Chapter Outline
Pelvic Anatomy
Bones of the Pelvis
The bones of the pelvis are the rigid foundation to which all of the pelvic structures are ultimately anchored. In the standing position, forces are dispersed to minimize the pressures on the pelvic viscera and musculature and disperse the forces to the bones that are better suited to the long-term, cumulative stress of daily life. In the upright position, the pubic rami are oriented in an almost vertical plane. Similar to the supports of an archway or bridge, the weight of the woman is transmitted along these bony supports to her femurs. Where the pubic rami articulate in the midline, they are nearly horizontal. Much of the weight of the abdominal and pelvic viscera is inferiorly supported by the bony articulation.
The pelvic bones are the ilium, ischium, pubic rami, sacrum, and coccyx ( Fig. 2.1 A and B ). The sacrum is composed of five sacral vertebrae that are fused together. The nerve foramina are positioned anterior and laterally. Overlying the middle of the sacrum is a rich neurovascular bed. The coccyx is inferiorly attached and is the posterior border of the pelvic outlet.
Several landmarks are important to the pelvic surgeon. The anterior superior iliac spine (ASIS) is easily identifiable and is anterior and laterally located on the superior ileum. The ischium is fused to the ilium. The medial surface of the ilium forms the lateral borders of the pelvic outlet. The superior greater sciatic notch and inferior lesser sciatic notch are medially separated by a projection, the ischial spine . The sacrospinous ligament, triangular in form, is laterally attached by its apex to the ischial spine; its broader base arises from the side of the lower sacral and coccygeal segments. This ligament converts the greater sciatic notch into the greater sciatic foramen (see Fig. 2.1 B ). Through this foramen pass the piriformis muscle and all of the vessels and nerves that leave the pelvis for the gluteal region and back of the thigh. The sacrospinous ligament and the dorsal sacrotuberous ligament also convert the lesser sciatic notch to the lesser sciatic foramen. Through this foramen pass the tendon and nerve of the internal obturator muscle, the pudendal nerve, and the internal pudendal vessels.
The ischial spine can be palpated easily through a vaginal, rectal, or retropubic approach, and many supportive structures attach to it. The ischial spine is useful as a fixed point to describe the relative position of other anatomic structures and as a landmark in various reconstructive surgeries for pelvic organ prolapse.
The superior and inferior pubic rami are anteriorly located and articulate in the midline at the pubic symphysis. The ridge along the superior, medial surface of the superior pubic rami is called the pectineal line , or Cooper’s ligament (see also Figure 21.2 , Figure 21.3 ).
In the standing position, the ASIS and pubic symphysis are in the same vertical plane ( Fig. 2.1 B ). This directs the pressure of the intra-abdominal and pelvic contents toward the bones of the pelvis instead of the muscles and endopelvic fascia attachments of the pelvic floor. The posterior surface of the pubic symphysis is located in a plane approximately 2 to 3 cm inferior to the ischial spine. Therefore, a line drawn connecting the two structures would be almost horizontal in the standing position. This has important implications for the support of pelvic organs, and it will be discussed later in the chapter.
Pelvic Floor and Sidewalls
The obturator internus and piriformis make up the pelvic sidewalls ( Fig. 2.2 ). The obturator membrane is a fibrous membrane that covers the obturator foramen. The obturator internus muscle lies on the superior (intrapelvic) side of the obturator membrane. The obturator internus origin is on the inferior margin of the superior pubic ramus and the pelvic surface of the obturator membrane. Its tendon passes through the lesser sciatic foramen to insert onto the greater trochanter of the femur to laterally rotate the thigh. The obturator internus receives its innervation from the obturator nerve originating from L5-S2. The obturator vessels and nerve pass through the anterior and lateral border of the obturator membrane, in the obturator canal, to their destination in the adductor compartment of the leg.
The piriformis is part of the pelvic sidewall and is located dorsal and lateral to the coccygeus. It extends from the anterolateral sacrum to pass through the greater sciatic foramen and insert onto the greater trochanter. Lying on top of the piriformis is a particularly large neurovascular plexus, the lumbosacral plexus.
There is a linear thickening of the fascial covering of the obturator internus muscle called the arcus tendineus levator ani . This thickened fascia forms an identifiable line from the ischial spine to the posterior surface of the ipsilateral superior pubic ramus. The muscles of the levator ani originate from this musculofascial attachment.
The pelvic diaphragm is defined as the levator ani muscles and the coccygeus muscle. It is stretched hammock-like between the pubis in front and the coccyx behind, and it is attached along the lateral pelvic walls to the arcus tendineus levator ani. The levator ani muscle complex consists of the puborectalis, pubococcygeus, and iliococcygeus muscles as classically described by and ( Fig. 2.2 ). The more medial puborectalis arises from the posterior inferior pubic rami and passes posteriorly forming a sling around the vagina, rectum, and perineal body to form the anorectal angle and contribute to fecal continence. Some of the fibers of the muscle blend with the muscularis of the vagina, perineal body, and anal canal and intermingle with the anal sphincter muscle. The pubococcygeus has a similar origin, but it inserts in the midline onto the anococcygeal raphe and the anterolateral borders of the coccyx. The iliococcygeus extends along the arcus tendineus levator ani from the pubis to the ischial spine to insert in the midline onto the anococcygeal raphe and coccyx.
Changes in the terminology of the levator ani muscle have more recently been proposed to be more in line with the origin and insertion of the various components ( ). The pubovisceral muscle encompasses all levator muscles medial to the ileococcygeus. It is the thick U-shaped muscle that arises from the pubic bone on each side of the midline and inserts into, or forms a sling around, the urethra, vagina, and rectum. On the basis of specific insertion points, segments of the pubovisceral muscle include the puboperinealis, pubovaginalis, and puboanalis. The puborectalis originates from the pubic bone and forms a sling dorsal to the anorectal angle, just cranial to the external anal sphincter.
The coccygeus, although not part of the levator ani, does make up the posterior part of the pelvic floor, and it plays a role in support. Its origin is on the ischial spine and sacrospinous ligament. It inserts on the lateral lower sacrum and coccyx, and it overlies the sacrospinous ligament. The muscle becomes thin and fibrous with age. The coccygeus often blends with the sacrospinous ligament, and it can be difficult to distinguish the two because they have the same origin and insertion.
The space between the levator ani musculature through which the urethra, vagina, and rectum pass is called the levator (genital) hiatus . The fusion of levator ani where they meet in the midline creates the levator plate . The levator plate forms the basis for pelvic support. The levator ani may be very thin and attenuated, especially in elderly patients and those with pelvic organ prolapse.
With the change from plantigrade to erect posture, the pelvis and vertebral column of humans underwent various evolutionary changes that restored the balance between intra-abdominal pressure and visceral support. The lumbosacral curve, a specific human characteristic, directs abdominal pressure forward onto the abdominal wall and the nearly horizontal, flattened pubic bones. Downward pressure is directed backward onto the sacrum and the rearranged pelvic muscles, which now fill in the pelvic cavity to form the pelvic floor and sidewalls.
The pelvic floor and sidewalls are made up of muscular and fascial structures that enclose the abdominal-pelvic cavity, the external vaginal opening (for intercourse and parturition), and the urethra and rectum (for elimination). The fascial components consist of two types of fascia: parietal and visceral (endopelvic). Parietal fascia covers the pelvic skeletal muscles and provides attachment of muscles to the bony pelvis; it is characterized histologically by regular arrangements of collagen. Visceral endopelvic fascia is less discrete and exists throughout the pelvis as a meshwork of loosely arranged collagen, elastin, and adipose tissue through which the blood vessels, lymphatics, and nerves travel to reach the pelvic organs. By surgical convention, condensations of the visceral endopelvic fascia of the pelvis have been described as discrete “ligaments,” such as the cardinal or uterosacral ligaments. The role of the endopelvic fascia in pelvic organ support will be discussed in detail later in this chapter.
The pelvic floor muscles have constant tone except during voiding and defecation, and they can be voluntarily contracted. This muscle tone serves as a constant support for the pelvic viscera. The levator muscles and the skeletal components of the urethral and anal sphincters all have the ability to contract quickly at the time of an acute stress, such as a cough or sneeze, to maintain continence. The muscles contain type I (slow-twitch) fibers to maintain constant tone, and type II (fast-twitch) fibers to provide reflex and voluntary contractions. Although the muscles of the pelvic floor were initially thought to have innervation from direct branches of the sacral nerves on the pelvic surface and via the pudendal nerve on the perineal surface, recent anatomic, neurophysiologic, and experimental evidence indicates that these standard descriptions are inaccurate and that the levator ani muscles are predominantly innervated by a nerve traveling on the superior (intrapelvic) surface of the muscles without contribution of the pudendal nerve. There are small branches that penetrate the body of each muscle as the nerve traverses them.
Abdominal Wall
The abdominal wall comprises several layers. Inferior to the umbilicus and beneath the skin, the subcutaneous tissue forms two distinct layers: the subcutaneous fat (Camper’s fascia) and an inner membranous layer (Scarpa’s fascia). These layers come together in the midline at the linea alba. The umbilicus consists of an umbilical ring through the linea alba, and it is the thinnest part of the abdominal wall. There are laterally three layers of muscles: the external oblique, the internal oblique, and the transversus abdominis muscles. Near the midline are the rectus abdominus and the pyramidalis muscles. The three lateral muscles come together near the midline to envelope the rectus abdominus muscles in the rectus sheath. The innermost layer of the sheath (posterior rectus sheath) is only present above the approximate position of halfway between the umbilicus and the pubic symphysis. This point of transition is called the arcuate line . Below the arcuate line, there is no posterior rectus sheath. The transversalis fascia and the peritoneum cover the internal surface of the entire anterior abdominal wall.
The ilioinguinal and iliohypogastric nerves are located in the space between the internal and external oblique muscles. Abdominal wall surgical sites below the level of the anterior superior iliac spine have the potential for ilioinguinal and iliohypogastric nerve injury.
Important folds located on the internal surface of the anterior abdominal wall include the median and medial umbilical folds. The paired obliterated remnants of the umbilical arteries course posterior to the rectus muscles. The single median umbilical fold is located in the midline and is the remnant of the embryologic urachus. Each inferior epigastric artery originates from the external iliac artery and ascends medial to the inguinal ring to lie along the lateral border of the rectus abdominus.
Nerves of the Pelvis
Although the muscles of the pelvic floor were initially thought to have innervation from direct branches of the sacral nerves on the pelvic surface and via the pudendal nerve on the perineal surface, evidence indicates that these standard descriptions are inaccurate and that the levator ani muscles are innervated solely by a nerve traveling on the superior (intrapelvic) surface of the muscles without contribution of the pudendal nerve ( Fig. 2.3 ).
In the pelvis, the sympathetic nerves to the pelvis originate at the T5 to L2 spinal level and act to promote storage by causing relaxation of the bladder and rectum and contraction of the smooth muscle components of the urethral and anal sphincters. The parasympathetic nerve supply to the pelvic viscera originates from the second, third, and fourth sacral nerves ( Fig. 2.4 ).
The sacral plexus is formed by the nerve roots of L4, L5, and S1 to S4 and is located on the anterior surface of the piriformis muscle on the lateral pelvic floor and sidewalls. The sacral plexus gives rise to multiple nerve branches. The sciatic and gluteal nerves innervate the lower limb and buttock. Other branches include the pudendal nerve (S2-S4), the anococcygeal nerves, the nerve to levator ani (S3, S4), the nerves to the piriformis and obturator internus, the pelvic splanchnic nerves, and the cutaneous nerves. The superior hypogastric plexus contains the sympathetic, parasympathetic, and visceral afferent fibers; it is located anterior to L5 between the sacral promontory and the aortic bifurcation before proceeding to the pelvic sidewalls ( Fig. 2.4 ). This plexus of nerves leaves the sacral surface to fan out on either side of the rectum approximately 3 to 4 cm superior to the pelvic floor muscles then disperses throughout the pelvis through the endopelvic fascia.
Uterus and Ovaries
The uterus is a muscular organ consisting of the uterine fundus, isthmus, and cervix. The uterus and cervix are continuous with the anterior apical portion of the vagina and are connected superiorly to the ovaries via the utero-ovarian ligaments. The ovaries receive their blood supply from the ovarian (gonadal) vessels that originate from the aorta. The uterus receives most its blood supply bilaterally from the uterine arteries. The uterine arteries originate at the anterior division of the internal iliac artery and reach the lower uterine segment within the cardinal ligament. The artery caudally gives off the vaginal branches and continues along the lateral surface of the uterus to anastomose with the ovarian blood supply (see Fig. 2.3 ).
The uterus is a midline organ positioned posterior to the bladder and anterior to the rectum. Condensations of peritoneum form the cardinal and broad ligaments, round ligament, and the suspensory ligament of the ovary (infundibulopelvic ligament).
The cardinal ligament is a condensation of the endopelvic fascia that is part of the supportive structure of the uterus. It extends outward and laterally from the uterine isthmus and cervix in a three-dimensional fan-like manner, and it is continuous with the endopelvic fascia of the pelvic sidewall. In addition to playing a role in upper vaginal support, it divides the deeper pelvis into avascular planes anteriorly called the paravesical space and posteriorly called the pararectal space. The space of Retzius is continuous with the paravesical space. The borders of the paravesical space are the obturator internus and neurovascular bundle laterally, the cardinal ligament posteriorly, the pubic symphysis anteriorly, and the obliterated umbilical artery medially. The borders of the pararectal space are the cardinal ligament anteriorly, the rectum medially, the internal iliac artery laterally, and the sacrum posteriorly.
Vagina
The vagina is a hollow, fibromuscular tube with rugal folds that extends from the vestibule to the uterine cervix. In the standing woman, the upper two thirds of the vagina is almost horizontal whereas the lower one third is nearly vertical. The vaginal wall is histologically composed of three layers. It is lined by a nonkeratinized stratified squamous epithelium that lies over a thin, loose layer of connective tissue, the lamina propria. The lamina propria contains no glands. Coursing through the lamina propria are small blood vessels. Vaginal lubrication is via a transudate from the vessels, cervix, and from the Bartholin’s and Skene’s glands. Beneath this is the vaginal muscularis, a well-developed fibromuscular layer consisting primarily of smooth muscle with smaller amounts of collagen and elastin. The muscularis is surrounded by an adventitial layer, which is a variably discrete layer of collagen, elastin, and adipose tissue containing blood vessels, lymphatics, and nerves. The adventitia represents an extension of the visceral endopelvic fascia that surrounds the vagina and adjacent pelvic organs, and it allows for their independent expansion and contraction.
The walls of the vagina are in contact except where its lumen is held open by the cervix. The vagina has an H-shaped lumen, with the principal dimension being transverse. In addition, the upper vagina is supported by connective tissue attachments to the sacrum, coccyx, and lateral pelvic sidewalls; these are identified at surgery as the cardinal and uterosacral ligament complex. The presence of a true fascia between the vagina and adjacent organs has been debated. Surgical terms such as pubocervical and rectovaginal fascia refer to layers that are developed as a result of separating the vaginal epithelium from the muscularis or by splitting the vaginal muscularis layer. Anteriorly, the vagina lies adjacent to and supports the bladder base, from which it is separated by the vesicovaginal adventitia (endopelvic fascia). The urethra is fused with the anterior vagina, with no distinct adventitial layer separating them. The terminal portions of the ureters cross the lateral fornices of the vagina on their way to the bladder base. Posteriorly, the vagina is related to the cul-de-sac, to the rectal ampulla, and inferiorly to the perineal body. Embryologically, an extension and fusion of the peritoneum from the cul-de-sac attached to the posterior surface of the vaginal muscularis forms the rectovaginal septum. A layer of adventitia separates the muscular layer of the rectum from the rectovaginal septum, except at the level of the perineal body, where there is fusion of the vaginal muscularis and connective tissue of the perineal body. The dense connective tissue of the perineal body extends 2 to 3 cm cephalad from the hymenal ring along the posterior vaginal wall and forms what some have called the rectovaginal fascia. Although at the time of surgery there appears to be an identifiable fascial plane, and have each concluded that between the adjacent organs is primarily vaginal muscularis and no fascia is histologically present.
Perineum
The perineum is divided into two compartments: superficial and deep. These are separated by a fibrous connective tissue layer called the perineal membrane. The perineal membrane is a triangular sheet of dense fibromuscular tissue that spans the anterior half of the pelvic outlet. It had previously been called the urogenital diaphragm; this change in name reflects the appreciation that it is not a two-layered structure with muscle in between, as had been thought. The perineal membrane provides support to the vagina and urethra as they pass through it. Cephalad to the perineal membrane lies the striated urogenital sphincter muscle, which compresses the mid- and distal urethra (see Fig. 2.7 ). The borders of the perineum are the ischiopubic rami, the ischial tuberosities, the sacrotuberous ligaments, and the coccyx. A line connecting the ischial tuberosities divides the perineum into the urogenital triangle anteriorly and the anal triangle posteriorly. The perineal body marks the point of convergence of the bulbospongiosus muscles, the superficial and deep transverse perinei, the perineal membrane, the external anal sphincter, the posterior vaginal muscularis, and the insertion of the puborectalis and pubococcygeus muscles. The bulbospongiosus originates on the inferior surface of the superior pubic rami and the crura of the clitoris. It inserts on the perineal body, where its fibers blend with the superficial transverse perinei and the external anal sphincter. It is innervated by the pudendal nerve. The superficial transverse perinei are bilateral muscles that extend from the medial ischial tuberosities to insert on the perineal body. Some fibers blend with the bulbospongiosus and the external anal sphincter. It is innervated by the pudendal nerve. The ischiocavernosus muscles originate from the medial ischial tuberosities and ischiopubic rami. They insert on the inferior aspect of the pubic angle and they are innervated by the pudendal nerve.
The deep perineal compartment is composed of the deep transverse perineus muscle, portions of the external urethral sphincter muscles (compressor urethrae and urethrovaginal sphincter), portions of the anal sphincter, and the vaginal musculofascial attachments.
The neurovascular anatomy of the perineum is illustrated in Figure 2.5 . The motor and sensory innervation of the perineum is via the pudendal nerve. The pudendal nerve originates from S2 to S4 and exits the pelvis through the greater sciatic foramen, hooks around the ischial spine, and then travels along the medial surface of the obturator internus through the ischiorectal fossa in a thickening of fascia called Alcock’s canal. It immerges posterior and medial to the ischial tuberosity and divides into three branches to supply the perineum: clitoral, perineal, and inferior rectal (inferior hemorrhoidal). The blood supply to the perineum is from the pudendal artery, which travels with the pudendal nerve to exit the pelvis. Similar to the nerve, there are three main branches with rich collateral anastomoses.
