Bones of the Pelvis

The bones of the pelvis are the rigid foundation to which all of the pelvic structures are ultimately anchored. It is important to understand and discuss the bony pelvis from the perspective of a standing woman. 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 supported by the bony articulation inferi-orly. In this way, increases in intraabdominal pressures are partially supported by the bony pelvis.

The pelvic bones are the ilium, ischium, pubic rami, sacrum, and coccyx (Figure 4-2.1). 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 attached inferiorly and is the posterior border of the pelvic outlet.

Attached to the sacrum are the ilium, ischium, and pubic rami. Several landmarks are important to the pelvic surgeon. The anterior superior iliac spine is located anterior and laterally on the superior ileum. This is easily identifiable in all patients and is a clinically useful landmark. The ischium is fused to the ilium. The medial surface of the ilium has two concavities forming the lateral borders of the pelvic outlet. The superior and larger of the two is the greater sciatic notch. Inferiorly is the lesser sciatic notch. They are separated by a projection medially, called the ischial spine. The ischial spine is important clinically and anatomically because it 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.

The superior and inferior pubic rami are located anteriorly 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.

In the standing position, the anterior superior iliac spine and pubic symphysis are in the same vertical plane (Figure 4-2.2). This directs the pressure of the intraabdominal 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 pubis 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.

Strong ligaments hold the bones together. The ligaments of the sacroiliac joint are rarely encountered during surgery for pelvic floor dysfunction and are not addressed here. The sacrospinous ligament is a strong, easily identifiable ligament that extends from the ischial spine to the distal sacrum. The ligament fans out to attach on the S1-S4 vertebrae. This ligament divides the lateral pelvic outlet into two foramina, the greater sciatic foramen superiorly and the lesser sciatic foramen inferiorly. This is an important location for identifying the course of the puden-dal nerve, artery, and vein, and will be discussed later in the chapter.

Figure 4-2.1. Bones and ligaments of the pelvis.a, Front view;b,lithotomy view. (Reprinted with the permission of The Cleveland Clinic Foundation.)

Figure 4-2.1. Bones and ligaments of the pelvis.a, Front view;b,lithotomy view. (Reprinted with the permission of The Cleveland Clinic Foundation.)

Figure 4-2.2. Bones and ligaments of the pelvis (sagittal view).The dashed line represents the vertical plane of the anterior superior iliac spine and pubic symphysis. The arrows represent the distribution of weight of the spinal column and abdominal contents and along the ilium. (Reprinted with the permission of The Cleveland Clinic Foundation.)

Figure 4-2.2. Bones and ligaments of the pelvis (sagittal view).The dashed line represents the vertical plane of the anterior superior iliac spine and pubic symphysis. The arrows represent the distribution of weight of the spinal column and abdominal contents and along the ilium. (Reprinted with the permission of The Cleveland Clinic Foundation.)

Muscles of the Pelvic Sidewalls and Pelvic Floor

The obturator internus and piriformis make up the pelvic sidewalls. 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 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 on 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 ipsi-lateral pubic ramus. The muscles of the levator ani originate from this musculofascial attachment (Figure 4-2.3).

The skeletal muscles of the pelvic floor include the levator ani muscles, the coccygeus muscle, the external anal sphincter, the striated urethral sphincter, and the superficial perineal muscles (bulbocavernosus, ischiocav-ernosus, and transverse perinea). The levator ani muscle

Figure 4-2.3. Muscles of the pelvis (sagittal view). (Reprinted with the permission of The Cleveland Clinic Foundation.)

complex consists of the puborectalis, pubococcygeus, and iliococcygeus muscles (Figure 4-2.4).

The puborectalis has an attachment to the posterior inferior pubic rami and arcus tendineus levator ani. Its fibers pass posteriorly forming a sling around the vagina, rectum, and perineal body to form the anorectal angle and con-

tribute to fecal continence. Some of the fibers of the muscle may blend with the muscularis of the vagina and rectum. The pubococcygeus has a similar origin, but 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.

The coccygeus, although not part of the levator ani, does make up the posterior part of the pelvic floor and has a role in support. Its origin is on the ischial spine and sacrospinous ligament. It inserts on the lateral lower sacrum and coccyx and overlies the sacrospinous ligament. The muscle becomes thin and fibrous with age. The coc-cygeus often blends with the sacrospinous ligament and, because they have the same origin and insertion, it can be difficult to distinguish the two.

The space between the levator ani musculature through which the urethra, vagina, and rectum pass is called the levator 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 as will be discussed in detail later in this chapter. The levator ani may be very thin and attenuated, especially in patients with pelvic organ prolapse.

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Constipation Prescription

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