PELVIC DIAPHRAGM II—FROM ABOVE
The pelvic diaphragm forms a musculotendinous, funnel-shaped partition between the pelvic cavity and the perineum and serves as one of the principal supports of the urethra, vagina, rectum, and pelvic viscera. It is composed of the levator ani and coccygeus muscles, sheathed in a superior and inferior layer of fascia. The muscles of the pelvic diaphragm extend from the lateral pelvic walls downward and medially to fuse with each other and are inserted into the terminal portions of the urethra, vagina, and anus. Anteriorly, they fail to meet in the midline just behind the pubic symphysis, exposing a gap in the pelvic ﬂoor, which is completed by the urogenital diaphragm. This gap is partially ﬁlled by the subpubic ligament that is pierced by the dorsal vein of the clitoris. In this area, the inferior fascia of the pelvic diaphragm fuses with the superior fascia of the urogenital diaphragm.
The levator ani muscles may be subdivided into an anterior pubococcygeus and a posterior iliococcygeus portion. They originate on each side at the posterior aspect of the pubis, the tendinous arch, and the ischial spine. They are inserted into the coccyx, the anococcygeal body, the lower end of the anal canal, the central point of the perineum, the lower vagina, and the posterolateral surface of the urethra. The levator ani muscles are primarily supporting structures, but they also contribute a sphincteric action on the anal canal and vagina. These muscles and their investing fascia are critical to maintaining support for the vagina and bladder. Rupture or stretch of this support system following pregnancy or childbirth is one of the major causes of pelvic support defects (hernias) and the attendant problems of urinary incontinence and fecal retention. It is to the tendinous arch (arcus tendineus) that some transabdominal approaches to the treatment of cysto-urethroceles provide anchorage or reattachment. The levator sling is also the plate on which pessaries must rest to provide mechanical support to prolapsing pelvic organs.
The pectineal ligament (also known as the inguinal ligament of Cooper) is an extension of the lacunar ligament that runs on the pectineal line of the pubic bone, seen as a ridge on the superior ramus of the pubic bone and forming part of the pelvic brim. Lying across it are ﬁbers of the pectineal ligament and the proximal origin of the pectineus muscle. This ﬁbrous line has been used clinically as an anchor point for incontinence procedures such as the Marshal-Marchetti-Krantz and Burch procedures.
The coccygeus muscles are triangular in shape, arise from the ischial spine, and are inserted into the lateral borders of the lower sacrum and upper coccyx. They lie on the pelvic aspect of the sacrospinous ligaments.
The fasciae of the pelvic diaphragm are continuous with the fascial layers of the perineal compartments— the endopelvic fascia, the obturator fascia, the iliac fascia, and the transversalis fascia of the abdomen.
Aside from the muscles of the pelvic diaphragm, two muscles—the obturator internus and the piriformis—cover the walls of the true pelvis. The piriformis is triangular and lies ﬂattened against the posterior wall of the pelvis minor. It originates from three or more processes lateral to the ﬁrst, second, third, and fourth anterior sacral foramina and leaves the pelvis through the greater sciatic foramen above the ischial spine to be inserted by a rounded tendon into the upper border of the greater trochanter of the femur. The obturator internus muscles are fan-shaped and cover the side walls of the pelvis.