Sacrum.

The sacrum and the coccyx (Fig. 4-1) make up the posterior midline portion of the bony pelvis. The sacrum transmits the weight of the body to the hip bones and then to the lower extremities. Normally, five sacral vertebrae fuse into one triangular mass, called the sacrum, that articulates with the fifth lumbar vertebra superiorly, the coccyx inferiorly, and the os coxae laterally. Where the sacrum meets the fifth lumbar vertebra, the sacrum is tilted posteriorly to form a lumbosacral angle. In some individuals the first sacral vertebra remains separate from the other four, or the fifth lumbar vertebra may fuse with the sacrum. Both conditions put a strain on the nearest intervertebral articulation, which may result in joint degeneration and cause low back pain.

When the five sacral vertebrae fuse together, they form a normal curvature so that the anterior, or pelvic, surface is concave. Anteriorly, the upper margin of the first sacral vertebra forms the sacral promontory, which marks the posterior portion of the true pelvic inlet, or pelvic brim. Both the anterior and posterior surfaces of the sacrum have two bilateral columns of four openings called sacral foramina. Branches of the sacral nerves pass through the sacral foramina. On the posterior surface, between the sacral foramina, poorly defined and fused spinous processes form the median crest. The fifth sacral vertebra has no spinous process or lamina. This deficiency in the neural arch leaves a midline opening, called the sacral hiatus. Local anesthetics may be injected through the sacral hiatus; this procedure is called extradural, epidural, or caudal anesthesia.

On each side of the upper portion of the sacrum is a rather large auricular surface for articulation with the iliac bones to form the sacroiliac (iliosacral) joint. The connection between the bones is further enhanced by strong interosseous ligaments, which act as cords to bind the bones together.

Coccyx.

The most inferior portion of the vertebral column is the coccyx, or tailbone, which is joined to the sacrum by cartilage. It usually consists of four rudimentary vertebrae with no processes or foramina, although the number varies from three to five. During adulthood, the bones of the coccyx usually fuse to form a single structure. The coccyx offers no support for the vertebral column but does provide attachment for a portion of the gluteus maximus muscle and some of the muscles of the pelvic floor. Sometimes, such as during childbirth or a fall, the sacrum and coccyx may separate, resulting in pain, especially when sitting.

Os Coxae.

The os coxae (innominate bones) are commonly called the hip bones. Each os coxa consists of an ilium, a pubis, and an ischium. In the child these are three separate bones joined together by hyaline cartilage, and each bone has its own ossification center in the cartilage. Ossification continues during childhood until the cartilage is replaced by bone. By puberty, only a small amount of cartilage remains as a Y-shaped region where the three bones meet in the acetabulum. When ossification is complete, the ilium, the ischium, and the pubis are fused together to form a single unit, called the os coxa (Fig. 4-2).

The ilium is the largest of the three bones of the os coxa. The superior part of the ilium presents a large, flaring, winglike surface called the ala. The inner aspect of the ala is the iliac fossa, which is the origin of the iliacus muscle. The iliac crest, the most superior portion of the hip bone, is the superior margin of the ala. The crest terminates both anteriorly and posteriorly in short projections, the superior and inferior iliac spines. Posteriorly, the ilium articulates with the sacrum at the sacroiliac joint.

The bodies of the two pubic bones meet in the anterior midline at the symphysis pubis. A small projection that is just lateral to the body forms the pubic tubercle, and from this point the superior pubic ramus extends laterally to meet the ilium. The pelvic surface of the superior margin of the superior pubic ramus is sharp and forms the pectineal line, which is continuous with the arcuate line of the ilium and the sacral promontory to mark the pelvic brim. The inferior pubic ramus extends inferiorly from the body to connect with the ischium. The inferior rami of the two pubic bones meet at the symphysis pubis to form the pubic arch, or subpubic angle. This angle is usually less than 70 degrees in the male and greater than 80 degrees in the female. The difference in the subpubic angle contributes to the variation in the contour of the pelvic cavity between the sexes. The male pelvis is deep and conical; the female pelvis is shallow and broad. In this way, the female pelvis is adapted for childbearing.

The inferior portion of the os coxa is formed by the ischium. Anteriorly, the ramus of the ischium meets the inferior pubic ramus at an indistinct point; together they often are called the ischiopubic ramus. The posterior and inferior border of the ischium is formed by a bulky, rough area called the ischial tuberosity. A sharp, pointed ischial spine divides the space between the ischial tuberosity and the ilium into the lesser sciatic notch, which is between the ischial tuberosity and the ischial spine, and the greater sciatic notch, which is between the ischial spine and the ilium. The notches are made into foramina by ligaments and are closed by muscles.

The ilium, ischium, and pubis meet in the acetabulum, which is a deep fossa for articulation with the head of the femur. The three bones also surround an opening called the obturator foramen, which is directed inferiorly. This foramen is closed by the obturator membrane and muscles. The pelvic outlet (inferior pelvic aperture) must also be covered to give support and maintain the pelvic viscera in position. The features of the os coxa are summarized in Table 4-1; see Fig. 4-2 and Fig. 4-3 to identify the features.

Muscles in the Wall of the Greater (False) Pelvis.

The muscles in the wall of the greater, or false, pelvis are actually abdominal muscles. The two principal muscles, the psoas major muscle and the iliacus muscle, extend throughout the whole pelvic region and continue into the anterior thigh. The computed tomography (CT) image in Fig. 4-4 illustrates these two muscles in a transverse plane.

The long, fleshy psoas major muscle appears as a muscle mass lateral to the vertebral bodies as it continues from the abdomen into the pelvis. The muscle passes deep to the inguinal ligament as it continues from the pelvis into the thigh, where it inserts on the lesser trochanter of the femur. The psoas major muscle acts with the iliacus muscle as a powerful flexor of the thigh. Lumbar nerves provide the innervation.

The iliacus muscle is a fan-shaped muscle that originates along the crest and fossa of the ilium and lines the iliac fossa. In the pelvis it appears lateral to the psoas major muscle. Fibers of the iliacus muscle insert on the femur with the psoas major muscle. Because the iliacus muscle and psoas major muscle appear to merge into one muscle and they have a close functional relationship, the two muscles often are referred to as the single iliopsoas muscle. The iliacus muscle is innervated by the femoral nerve, which is a branch of the lumbar plexus.

Muscles in the Wall of the True Pelvis.

Most of the inner surface of the bony true pelvis is lined with muscle. The obturator internus and piriformis muscles are the principal muscles that make up the wall of the true pelvis and form this lining. The line drawing in Fig. 4-5 illustrates the two muscles, and Fig. 4-6 is a CT image of these muscles.

The obturator internus muscle is a fan-shaped muscle that covers most of the lateral wall of the true pelvis. It originates on the inner surface of the pelvic bones, crosses over the obturator foramen to close off the opening, and then leaves the pelvis through the lesser sciatic notch. As it passes through the lesser sciatic notch, it becomes tendinous and makes a sharp turn to insert on the medial surface of the greater trochanter. The obturator internus muscle is innervated by branches from the L5 and S1 spinal nerves. This muscle rotates the thigh.

The piriformis muscle is located partially on the posterior wall of the true pelvis and partially external to the pelvis, posterior to the hip joint. It originates on the anterior surface of the sacrum and passes through the greater sciatic notch to insert on the greater trochanter of the femur. The piriformis muscle is closely associated with the sacral nerve plexus and is innervated by sacral nerves. It rotates and abducts the thigh.

Muscles of the Pelvic Outlet.

The pelvic outlet (inferior pelvic aperture) must be covered to give support to and maintain the position of the pelvic viscera. The floor of the pelvis includes all structures that contribute to this support; specifically, these are the peritoneum, pelvic diaphragm, urogenital diaphragm, and muscles of the perineum.

The principal structure supporting the pelvic viscera is the pelvic diaphragm, which forms a muscular pelvic floor. It is a hammock-like structure consisting primarily of the levator ani and coccygeus muscles. Of the two muscles that form the pelvic diaphragm, the levator ani muscle is the larger and more important. The integrity of the pelvic floor depends on the appropriate function of the levator ani muscles. In females, these muscles are particularly vulnerable during a strenuous delivery. If the muscles are damaged, support for the pelvic viscera is weakened, and urinary incontinence and prolapse of the uterus may result. The levator ani muscles originate on the pelvic surface of the pubis and the spine of the ischium. The fibers converge to insert on the coccyx, and some fibers insert on the muscle of the opposite side. The levator ani muscles are innervated by the pudendal nerve.

The coccygeus muscle is the smaller of the two muscles that form the pelvic diaphragm. From its origin on the spine of the ischium, the fibers fan out to form a triangular sheet that inserts on the sacrum and coccyx. Branches of the pudendal nerve innervate the coccygeus muscle.

The urogenital diaphragm lies immediately deep to the external genitalia. It consists of the sheetlike transversus perinei muscle and the external anal sphincter.

External to the pelvic and urogenital diaphragms are the muscles of the perineum. The outline of this region is roughly diamond shaped (Fig. 4-7). Anteriorly to posteriorly, the region extends from the pubic arch to the tip of the coccyx. The two ischial tuberosities form the two lateral points. A line drawn between the ischial tuberosities divides the perineum into two regions: a posterior anal region and an anterior urogenital region. The arrangement of the muscles in the anal region is the same in both sexes; however, the muscles in the urogenital region are arranged differently in the male and female.

The muscles found in the anal region are the levator ani and the sphincter ani. The levator ani muscle has been described previously as a primary muscle of the pelvic diaphragm. The sphincter ani muscle is part of the urogenital diaphragm and surrounds the anal canal. The musculature of the urogenital region consists of the bulbospongiosus, ischiocavernosus, and transversus perinei muscles. Fig. 4-8 illustrates the arrangement in the female, and Fig. 4-9 shows the arrangement in the male. The transversus perinei muscles are horizontal, arising on the ischial tuberosities and passing medially to insert on the central perineal tendon. In other words, they pass along the line that divides the perineum into the urogenital and anal regions. The ischiocavernosus muscles also arise on the ischial tuberosities but pass forward to insert on the pubic arch and the crus of the penis in the male or the clitoris in the female. The bulbospongiosus muscle is in the median line of the urogenital region. In the female, the two parts of this muscle are separated by the urethra and the vagina. In the male, the fibers of the two muscles unite in the midline and encircle the corpus spongiosum of the penis.

Extrapelvic Muscles Seen in Pelvic Sections.

Numerous muscles, such as the gluteal muscles and the thigh muscles, are external to the pelvis but are apparent in sections through the pelvis. These muscles are associated with the hip joint and movement of the lower extremity. Some of the more important and obvious extrapelvic muscles are named and described briefly in Table 4-3. They are discussed in more detail with the lower extremity. Some of these muscles are illustrated with the pelvic wall muscles in Fig. 4-5 and Fig. 4-6.