Pelvic and Support Anatomy
Polina Sawyer
Marlene M. Corton
Introduction
Support of the pelvic organs is provided by the complex interactions between pelvic viscera, musculature, and connective tissues and their attachments to the inner surface of the bony pelvis. The bony pelvis serves as a unique scaffolding that enables the pelvic viscera to expand, store and expel contents, all while maintaining the dynamic positions of the organs in three-dimensional space.
BONY PELVIS
The paired hip or coxal bones are formed by the fusion of three bones: ilium, ischium, and pubic bones (Fig. 2.1). These bones fuse at the acetabulum, the cupshaped structure which articulates with the head of the femur. Anteriorly, the pubic bones articulate with each other at the pubic symphysis, and posteriorly, the ilium articulates with the sacrum at the sacroiliac joint.
The pelvic inlet separates the pelvic cavity into a true pelvis (below the inlet) and a false pelvis (above the inlet) (Fig. 2.2). The pelvic brim is the edge of the pelvic inlet. The brim is circumscribed by the sacral promontory, the arcuate and pectineal lines, pubic crest, and the upper margin of the pubic symphysis. The false pelvis is bounded laterally by the alae of the ilium, which serve as attachment sites for the large muscles of locomotion. The pelvic outlet is bordered by the pubic arch anteriorly, the ischial tuberosities laterally, and the coccyx posteriorly (Fig. 2.3).
In the anatomic or standing position, the pelvic cavity tilts anteriorly. This adaption, which evolved to allow humans bipedal locomotion, brings the internal surface of the sacrum forward and downward and tilts the pelvic outlet so that it lies in a plane 60° to 65° from the horizontal. In this position, the anterior superior iliac spines and anterior surface of the pubic symphysis are in the same vertical plane and the ischiopubic rami and genital hiatus are parallel to the ground. This orientation directs the pressure of the intra-abdominal and pelvic contents toward the bones of the pelvis rather than toward the muscles and connective tissue of the pelvic floor.1
Deviations from stated configuration may predispose to pelvic organ prolapse.2,3 For example, a loss of lumbar lordosis, which can occur following spinal fusion, shifts the orientation of the pelvic outlet into a more horizontal plane, and this has been more commonly noted in women with prolapse.4,5
LIGAMENTS AND OPENINGS OF THE PELVIS
The sacrospinous, sacrotuberous, sacroiliac, and anterior longitudinal ligaments of the sacrum are dense connective tissue condensations which join bony structures and contribute significantly to the stability of the bony pelvis.
Sacrospinous and Sacrotuberous Ligaments
Both sacrospinous and sacrotuberous ligaments attach to the lateral and anterior aspect of the lower sacrum and coccyx medially and are fused in this region (see Fig. 2.2). Laterally, the sacrospinous ligament attaches to the ischial spine and the sacrotuberous ligament to the ischial tuberosity.
Greater and Lesser Sciatic Foramen
The sacrospinous and sacrotuberous ligaments divide the greater and lesser sciatic notches of the ischium into the greater sciatic foramen and lesser sciatic foramen.
Structures that pass through the greater sciatic foramen include the piriformis muscle, sciatic nerve, internal pudendal vessels and pudendal nerve, superior and inferior gluteal vessels and nerves, and other branches of the lumbosacral nerve plexus. These structures pass through the foramen dorsal to the sacrospinous ligament and near the ischial spine. Understanding these anatomic relationships is critical to avoiding neurovascular injury during sacrospinous ligament fixation and pudendal nerve blocks.
Structures which pass through the lesser sciatic foramen include the internal pudendal vessels and pudendal nerve and the obturator internus tendon.
 FIGURE 2.1 Left hip bone. |
 FIGURE 2.2 Pelvic ligaments and openings. *, ischial spine; PS, pubic symphysis. |
 FIGURE 2.3 Perineal triangles. mm., muscle. |
Anterior Longitudinal Ligament
The anterior longitudinal ligament consists of a broad and strong band of connective tissue fibers that extends along the anterior surfaces of the vertebral bodies and intervertebral discs, from the base of the skull to the sacrum. This ligament is composed of several layers of fibers of varying length and thickness that are closely interlaced with each other. These fibers limit extension of the vertebral column and reinforce the intervertebral disc, to which it is densely adherent. On the anterior surface of the first sacral vertebra, the average ligament thickness is 1.2 to 2.5 mm.6 It is in this region that the ligament serves as the optimal site for mesh fixation during sacrocolpolpopexy.7,8
Obturator Membrane and Obturator Canal
The obturator membrane is a dense fibrous structure that almost completely covers the obturator foramen between the ischium and pubis. Within the superior portion of this membrane is a small aperture, the obturator canal, through which the obturator neurovascular bundle exits the pelvis to reach the medial compartment of the thigh. This canal is found 5 to 6 cm from the midline of the upper pubic symphysis and 1.5 to 2 cm below the upper margin of the pectineal ligament.9,10
The pectineal (Cooper) ligament is a thickening in the pubic bone periosteum formed by the deep and medial fibers of the inguinal ligament; it serves as the anchoring site for retropubic colposuspension procedures.
PELVIC WALL MUSCLES
The lateral, inferior, and posterior walls of the pelvis are partially covered by striated muscles and their investing layers of fascia. These muscles include the piriformis, obturator internus, and the muscles of the pelvic diaphragm.
Piriformis Muscle
The piriformis muscle lines the posterolateral walls of the pelvis and is found directly posterior to the sacral nerves (Fig. 2.4). It arises from the anterior and lateral surface of the sacrum and exits the pelvis through the greater sciatic foramen to attach to the greater trochanter of the femur, where it functions as an external hip rotator. This muscle has a precise and intimate relationship with the sciatic nerve and has been implicated as a cause of nondiscogenic sciatica in cases where the muscle impinges on the nerve.11,12
Beaton and Anson13 described the existence of six different anatomic variations between the sciatic nerve and the piriformis muscle. In this cadaveric study, an undivided sciatic nerve emerging inferior to the piriformis
muscle was the most common and accounted for 90% of specimens examined. However, a recent magnetic resonance imaging study showed 20% aberrant anatomic relationships between the piriformis muscle and sciatic nerve.14 Although the clinical significance of these findings is uncertain, it has been suggested that these anatomic aberrations may have a role in pelvic floor dysfunction.15
muscle was the most common and accounted for 90% of specimens examined. However, a recent magnetic resonance imaging study showed 20% aberrant anatomic relationships between the piriformis muscle and sciatic nerve.14 Although the clinical significance of these findings is uncertain, it has been suggested that these anatomic aberrations may have a role in pelvic floor dysfunction.15
 FIGURE 2.4 Posterior view of pelvis. m., muscle; n., nerve. |
Obturator Internus Muscle
The obturator internus muscle arises from the pelvic surfaces of the ischium and ilium and from the obturator membrane and fills the lateral sidewalls of the pelvis (Fig. 2.5). The muscle tendon exits the pelvis through the lesser sciatic foramen and attaches to the greater trochanter of the hip. Like the piriformis muscle, it serves as an external hip rotator.
Along its medial pelvic surface, the obturator internus fascia condenses into two structures that are critical to pelvic organ support: the arcus tendineus fascia pelvis (ATFP) and the arcus tendineus levator ani (ATLA). The ATLA serves as origin for part of the levator ani muscles, and the ATFP serves as the attachment site for the mid anterior vaginal wall. In the perineum, a splitting of the obturator internus fascia forms the pudendal (Alcock) canal, through which the pudendal nerve and internal pudendal vessels pass before separating into the three terminal branches that innervate structures in the perineum.9,16
Pelvic Diaphragm
The muscles that span the pelvic floor are collectively known as the pelvic diaphragm (Fig. 2.6). The diaphragm consists of the levator ani and coccygeus muscles as well as their investing layers of fascia.
The levator ani muscle represents the “active” support of the pelvis and is functionally composed of three distinct parts: the iliococcygeus, pubococcygeus, and puborectalis. The connective tissue covering both the superior and inferior surfaces of these muscles is called the superior and inferior fascia of the levator ani. Below the pelvic diaphragm, the perineal membrane and perineal body also provide contributions to pelvic floor support.
Coccygeus muscle
The coccygeus muscle lies on the superior (pelvic) surface of the sacrospinous ligament and together with the levator ani muscles form the pelvic diaphragm. This muscle has the same bony attachments and runs an identical course to the sacrospinous ligament; thus, these two structures are commonly referred to as the coccygeus-sacrospinous ligament complex (C-SSL). Nerves which supply the coccygeus and levator ani muscles course over the midportion of the C-SSL complex, making these nerves particularly vulnerable during sacrospinous ligament fixation procedures (see Fig. 2.6).17,18 Postoperative dyspareunia and pelvic pain may potentially result from nerve or muscle disruption; thus, examination of the pelvic floor muscles is warranted when these symptoms present.
Pubococcygeus muscle
The pubococcygeus part of the levator ani muscle arises from the inner surface of the pubic bone and the ATLA
and attaches medially to the distal walls of the vagina, rectum, and perineal body (Fig. 2.7). Accordingly, this muscle consists of three subdivisions: The puboperinealis attaches to the perineal body, the pubovaginalis to the vaginal wall, and the puboanalis to the anus at the intersphincteric groove between the internal and external anal sphincters. Some fibers of the pubococcygeus also attach to the coccyx. However, given the significant attachments of muscle fibers to the pelvic viscera, the pubococcygeus is also known as the pubovisceralis muscle.19
and attaches medially to the distal walls of the vagina, rectum, and perineal body (Fig. 2.7). Accordingly, this muscle consists of three subdivisions: The puboperinealis attaches to the perineal body, the pubovaginalis to the vaginal wall, and the puboanalis to the anus at the intersphincteric groove between the internal and external anal sphincters. Some fibers of the pubococcygeus also attach to the coccyx. However, given the significant attachments of muscle fibers to the pelvic viscera, the pubococcygeus is also known as the pubovisceralis muscle.19
 FIGURE 2.5 Pelvic sidewall and pudendal canal. n., nerve; nn., nerves; LST, lumbosacral trunk; PS, pubic symphysis; m., muscle. |
 FIGURE 2.6 Levator ani muscles and innervation. *, ischial spine; a., artery; m., muscle; n., nerve; PS, pubic symphysis. |
Although the levator ani muscles have no direct attachments to the urethra in women, the fibers of the pubococcygeus muscle that attach to the vagina are responsible for elevating the anterior vaginal wall and indirectly the urethra and hence may contribute to urinary continence, as discussed later.20,21
 FIGURE 2.7 Pelvic floor muscles and urethral support. m., muscle; n., nerve. |
Iliococcygeus muscle
The iliococcygeus, the most posterior part of the levator ani muscles, arises from the ATLA and the ischial spines. Muscle fibers from one side join those from the opposite side in the midline between the anus and coccyx. This meeting line is termed the iliococcygeal raphe, which contributes to the anococcygeal body. This portion forms a relatively flat, horizontal shelf, which spans the potential gap from one pelvic sidewall to the other. In addition to the iliococcygeus, some fibers of the pubococcygeus muscle pass behind the rectum and attach to the coccyx. These fibers course cephalad to the iliococcygeus muscle, and along with attachments of the superficial part of the external anal sphincter, also contribute to the anococcygeal body. The levator plate is a clinical term used to describe the anococcygeal body (Fig. 2.8); it forms a supportive shelf on which the rectum, upper vagina, and uterus rest.22
Puborectalis muscle
The puborectalis represents the medial and most inferior fibers of the levator ani muscle that arise on either side from the pubic bone and form a U-shaped sling behind the anorectal junction. Fibers of this muscle blend with the deep part of the external anal sphincter muscle. The puborectalis muscle fibers are oriented such that muscular contraction draws the anorectal junction toward the pubic bones and contributes to the anorectal angle.20 The importance of the puborectalis and anorectal angle in anal continence is best appreciated in women with chronic fourth-degree lacerations who maintain fecal continence.
Urogenital hiatus and levator hiatus
Within the pelvic diaphragm is a U-shaped opening in the pelvic floor muscles, called the levator hiatus, through which the urethra, vagina, and rectum pass. This is also the opening through which pelvic organ prolapse occurs. The opening through which the urethra and vagina pass is the urogenital hiatus (see Fig. 2.6). This latter hiatus is separated from the anus by the perineal body (see Fig. 2.8).
Innervation of the pelvic diaphragm
The pelvic diaphragm muscles are primarily innervated by direct somatic efferents from the third through the fifth sacral nerve roots (S3-S5). These are appropriately called the nerve to the coccygeus and nerve to the levator ani muscles (see Fig. 2.6). However, a dual innervation to the levator muscle has traditionally been described, where the perineal (or inferior) surface of the muscle is supplied by branches of the pudendal nerve.23,24 Recent data, however, suggests the pudendal does not contribute significantly to levator muscle innervation.16,25
CONNECTIVE TISSUE ELEMENTS OF SUPPORT
The pelvic connective tissue represents the “passive” support of the pelvis. It consists of parietal fascia, which lines the walls of the abdominopelvic cavity and
covers the pelvic sidewall and pelvic floor muscles; the visceral fascia, which surrounds the pelvic viscera and allows for independent organ function; and the extraserosal fascia, which surrounds blood vessels, nerves, and lymph channels in their path from the pelvic sidewall to the pelvic organs. The most obvious example of extraserosal fascia in the pelvis is the cardinal ligament of the uterus.
covers the pelvic sidewall and pelvic floor muscles; the visceral fascia, which surrounds the pelvic viscera and allows for independent organ function; and the extraserosal fascia, which surrounds blood vessels, nerves, and lymph channels in their path from the pelvic sidewall to the pelvic organs. The most obvious example of extraserosal fascia in the pelvis is the cardinal ligament of the uterus.
Pelvic Fascia
All connective tissue within the pelvis is interconnected and is collectively known as the endopelvic fascia. However, the composition of this tissue varies depending on function.
The fascia that invests the striated muscles that lines the walls of the abdomen and pelvis is known as parietal fascia. This fascia consists of a mechanically dense matrix of predominantly collagen fibers which weave together to form a tough three-dimensional sheet. A common example of parietal fascia in the abdomen is the rectus fascia, which can be easily dissected away from the rectus muscles and reapproximated with sutures during closure of an abdominal incision. In the pelvis, condensations of the obturator fascia, such as the ATFP and ATLA, serve important functions, as discussed earlier.
In contrast to parietal fascia, visceral pelvic fascia surrounds the pelvic viscera, and it allows for independent and proper function of each organ. It consists of a loose mesh-like matrix of collagen, elastin, and smooth muscle, and it lacks the dense collagen composition associated with conventional fascia, such as the rectus fascia.26 Nonetheless, it contributes to pelvic organ support.
Lastly, extraserosal fascia is a term used to describe condensations of connective tissue that join pelvic viscera to pelvic side wall structures. It is a generic term of exclusion used to describe any other fascia lying inside the parietal fascia and outside the visceral fascia. Examples of this tissue include the cardinal and uterosacral ligaments. Extraserosal fascia provides a flexible conduit for pelvic nerves, vessels, and lymphatics, and thus, it resembles a mesentery-like structure.27 A critical role of the cardinal and uterosacral ligaments is to mechanically suspend the pelvic viscera over the pelvic floor.
 FIGURE 2.8 Pelvic organs and surgical spaces. USL, uterosacral ligament; PS, pubic symphysis. |
Levels of Support
Although the connective tissue support of the pelvis is a seamless, interdependent web, pelvic organ support can be better understood when separated into three levels described by DeLancey (Fig. 2.9).21
Level 1 (apical support)
Level 1 refers to support of the cervix and upper onethird of the vagina and is provided by the parametrium (cardinal and uterosacral ligaments) and its distal extensions, the paracolpium. This tissue attaches the upper vagina and cervix to the pelvic wall, suspending it over the pelvic floor. Thus, level 1 support is also known as the suspensory axis, and it provides connective tissue support to the vaginal apex after hysterectomy.28 In the standing or anatomic position, level 1 support fibers are both vertically oriented (cardinal ligaments) and horizontally oriented (uterosacral ligaments).29
Uterosacral ligament
The uterosacral ligaments attach broadly at the presacral and coccygeus fascia proximally and to the posterolateral walls of the cervix and upper vagina distally. They define the lateral boundaries of the rectouterine
pouch (posterior cul-de-sac) and are positioned lateral to the rectum and medial to the ureters. The ureters are thus susceptible to injury during surgical procedures that involve the uterosacral ligaments.
pouch (posterior cul-de-sac) and are positioned lateral to the rectum and medial to the ureters. The ureters are thus susceptible to injury during surgical procedures that involve the uterosacral ligaments.
 FIGURE 2.9 Pelvic organ support structures. a., artery; mm., muscles. |
The ureter courses lateral to the uterosacral ligament and lies closest to the ligament at the level of the cervix. In one cadaveric study, the mean distance from the ureter to the uterosacral ligament was 4.1, 2.3, and 0.9 cm at the levels of the sacrum, ischial spine, and cervix, respectively.17,30,31
The sacral nerves lie close to the ligament at the level of the ischial spine, where suspension sutures are typically placed. Cadaveric studies have shown that sacral nerves S1, S2, and S3 are in proximity to the ligament, but S3 seems most susceptible to entrapment during uterosacral ligament suspensions.17,32,33
The uterosacral ligament is divided into a superficial and a deep component. Although the superficial portion of the ligament is largely composed of smooth muscle and connective tissue, higher content of autonomic nerve fibers is found in the deep part.27
Cardinal ligament
In contrast to the uterosacral ligaments, the attachment points of the cardinal ligament are less well defined. They extend from the pelvic sidewalls, near the origin of the internal iliac artery to the lateral walls of the pelvic organs. The cardinal ligaments consist of a “mesentery-like” connective tissue network that convey blood vessels and nerves to target organs.34
The cranial part of the ligament consists of a perivascular sheath of connective tissue that conveys internal iliac branches to the pelvic viscera. The caudal or deeper portion of the ligament is relatively less vascular and has higher density of nerve fibers from the inferior hypogastric plexus.
The ureter passes within a parametrial “tunnel” before entering the bladder wall at the ureterovesical junction. This tunnel roughly separates the anterior fibers of the cardinal ligament from the posterior fibers of the uterosacral ligament. A higher density of nerve tissue is found posterior to the ureter.31
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