Structural and Functional Anatomy of Female Urogenital Organs and Pelvic Floor
1. Urogenital female organs & back passage (anorectum)
A- Urinary bladder
C- Pelvic ureter
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
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
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).
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).
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.
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
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).
Rectovaginal fascia (Denonvillier’s fascia) was noted to attach to the pelvic sidewall. This attachment amounts to a fusion of the rectovaginal fascia with the aponeurosis of the levator ani muscle. It occurs along a well-defined line that begins at the perineal body. This line of attachment converges to the arcus tendineus fasciae pelvis at a point approximately midway between the pubic symphysis and the ischial spine to form a Y configuration on the sidewall of the pelvis (Figure 6). The rectovaginal fascia supports the posterior compartment analogous to the pubocervical fascia in the anterior compartment (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).
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).