Pelvis and Perineum

Bony Pelvic Girdle

The pelvic girdle is the attachment point of the lower limb to the body’s trunk. (The pectoral girdle is its counterpart for the attachment of the upper limb.) The bones of the pelvis include the following ( Fig. 5.2 ):

• Right and left pelvic bones (coxal or hip bones): fusion of three separate bones—the ilium, ischium, and pubis— that join in the acetabulum (cup-shaped surface where the pelvis articulates with the head of the femur).

• Sacrum: fusion of the five sacral vertebrae; the two pelvic bones, at the medial aspect of the ilium on each side, articulate with the sacrum posteriorly ( sacroiliac joint ). Superiorly, the sacrum articulates with the fifth lumbar vertebra ( lumbosacral joint ) ( Table 5.1 ).

  TABLE 5.1

  Joints and Ligaments of the Pelvis

  LIGAMENT	ATTACHMENT	COMMENT
  Lumbosacral Joint *
  Intervertebral disc	Between L5 and sacrum	Allows little movement
  Iliolumbar	Transverse process of L5 to crest of ilium	Can be involved in avulsion fracture
  Sacroiliac (Plane Synovial) Joint
  Sacroiliac	Sacrum to ilium	Allows little movement; consists of posterior (strong), anterior (provides rotational stability), and interosseous (strongest) ligaments
  Sacrococcygeal (Symphysis) Joint
  Sacrococcygeal	Between coccyx and sacrum	Allows some movement; consists of anterior, posterior, and lateral ligaments; contains intervertebral disc between S5 and Co1
  Pubic Symphysis
  Pubic	Between pubic bones	Allows some movement, fibrocartilage disc
  Accessory Ligaments
  Sacrotuberous	Iliac spines and sacrum to ischial tuberosity	Provides vertical stability
  Sacrospinous	Ischial spine to sacrum and coccyx	Divides sciatic notch into greater and lesser sciatic foramina

  * Other ligaments include those binding any two vertebrae and facet joints.

• Coccyx: terminal end of the vertebral column; a remnant of our embryonic tail; articulates with the last fused sacral vertebra at the sacrococcygeal joint (see Tables 5.1 and 2.2 ).

Clinical Focus 5.1

Pelvic Fractures

Clinically, the term pelvic fractures is used to describe fractures of the pelvic ring and does not typically include acetabular fractures, which are a separate type of fracture, usually from high-impact falls or automobile crashes. Pelvic fractures may be high or low impact; high-impact fractures often involve significant bleeding and may be life threatening. Pelvic ring fractures are classified as stable, involving only one side of the ring, or unstable, involving both parts of the pelvic ring.

The pelvis protects the pelvic viscera, supports the weight of the body, aids in ambulation by swinging side to side in a rotary movement at the lumbosacral articulation, provides for muscle attachments, and provides a bony support for the lower birth canal. The pelvic inlet is the circular opening where the lower abdominal cavity is continuous with the pelvic cavity; the promontory of the sacrum protrudes into this opening and represents its posterior midline boundary ( Fig. 5.3 ). The pelvic outlet is diamond shaped and bounded by the pubic symphysis anteriorly, the pubic arches, the inferior pubic rami and ischial rami, the sacrotuberous ligament, and the coccyx. The perineum is enclosed by these boundaries and lies below the pelvic floor (see Fig. 5.18 ).

The pelvic girdle forms a stable articulation to support the transfer of weight from the trunk to the lower limb. Weight is transferred from the lumbar vertebral column to the sacrum, across the sacroiliac joints to the coxal (pelvic or hip) bones, and then to the femur (thigh bone). The joints and ligaments reflect this stability ( Fig. 5.3 and Table 5.1 ). The sacroiliac ligaments are strong, especially posteriorly, as they support the weight of the upper body.

Anatomical differences in the female bony pelvis reflect the adaptations for childbirth. The differences from the male pelvis include the following:

• The bones of the female pelvis usually are smaller, lighter, and thinner.

• The pelvic inlet is oval in the female and heart shaped in the male.

• The female pelvic outlet is larger because of everted ischial tuberosities.

• The female pelvic cavity is wider and shallower .

• The female pubic arch is larger and wider.

• The greater sciatic notch is wider in females.

• The female sacrum is shorter and wider .

• The obturator foramen is oval or triangular in females and round in males.

The female pelvis may assume variable shapes, as follows:

• Gynecoid: normal, and most suitable shape for childbirth.

• Android: a masculine pelvic type, with a heart-shaped inlet, prominent ischial spines, and a narrower pelvic outlet.

• Platypelloid: foreshortened in the anteroposterior dimension of the pelvic inlet and wider in the transverse dimension.

• Anthropoid: resembling the pelvis of an anthropoid ape, with an oval-shaped inlet with a greatly elongated anteroposterior dimension and a shortened transverse dimension. It possesses a larger outlet.

Various asymmetric shapes may also result from scoliosis, poliomyelitis, fractures, and other pathologies. The characteristic shapes of the female pelvic inlet and outlet are important from the gynecological point of view.

Muscles of the Pelvis

The muscles of the true pelvis line its lateral wall and form a floor over the pelvic outlet. (The pelvic inlet is demarcated by the pelvic brim.) Two muscles line the lateral wall ( obturator internus and piriformis muscles ) and attach to the femur (see Table 6.5 ), and two muscles form the floor, or pelvic diaphragm (levator ani and coccygeus muscles) ( Fig. 5.4 and Table 5.2 ). The levator ani muscle consists of three muscle groups intermingled to form a single sheet of muscle ( iliococcygeus, pubococcygeus, and puborectalis muscles ). The levator ani muscle is an important support structure for the pelvic viscera in bipeds (upright-walking humans) and helps maintain closure of the vagina and rectum. Bipedalism places greater pressure on the lower pelvic floor, and the coccygeus and levator ani muscles have been “co-opted” for a different use than originally intended in most land-dwelling quadruped mammals. Thus, the muscles once used to tuck the tail between the hind legs (coccygeus) and wag the tail (levator ani) now subserve an important support function during our evolution as bipeds.

Distal Gastrointestinal Tract

In both genders the distal gastrointestinal tract passes into the pelvis as the rectum and anal canal . The rectosigmoid junction superiorly lies at about the level of the S3 vertebra, and the rectum extends inferiorly to become the anal canal just below the coccyx ( Fig. 5.5 ). As the rectum passes through the pelvic diaphragm, it bends posteriorly at the anorectal flexure and becomes the anal canal. The anorectal flexure helps maintain fecal continence through the muscle tone maintained by the puborectalis portion of the levator ani muscle. During defecation this muscle relaxes, the anorectal flexure straightens, and fecal matter can then move into the anal canal. Superiorly, the rectum is covered on its anterolateral surface with peritoneum, which gradually covers only the anterior surface, while the distal portion of the rectum descends below the peritoneal cavity (subperitoneal) to form the anorectal flexure . Features of the rectum and anal canal are summarized in Table 5.3 .