Figure 5.3 (Continued ) C: A lateral, sagittal view of the pelvic diaphragm and superior fascia of the urogenital diaphragm. The muscles include the deep transverse perineal and sphincter urethrae.
The pelvic diaphragm is a funnel-shaped fibromuscular partition that forms the primary supporting structure for the pelvic contents (Fig. 5.4). It is composed of the levator ani (pubococcygeus, puborectalis, iliococcygeus) and the coccygeus muscles, along with their superior and inferior fasciae (Table 5.1). It forms the ceiling of the ischiorectal fossa.
The levator ani muscles are composed of the pubococcygeus (including the pubovaginalis and pubourethralis, puborectalis, and the iliococcygeus). The levator ani is a broad, curved sheet of muscle stretching anteriorly from the pubis and posteriorly from the coccyx and from one side of the pelvis to the other. It is perforated by the urethra, vagina, and anal canal. Its origin is from the tendinous arch extending from the body of the pubis to the ischial spine.
This tendineus arch, called the arcus tendineus levator ani, is formed by a thickening of the obturator fascia and serves as a lateral landmark and point of attachment for some vaginal suspension procedures. The levator ani is inserted into the central tendon of the perineum, the wall of the anal canal, the anococcygeal ligament, the coccyx, and the vaginal wall.
The levator ani assists the anterior abdominal wall muscles in containing the abdominal and pelvic contents. It supports the vagina, facilitates defecation, and aids in maintaining fecal continence. During parturition, the levator ani supports the fetal head while the cervix dilates. The anterior portion of the levator ani complex serves to close the urogenital hiatus and pull the urethra, vagina, perineum, and anorectum toward the pubic bone, whereas the horizontally oriented posterior portion (levator plate) serves as a supportive diaphragm or “backstop” behind the pelvic viscera. Loss of normal levator ani tone, through denervation or direct muscle trauma, results in laxity of the urogenital hiatus, loss of the horizontal orientation of the levator plate, and a more bowl-like configuration. These changes can be bilateral or asymmetric (8). Such configurations are seen more often in women with pelvic organ prolapse than in those with normal pelvic organ support (9).
Table 5.1 Muscles of the Pelvic Floor
Traditional teaching is that the levator ani muscles are innervated by the pudendal nerve on the perineal surface and direct branches of the sacral nerves on the pelvic surface. Evidence indicates that the levator ani muscles are innervated solely by a nerve traveling on the superior (intrapelvic) surface of the muscles without the contribution of the pudendal nerve (10–15). This nerve, referred to as the levator ani nerve, originates from S3, S4, and/or S5 and innervates both the coccygeus and the levator ani muscle complex (10). After exiting the sacral foramina, it travels 2 to 3 cm medial to the ischial spine and arcus tendineus levator ani across the coccygeus, iliococcygeus, pubococcygeus, and puborectalis. Occasionally, a separate nerve comes directly from S5 to innervate the puborectalis muscle independently. Given its location, the levator ani nerve is susceptible to injury through parturition and pelvic surgery, such as during sacrospinous or iliococcygeus vaginal vault suspensions.
Figure 5.4 The ligaments and fascial support of the pelvic viscera.