas are continuous but depicted distinctly to demonstrate specialization of
levator fascia as it provides support of bladder and urethra (Quoted from
Safir et al, 1999).
Figure 6: Illustration of y configuration formed by the lateral
attachment of the rectovaginal fascia as it converges to the arcus tendineus
fasciae pelvis (quoted from Kenneth et al, 2001).
D- Connective
tissues
It is composed primarily
of elastin and collagen fibers in a polysaccharide ground substance. Connective
tissue is not static; instead, it is a dynamic tissue which undergoes constant
turnover and remodeling in response to stress. Hormonal changes seem to have
significant effects on collagen, and these effects are probably of great
importance during pregnancy and parturition, as well as in aging (Bird,
1984).
E-Perineal body
The body is a
pyramid-shaped structure made up of smooth muscle, skeletal muscle, fibrous and
elastic tissue, as well as nerve fibers and ganglia. The large amount of the
smooth muscle and elastic fibers distends, allowing significant distortion
followed by elastic distensibility is lost, which may occur with surgical or
obstetrical trauma, the vaginal outlet can become physiologically unstable. It
is generally accepted that weakness in this area is a precursor to, or
reflection of, significant problems at one or more levels of pelvic support. The perineal
body represents the point at dorsal attachment of the three muscles of the
perineum: the bulbocavernosus, ischiocavernosus and superficial transverse
perinei. Also attaching at the perineal body are slips of the puborectalis and
pubococcygeus muscles from the pelvic floor as well as fibers from the external
anal sphincter. Superficially, the perineal body is associated with Colles’
fascia (Nichols and Milley, 1973).
F- Urogenital Diaphragm
The muscles of the
urogenital diaphragm reinforce the pelvic diaphragm anteriorly and are
intimately related to the vagina and the urethra. They are enclosed between the
inferior (perineal membrane) and superior fascia of the urogenital diaphragm.
The muscles include the deep transverse perineal muscle and sphincter urethrae (Anderson
and Genadry, 1996).
