SOFT TISSUES OF THE PELVIS

Pelvic Musculature

Muscles and fascia line the true pelvis and form its floor. The obturator internus arises from the inner surface of the obturator foramen and the obturator membrane and passes through the lesser sciatic foramen to insert on the femur (see Fig. 2-8 ). The fascia on the pelvic surface of this muscle is thickened into a tough line extending from the lower half of the pubis to the ischial spine. This tendinous arc of the levator ani serves as the origin of the muscles of the pelvic diaphragm: pubococcygeus and iliococcygeus ( Fig. 2-9 ). These muscles are not truly separable, and they form a diaphragm that closes the pelvic outlet. Anteriorly, a narrow U-shaped hiatus remains through which the urethra and rectum exit in the male and the urethra, vagina, and rectum exit in the female ( Fig. 2-10 ). The muscle bordering this hiatus has been referred to as pubovisceral because it provides a sling for (pubourethralis, puborectalis), inserts directly into (pubovaginalis, puboanalis, levator prostatae), or inserts into a structure intimately associated with the pelvic viscera ( Lawson, 1974 ). The pubovisceral group provides strong fixation and support for the pelvic viscera. The coccygeus muscle extends from the sacrospinous ligament to the lateral border of the sacrum and coccyx to complete the pelvic diaphragm. Muscles of the pelvic diaphragm contain type I (slow-twitch) fibers, which provide tonic support to pelvic structures, and type II (fast-twitch) fibers, for sudden increases in intra-abdominal pressure ( Gosling et al, 1981 ). The piriformis muscle arises from the lateral aspect of the sacrum and passes through and fills the greater sciatic foramen to form the posterolateral wall of the pelvis.

Figure 2-9 Muscles of the true pelvis (three-quarter view).

Figure 2-10 Location and contour of the levator ani and pelvic viscera. A, Anterior view demonstrating the near-vertical orientation of the lateral walls of the levator ani and the horizontal wings at its posterior superior aspect. B, Lateral view in which the levator ani has been made transparent. The perineal membrane bridges the urogenital hiatus, and the urethral sphincter fills much of the hiatus. C, View of the levator ani from below showing the urogenital hiatus and the thickened inferior border of the levator ani. The perineal body and related structures are not shown. (From Brooks JD, Chao W-M, Kerr J: Male pelvic anatomy reconstructed from the Visible Human data set. J Urol 1998;159:868-872.)

It is important to recognize that the pelvic diaphragm is not flat or bowl shaped, as it is frequently depicted. At the urogenital and anal hiatus, the muscles lie in a near-vertical configuration and are thickened inferiorly (see Fig. 2-10 ) ( Brooks et al, 1998 ; Myers et al, 1998 ). Behind the anus, they flatten to form a nearly horizontal diaphragm, referred to as the levator plate. In the female, the levator plate provides critical support to the pelvic viscera, as discussed later.

Pelvic Fasciae

The pelvic fasciae are not merely collagenous; they are also rich in elastic tissue and smooth muscle. This suggests that they are active in the support, and possibly the function, of the pelvic viscera. The pelvic fasciae are continuous with the retroperitoneal fasciae and have been categorized somewhat arbitrarily into outer, intermediate, and inner strata. The outer stratum, or endopelvic fascia, lines the inner surface of the pelvic muscles and is continuous with the transversalis layer of the abdomen. It is fixed to the arcuate line of the pelvis, Cooper's ligament, the sacrospinous ligament, the ischial spine, and the tendinous arc of the levator ani. The intermediate stratum embeds the pelvic viscera in a fatty, compressible layer that accommodates their filling and emptying. Its tissues are easily swept aside to reveal the retropubic, paravesical, rectogenital, and retrorectal potential spaces. All pelvic vessels and some pelvic nerves travel in this stratum and are subject to injury when these potential spaces are developed at surgery. The intermediate stratum coalesces around vessels and nerves supplying the pelvic organs to form named ligaments (e.g., cardinal, uterosacral, lateral, and posterior vesical) that suspend and tether these organs in the pelvis. This fascia also thickens around the pelvic urogenital organs to form their visceral fascia. These are not true ligaments but are a meshwork of connective tissue and smooth muscle investing the visceral neurovascular pedicles ( DeCaro et al, 1998 ). The inner stratum lies just beneath the peritoneum and is associated with the entire gastrointestinal tract. In the pelvis, it covers the rectum and the dome of the bladder and forms the rectogenital septum (Denonvilliers' fascia). This septum is the developmental remains of the rectogenital pouch of peritoneum that extended between the rectum and internal genitalia to the pelvic floor.

The pelvic fasciae have been given a confusing array of appellations by anatomists and surgeons interested in female pelvic organ prolapse. To add to the confusion, the strength of pelvic fasciae can differ significantly between individuals and races, and these differences may predispose some individuals to pelvic prolapse ( Zacharin, 1985 ). There are three important components of the pelvic fasciae: (1) Anteriorly, the puboprostatic ligaments attach to the lower fifth of the pubis, lateral to the symphysis and to the junction of the prostate and external sphincter (see Fig. 2-40 ). They are called the pubourethral ligaments in the female and insert on the proximal third of the urethra ( Fig. 2-11 ). (2) Laterally, the arcus tendineus fascia pelvis extends from the puboprostatic (pubourethral) ligament to the ischial spine (see Fig. 2-11 ). This fascia forms at the junction of the endopelvic and visceral fasciae. It should not be confused with the arcus tendineus levator ani, which lies above its anterior portion ( Fig. 2-12 ). In the male, the arcus tendineus fascia pelvis is found at the base of a sulcus between the pelvic side wall and the prostate and bladder. In the female, it corresponds to the lateral attachment of the anterior bladder wall to the pelvic side wall. Paravaginal suspension procedures for stress urinary incontinence entail lateral reapproximation of the vaginal wall to this tendinous arc ( Richardson et al, 1981 ). The lateral branches of the dorsal venous complex are directly beneath the arcus tendineus fascia pelvis; thus, the endopelvic fascia should be opened lateral to this landmark. In the female, fascia extending medially from this arch carries a variety of names (pubovesical, periurethral, urethropelvic ligament) and provides important support to the urethra and anterior vaginal wall. Damage to this fascia and its attachments has been implicated in urethrocele, cystocele, and stress urinary incontinence. (3) Posterior to the ischial spine, the fascia fans out to either side of the rectum and attaches to the pelvic side wall as the lateral and posterior vesical ligaments. In the female, these are the strong cardinal and uterosacral ligaments. They are not true ligaments; rather, they are condensations of intermediate stratum around visceral neurovascular pedicles. The peritoneum over these ligaments forms discrete folds (rectovesical in the male and rectouterine in the female) that can be appreciated at cystectomy ( Fig. 2-13 ). Taken as a whole, the pelvic fasciae form a Y-shaped scaffolding for the pelvic viscera (see Fig. 2-12 ).

Figure 2-40 Accessory pudendal arteries, as seen in the retropubic space. (From Polascik TJ, Walsh PC: Radical retropubic prostatectomy: The influence of accessory pudendal arteries on the recovery of sexual function. J Urol 1995;153:150-152.)

Figure 2-11 Floor of the space of Retzius in a thin, elderly female cadaver. The fat has been removed to show the continuous sheet of endopelvic fascia, and the bladder has been retracted posteriorly. 1, Symphysis pubis; 2, right pubourethral ligament; 3, lateral condensation of endopelvic fascia forming the right arcus tendineus fasciae pelvis; 4, condensation of the endopelvic fascia, which forms a firm, whitish aponeurosis over the proximal urethra and internal vesical orifice. (From Mostwin JL: Current concepts of female pelvic anatomy and physiology. Urol Clin North Am 1991;18:175-195.)

Figure 2-12 Vagina and supportive structures after removal of the bladder and uterus. The arcus tendineus fascia pelvis and the cardinal and uterosacral ligaments (paracolpium) form a continuous structure that supports the pelvic viscera. (From DeLancey JO: Structural support of the urethra as it relates to stress urinary incontinence: The hammock hypothesis. Am J Obstet Gynecol 1994;170:1713-1720.)

Figure 2-13 Peritoneal surfaces of the female and male pelves. In the female, the ureter passes medial to the ovarian vessels, then deep to the uterine artery within the substance of the cardinal ligament. The sacrogenital and sacrouterine folds represent the posterior portions of pelvic fascial support.

Fasciae of the Perineum and the Perineal Body

The weakest point in the pelvic floor, the urogenital hiatus, is bridged by the urogenital diaphragm, a structure unique to humans (see Fig. 2-10 ). The fibrous perineal membrane lies at the center of, and defines, the urogenital diaphragm (see Figs. 2-3, 2-10, and 2-30 [3] [10] [30]). It is triangular and spans the inferior ischiopubic rami from the pubis to the ischial tuberosities. Posteriorly, it ends abruptly; the superficial and deep transverse perinei run along its free edge ( Fig. 2-14 ). The external genitalia attach to its inferior surface; superiorly, it supports the urethral sphincter (discussed later). The perineal body represents the point of fusion between the free posterior edge of the urogenital diaphragm and the posterior apex of the urogenital hiatus. This pyramid-shaped structure lies at the hub of pelvic support. Virtually every pelvic muscle (superficial and deep transverse perinei, bulbocavernosus, levator ani, rectourethralis, external anal sphincter, striated urethral sphincter) and fascia (perineal membrane, Denonvilliers', Colles', and endopelvic) insert into the perineal body. At its core are abundant elastin and richly innervated smooth muscle, which suggests that it may have a dynamic role in support. Damage to the perineal body during perineal prostatectomy risks postoperative urinary incontinence.

Figure 2-30 Structure of the male striated urethral sphincter. A, Anterior projection shows the cone shape of the sphincter and the smooth muscle of the sphincter. B, Viewed laterally, the anterior wall of the sphincter is nearly twice the length of the posterior wall, although both are of comparable thickness. (From Brooks JD, Chao W-M, Kerr J: Male pelvic anatomy reconstructed from the Visible Human data set. J Urol 1998;159:868-872.)

Figure 2-14 Muscles of the male perineum. The transversus perinei and ischiocavernosi frame the urogenital diaphragm. (From Williams PL, Warwick R: Gray's Anatomy, 35th British ed. Philadelphia, WB Saunders, 1973, p 530.)