The pelvic outlet is diamond shaped with the apices defined by bony landmarks – inferior aspect of the symphysis pubis anteriorly, ischial tuberosities laterally, and tip of the coccyx posteriorly. The outlet can be subdivided into the anterior urogenital triangle and the posterior anal triangle. The lateral edges of the anterior triangle are the ischiopubic rami – which have assumed significance in delineating the obturator foramen for graft anchorage (Fig. 1.2).
Obturator Foramen
The obturator foramen is a large oval window, bounded by the superior and inferior pubic ramus, body of the pubis, ischial ramus, and body of the ischium. The obturator membrane covers this opening almost completely except for a small opening in the superolateral portion of the foramen, the obturator canal through which the obturator neurovascular bundle passes from the pelvis to the medial compartment of the thigh. The obturator internus muscle on the inner side of the obturator membrane originates from the bony margin of the obturator foramen and partly from the pelvic side of the obturator membrane. A curvilinear thickening of the parietal fascia overlying the belly of the obturator internus known as the arcus tendineus levator ani (ATLA) extends from the posterior pubic symphysis to the ischial spine. The obturator externus arises from the outer surface of the obturator membrane and from the pubic and ischial rami and from the medial two-third of the obturator membrane, attaching itself to the greater trochanteric fossa of the femur (Fig. 1.3).
Fig. 1.3
Obturator foramen covered with obturator membrane and the location of obturator canal (Reproduced from: www.teachmeanatomy.com; with permission)
Applied Anatomy
The potential safety of the obturator foramen with the neurovascular bundle occupying only its superolateral portion, has made it a safe zone for insertion of the trocars in the transobturator mesh anchoring techniques.
The Ischial Spine
The ischial spines are bony prominences projecting from the medial surface of ischium at the anterior border of greater sciatic notch. The sacrospinous ligament (SSL) passes medially and posteriorly from the ischial spine to the lateral aspect of lower portion of the sacrum and coccyx. The sacrotuberous ligament extends from the posterior surface of sacrum to the ischial tuberosity, and this along with the SSL separates the greater sciatic foramen from the lesser sciatic foramen.
Fig. 1.4
Sagittal section of the bony pelvis. ATLA arcus tendineus levator ani, ATFP arcus tendineus fascia pelvis, C-SSL coccygeus–sacrospinous ligament complex
Applied Anatomy
The ischial spines are important surgical reference points for several pelvic structures (Fig. 1.4):
The pelvic ureter usually leaves the pelvic sidewall about 1–2 cm from the ischial spine to pass medially on the pubocervical fascia before entering the bladder.
The pudendal nerve and vessel exit the pelvis through the greater sciatic foramen and course beneath the ischial spine and sacrospinous ligament before reentering the lesser sciatic foramen. Identification of ischial spine is essential when performing sacrospinous fixation and planning suture placement to avoid injury to the pudendal neurovascular bundle.
The arcus tendineus fascia pelvis (ATFP) and arcus tendineus levator ani (ATLA) both extend from the posterior surface of the pubic bone and end at the ischial spine.
Muscular Support
The levator ani (LA) muscle constitutes the primary muscular support to the pelvic organs. The LA muscle consists of three components – the pubococcygeus, puborectalis, and iliococcygeus, nomenclatures based on the origin and insertion of the muscle components (Fig. 1.5).
The anterior division of levator ani, the pubococcygeus, sweeps downward from the inner surface of the pubic bone along the sides of the urethra, vagina, rectum, and perineal body. This is further subdivided into the pubovaginalis, puboperinealis, and puboanalis. The pubovaginalis portion attaches to the lateral wall of vagina, the puboperinealis attaches to the perineal body, and the puboanalis portion attaches to the anus at intersphincteric groove.
The puborectalis, most caudal part of levator ani, originates from the inner surface of pubic bone and forms a U-shaped sling behind the anorectal junction and contributes to the anorectal angle.
The iliococcygeus portion arises from the ATLA and ischial spine on both sides and joins with each other at the iliococcygeal raphe and coccyx. The iliococcygeal raphe between the anus and coccyx is referred to as the levator plate and provides support to the rectum, upper vagina, and uterus. The openings between the levator ani muscles through which the urethra, vagina, and rectum pass is known as the urogenital hiatus. The whole expansion of the levator ani along with the coccygeus muscle, perineal membrane, and perineal body is the pelvic diaphragm.
Fig. 1.5
Levator ani muscle – pubococcygeus, puborectalis, and Iliococcygeus. U urethral opening, V vaginal opening, R rectum
Applied Anatomy
The normal resting levators maintain a constant state of contraction and relaxation occurs only during the elimination process (micturition, defecation, and parturition). Contraction of the pubococcygeus elevates the urethra and the anterior vaginal wall helping in urinary continence. Pubococcygeus and puborectalis contraction also elevates the anus and keeps the urogenital hiatus closed. Contraction of the levator ani can be assessed on rectovaginal examination while instructing the patient to squeeze the muscles as if holding bowels. The “U-shaped” muscle is felt along the side and posterior vaginal wall.
Neuromuscular injury to the levators such as during childbirth can lead to widening of the urogenital hiatus and lead to vertical inclination of the levator plate leading to dysfunction or prolapse of the pelvic organs (Fig. 1.6a, b).
Fig. 1.6
Levator plate in horizontal orientation (a) and levator plate with vertical inclination (b) (From: Beco [2]; with permission)
Levator avulsion, a documented injury of childbirth, was first reported as early as 1907. Using ultrasound imaging Dietz identified that levator avulsion involves detachment of the puborectalis portion from pelvic sidewall and it occurs in about 36 % after vaginal delivery. Avulsion can be diagnosed digitally by palpating the inferior pubic ramus and feeling for insertion of the puborectalis portion. In the presence of levator avulsion, 2–3 cm lateral to the urethra, bony surface of the pubic ramus can be palpated devoid of the muscle.
Perineal Membrane
A thick fibromuscular sheet that stretches across the anterior urogenital triangle of pelvic outlet, caudal to levator ani, is the perineal membrane (formerly known as the urogenital diaphragm). It attaches laterally to the ischiopubic rami and has a free posterior margin with anchorage at the perineal body. The urethra and vagina pass through the hiatus in perineal membrane (Fig. 1.7). The perineal membrane hence fixes distal urethra, distal vagina, and the perineal body to bony pelvis at the ischiopubic rami. The superficial perineal space lies external to the perineal membrane and contains the superficial perineal muscles, ischiocavernosus, bulbospongiosus and superficial transverse perineal muscles. The deep perineal pouch lies between the perineal membrane and levator ani and contains the external urethral sphincter, compressor urethrae, urethrovaginalis, and the deep transverse perineal muscles (Fig. 1.8).
Fig. 1.7
Perineal membrane, direction of the fibers and its attachment (From DeLancey [6]; with permission)
Fig. 1.8
Muscles of the deep perineal pouch
Perineal Body
The perineal body situated between distal vagina and anus is a point of convergence for different structures. The superficial perineal muscles, bulbospongiosus and superficial transverse perineal muscles, external anal sphincter, perineal membrane, deep transverse perineal muscles, distal part of the rectovaginal fascia, pubococcygeus and puborectalis portion of the levator ani all insert into this mass of connective tissue (Fig. 1.9). The perineal body plays an important role in the support of distal vagina and maintaining normal rectal function. In reconstructive surgeries, it is therefore important to restore the perineal body anatomy by proper re-approximation.
Fig. 1.9
Perineal body with its muscular attachments
Applied Anatomy
Orientation of the superficial perineal muscles in relation to vaginal outlet is important in reconstruction of the perineum (perineorrhaphy) and in episiotomy repair. In an ideal reconstruction, the widened genital hiatus is narrowed, and perineal body length is maintained.
Connective Tissue Support: Pelvic Fascia and Ligaments
The pelvic fascia has two components: parietal and visceral fascia. The parietal fascia covers the muscles along lateral pelvic wall and on the superior surface of pelvic diaphragm. The fascia covering the obturator internus muscle, obturator fascia, has two thickened portions: arcus tendineus levator ani (ATLA) and arcus tendineus fascia pelvis (ATFP), extending from the ischial spines to posterior surface of pubic bone. Portions of levator ani originate from ATLA, while the ATFP provides the lateral point of endopelvic fascial attachment (see Fig. 1.4).
The existence of the visceral fascia, in certain areas of pelvis and its various nomenclatures, has been an area of controversy in pelvic anatomy. The bladder and vagina are not enclosed in their own fascial layer, and the vagina appears to be separated from bladder anteriorly, only by the adventitial layer of vagina. The existence of a separate fascial layer between anterior vaginal wall and bladder has been of dispute on histological studies, and the nomenclature of pubocervical/pubovaginal fascia is being questioned. On the other hand condensation of the visceral fascia between rectum and vagina, the rectovaginal fascia, is an identifiable separate layer. This extends from the perineal body proximally to about 2–3 cm above hymenal ring. Above this level there is no separate fascial layer, and the endopelvic fascia attaches posterior vaginal wall laterally to pelvic side wall.
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