Basic understanding of pelvic anatomy is key to understanding pathologic processes. Pelvic anatomy must be understood as relationships among visceral organs, connective tissues, muscles, and nerves. Conceptualizing the three-dimensional (3D) aspects of these structures is useful when performing reconstructive pelvic surgery.
The female external genitalia, or vulva, include the mons pubis, labia majora and minora, clitoris, vestibule, vestibular bulbs, greater (Bartholin) and lesser vestibular glands, Skene glands, and the distal urethral and vaginal openings (Figure 2-1).
The skin over the mons pubis and labia majora contains hair, and the subcutaneous layer is similar to that of the anterior abdominal wall. The subcutaneous layer consists of a superficial fatty layer similar to Camper’s fascia, and a deeper membranous layer, Colles’ fascia, which is continuous with Scarpa’s fascia of the anterior abdominal wall (Figure 2-2).
The firm attachments of Colles’ fascia to the ischiopubic rami laterally and the perineal membrane posteriorly prevent the spread of blood or infection from the superficial perineal space to the thighs or posterior perineal triangle. Anteriorly, the continuity of Colles with Scarpa fascia may allow the spread of blood and infection between these compartments. This continuity may also be apparent when subcutaneous gas is noted in the vulva during laparoscopic cases.
The inguinal canal allows communication between the intra-abdominal cavity and the subcutaneous tissue of the mons and labia majora. The round ligament and obliterated processus vaginalis exit the inguinal canal through the external or superficial inguinal ring and attach to the subcutaneous tissue or skin of the labia majora. An abnormally patent processus vaginalis, also known as the canal of Nuck, can result in hydroceles or inguinal hernias. Therefore, the differential diagnosis of a mass in the labium majus should include a round ligament leiomyoma and an inguinal hernia.
In contrast to the skin that overlies the labia majora, the skin of the labia minora does not contain hair and the subcutaneous tissue consists primarily of loose connective tissue (Figure 2-3). The labia minora lie between the labia majora and contribute to the lateral boundaries of the vestibule as described below. Anteriorly, each labium minus separates to form two folds that surround the glans of the clitoris. The prepuce is the anterior fold that overlies the glans, and the frenulum is the fold that passes below the clitoris. Posteriorly, the labia minora end at the fourchette.
The loose connective tissue underlying the skin of the labia minora allows mobility of the skin during sex and justifies the ease of dissection during vulvectomy.
Chronic dermatologic diseases such as lichen sclerosus may lead to significant atrophy of the labia minora and vulvar pain conditions such as vestibulitis can lead to significant vulvar irritation and pain.
This is the female erectile structure that is homologous to the penis. It consists of a glans, a body, and two crura. The glans contains many nerve endings and is covered by a mucous membrane. The body measures approximately 2 cm and is attached to the pubic ramus by the crura.
This area is bounded by Hart’s line (Figure 2-1) on the labia minora laterally, the hymeneal ring medially, the prepuce anteriorly, and the fourchette posteriorly (Figure 2-4). The Hart line represents the line of transition between the darker skin of the labia minora and the lighter mucous membrane on the inner surface of the labia minora. The vestibule contains the openings of the urethra, vagina, greater (Bartholin) and lesser vestibular glands, and Skene glands. A shallow posterior depression, known as the navicular fossa, lies between the vaginal opening and the fourchette.
Localized vestibulitis is characterized by pain with vaginal penetration, localized point tenderness, and erythema of the vestibular mucosa.
The incision for Bartholin gland drainage or marsupialization should be kept medial or inside Hart line in attempts to restore normal gland duct anatomy1 and avoid visible scars on the vulva.
These are homologues to the bulb of the penis and corpus spongiosum of the male. They are two richly vascular erectile masses that surround the urethra and vaginal opening, and are partially covered by the bulbocavernosus muscles (Figure 2-4). They are found superficial to the perineal membrane and their posterior ends are in contact with Bartholin glands. Anteriorly, they are joined to one another and to the clitoris.
The proximity of the Bartholin glands to the vestibular bulbs accounts for the significant bleeding often encountered with Bartholin gland excision.
These are the homologues of the male bulbourethral or Cowper glands. They are in contact with, and often overlapped by, the posterior ends of the vestibular bulbs. Each gland duct opens at the vaginal vestibule at approximately 5- and 7-o’clock positions.
Contraction of the bulbocavernosus muscle during sexual arousal stimulates gland secretion of small amounts of mucous material, which may serve to lubricate the vaginal opening.
Obstruction of the Bartholin ducts from inflammation or infection can lead to symptomatic cysts or abscesses, which are surgically drained. As discussed above, the incision should be made inside the Hart’s line. Symptomatic or recurrent cysts may require marsupialization or gland excision.
Key Point
An arbitrary line joining the ischial tuberosities divides the perineum into the anterior or urogenital triangle, and a posterior or anal triangle.
The perineum is the diamond-shaped area between the thighs that extends from the skin in this area to the inferior fascia of the pelvic diaphragm superiorly. The anterior, posterior, and lateral boundaries of the perineum are the same as those of the bony pelvic outlet. These include the pubic symphysis anteriorly, ischiopubic rami and ischial tuberosities anterolaterally, coccyx posteriorly, and sacrotuberous ligaments posterolaterally. An arbitrary line joining the ischial tuberosities divides the perineum into the anterior or urogenital triangle, and a posterior or anal triangle (Figure 2-3).
Key Point
The perineal membrane further divides the anterior perineal triangle into a superficial and a deep space. The deep space is bounded superiorly by the inferior fascia of the levator ani muscles.
The structures that comprise the external female genitalia or vulva lie in the anterior perineal triangle. The base of this triangle lies between the ischial tuberosities and generally overlies the superficial transverse perineal muscles. The perineal membrane further divides the anterior perineal triangle into a superficial and a deep space (Figure 2-5). The superficial perineal space lies below or inferior to the perineal membrane and the deep space lies above or superior to the membrane.
This space lies between Colles’ fascia inferiorly and the perineal membrane superiorly. It contains the striated ischiocavernosus, bulbocavernosus, and superficial transverse perineal muscles, and branches of the pudendal vessels and nerve. In addition, the clitoris, vestibular bulbs, and Bartholin glands lie within the space and the urethra and vagina traverse it (Figure 2-5).
This muscle attaches to the medial aspect of the ischial tuberosities posteriorly and the ischiopubic rami laterally. Anteriorly, it attaches to the crus of the clitoris. The ischiocavernosus muscle is innervated by muscular branches of the perineal nerve, which is a branch of the pudendal nerve. The dorsal nerve of the clitoris courses deep into this muscle.1
Contraction of the ischiocavernosus muscle during sexual arousal may help maintain clitoral erection by compressing the crus of the clitoris, thus delaying venous drainage.
These muscles, also termed bulbospongiosus muscles, surround the vaginal opening and partially cover the superficial portion of the vestibular bulbs and Bartholin glands. They attach to the body of the clitoris anteriorly and the perineal body posteriorly.
Contraction of the bulbocavernosus muscles may contribute to the release of secretions from Bartholin glands. They may also contribute to clitoral erection by compressing the deep dorsal vein of the clitoris.
These are narrow strips of muscles that attach to the ischial tuberosity laterally and the perineal body medially.
The superficial transverse perineal muscle may be attenuated or even absent, but when present, contribute to the perineal body as discussed later.
This “space” lies deep into the perineal membrane. It is partially bounded superiorly by the inferior fascia of the levator ani muscles. In women, the levator muscles lack direct attachments to the urethra and thus, the “deep space” is continuous with the pelvic cavity as described below.2 Parts of urethra and vagina, branches of the internal pudendal artery, and portions of the dorsal nerve and vein of the clitoris are found within this area. It also contains the compressor urethrae, urethrovaginal sphincter, and external urethral sphincter muscles as described later in the chapter.
Traditionally, the urogenital diaphragm is described as consisting of the deep transverse perineal muscles and sphincter urethrae muscles between the inferior fascia of the urogenital diaphragm (perineal membrane) and the superior fascia of the urogenital diaphragm. However, the term “diaphragm” generally implies a closed compartment. As described above, the deep space is an open compartment. It is bounded inferiorly by the perineal membrane and extends up into the pelvis.2 As a result, when describing perineal anatomy, the terms urogenital diaphragm and inferior fascia of the urogenital diaphragm are misnomers and have been replaced by the anatomically correct term perineal membrane.3
The perineal membrane has recently been shown to consist of two histologically, and probably functionally, distinct portions that span the opening of the anterior pelvic outlet.4 The dorsal or posterior portion consists of a sheet of dense fibrous tissue that attaches laterally to the ishiopubic rami and medially to the distal third of the vagina and to the perineal body. The ventral or anterior portion of the perineal membrane is intimately associated with the compressor urethrae and urethrovaginal sphincter muscles, previously called the deep transverse perineal muscles in the female.2 In addition, the ventral portion of the perineal membrane is continuous with the distal insertion of the arcus tendineus fascia pelvis, which can best be appreciated during dissection of the retropubic space (Figure 2-6). In the above-mentioned histology study, the deep or superior surface of the perineal membrane was shown to have direct connections to the levator ani muscles and the superficial or inferior surface of the membrane was fused with the vestibular bulb and clitoral crus. A follow-up magnetic resonance imaging (MRI) study showed that many of the distinct anatomic features of the perineal membrane described above could be seen with MRI.5
The perineal membrane provides support to the distal vagina and urethra by attaching these structures to the bony pelvis. In addition, its attachments to the levator ani muscles suggest that the perineal membrane may play a more active role in support than what was previously thought.
During cadaver dissections, attachments of the perineal membrane to the lateral vaginal walls can generally be identified approximately at the level of the hymeneal remnants.
The perineal body is a mass of dense connective tissue found between the distal third of the posterior vaginal wall and the anus below the pelvic floor (Figure 2-7). It is largely formed by the midline connection between the two halves of the perineal membrane.6 Distal or superficial to the perineal membrane, the medial ends of the bulbocavernosus and superficial transverse perineal muscles also contribute to the perineal body. Deep into the perineal membrane, fibers of the pubovisceral portion of the levator ani attach to the perineal body. The perineal body has direct attachments to the posterior vaginal wall anteriorly and the external anal sphincter posteriorly. In the sagittal plane, the perineal body is triangular in shape with a base that is much wider than its apex. The apex of the perineal body extends 2 to 3 cm above the hymeneal ring. The relationships of the perineal body in reference to posterior compartment anatomy were demonstrated in a recent MRI study.7
Clinical assessment of perineal body length takes into account the anterior portion of the external anal sphincter as well as the posterior vaginal wall and anterior anal wall thickness.
The perineal body contributes to support the distal vagina and rectum; therefore, during episiotomy repairs and perineal reconstructive procedures emphasis should placed on reapproximation of the torn ends of the anatomic structures that form the perineal body.
Key Point
The pudendal or Alcock canal is a splitting of the obturator internus fascia on the lateral walls of the posterior perineal triangle through which the pudendal neurovascular bundle passes.
This triangle contains the anal canal, anal sphincter complex, ischioanal fossa, and branches of the internal pudendal vessels and pudendal nerve (Figure 2-8). It is bounded deeply by the fascia overlying the inferior surface of the levator ani muscles, and laterally by the fascia overlying the medial surface of the obturator internus muscles.
The pudendal or Alcock canal (Figure 2-9) is a splitting of the obturator internus fascia on the lateral walls of the posterior perineal triangle. It allows path of the internal pudendal vessels and pudendal nerve before these structures split into terminal branches to supply the structures of the vulva and perineum (Figure 2-9).
The inferior rectal nerve often courses through the ischioanal fossa without entering the pudendal canal.1
The ischioanal fossa, formerly known as the ischiorectal fossa, is primarily filled with adipose tissue and contains branches of the pudendal vessels and pudendal nerve (Figure 2-10). The anal canal and anal sphincter complex lie in the center of this fossa. The boundaries of the fossa include (1) the inferior fascia of the levator ani muscles superior and medially, (2) the fascia covering the medial surface of the obturator internus muscles and ischial tuberosities laterally, and (3) the lower border of the gluteus maximus muscles and sacrotuberous ligaments posterior and laterally. The contents of the ischioanal fossa extend to the anterior perineal triangle deep into the perineal membrane. Posterior to the anus, the contents of the fossa are continuous across the midline except for the attachments of the external anal sphincter fibers to the coccyx.
The continuity of the ischioanal fossa across perineal compartments and across the midline posteriorly allows fluid, infection, and malignancy to spread from one side of the anal canal to the other, and also into the anterior perineal triangle deep into the perineal membrane.
Key Point
The anal sphincter complex consists of the external and internal anal sphincters and the puborectalis muscle.
The anal sphincter complex consists of the external and internal anal sphincters and the puborectalis muscle (Figure 2-10).
This sphincter consists of striated or skeletal muscle and is responsible for the squeeze pressure of the anal canal. It surrounds the distal anal canal and consists of a superficial and a deep portion. The more superficial fibers lie distal or below the internal sphincter and are separated from the anal epithelium only by submucosa. The deep fibers blend with the lowest fibers of the puborectalis muscle. The external anal sphincter is primarily innervated by the inferior anal nerve, also known as the inferior rectal or inferior hemorrhoidal nerve. This nerve can be a branch of the pudendal nerve or may arise directly from the sacral plexus.1 The inferior anal nerve communicates with the perineal branch of the posterior femoral cutaneous nerve, which also contributes to the innervation of the perianal skin.
This sphincter represents the distal thickening of the circular smooth muscle layer of the anal wall. It is under the control of the autonomic nervous system and is responsible for approximately 80% of the resting pressure of the anal canal.8
This muscle comprises the medial portion of the levator ani muscle that arises on either side from the inner surface of the pubic bones. It passes behind the rectum, and forms a sling behind the anorectal junction, contributing to the anorectal angle and possibly to fecal continence.
The bony pelvis comprises the two hipbones, also known as the innominate or coxal bones; the sacrum; and the coccyx (Figure 2-11). The hipbones consist of the ilium, ischium, and pubis, which fuse at the acetabulum, a cup-shaped structure that articulates with the head of the femur. The ilium articulates with the sacrum posteriorly at the sacroiliac joint, and the pubic bones articulate with each other anteriorly at the symphysis pubis. The sacroiliac joint is a synovial joint that connects the articular surfaces of the sacrum and ilium. This joint and its ligaments contribute significantly to the stability of the bony pelvis. The symphysis pubis is a cartilaginous joint, which connects the articular surfaces of the pubic bones through a fibrocartilaginous disc.
The ischial spines are clinically important bony prominences that project posteromedially from the medial surface of the ischium approximately at the level of the fifth sacral vertebra (S5).
The posterior, lateral, and inferior walls of the pelvis have several openings through which many important structures pass. The large obturator foramen between the ischium and pubis is filled almost completely by the obturator membrane (Figure 2-12). In the superior portion of this membrane, a small opening known as the obturator canal, allows passage of the obturator neurovascular bundle into the medial or adductor compartment of the thigh.
The posterolateral walls of the pelvis are not covered by bone. Two important ligaments, the sacrospinous and sacrotuberous, convert the greater and lesser sciatic notches of the ischium into the greater sciatic foramen and lesser sciatic foramen. The piriformis muscle, superior and inferior gluteal vessels (Figure 2-12A), internal pudendal vessels and pudendal nerve, sciatic nerve, and other branches of the sacral nerve plexus pass through the greater sciatic foramen (Figure 2-12B). The internal pudendal vessels, pudendal nerve, and obturator internus muscle tendon pass through the lesser sciatic foramen.
Posteriorly, four pairs of pelvic sacral foramina allow passage of the anterior divisions of the first four sacral nerves and lateral sacral arteries and veins (Figure 2-13).
The urogenital hiatus is the U-shaped opening in the pelvic floor muscles through which the urethra, vagina, and rectum pass.
Understanding the anatomy related to the greater sciatic foramen is critical to avoid neurovascular injury during sacrospinous ligament fixation procedures and when administering pudendal nerve blockade.9
Weakening and opening of the urogenital hiatus from neuromuscular injury to the pelvic floor muscles is thought to contribute to urogenital prolapse as described later in the chapter.
Key Point
The round and broad ligaments of the uterus consist of smooth muscle and loose areolar tissue, respectively, and do not contribute to the support of the uterus and adnexa. In contrast, the cardinal and uterosacral ligaments do contribute to pelvic organ support.
Although the term ligament is most often used to describe dense connective tissue that connects two bones, the “ligaments” of the pelvis are variable in composition, site of attachments, and function. The pelvic ligaments range from connective tissue structures that support the bony pelvis and pelvic organs to smooth muscle and loose areolar tissue that add no significant support. The sacrospinous, sacrotuberous, and anterior longitudinal ligament of the sacrum consist of dense connective tissue that join bony structures and contribute to the stability of the bony pelvis. The round and broad “ligaments” of the uterus consist of smooth muscle and loose areolar tissue, respectively, and do not contribute to the support of the uterus and adnexae. In contrast, the cardinal and uterosacral “ligaments” do contribute to the support of the uterus and upper third of the vagina. The cardinal ligaments primarily consist of perivesical connective tissue and nerves and are vertically oriented in the anatomic or standing position. The uterosacral ligaments consist primarily of smooth muscle and contain some of the pelvic autonomic nerves. In the anatomic position, the uterosacral ligaments are directed posteriorly and oriented almost horizontal to the floor.
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