The Physiology of the Pelvis
The pelvic organs are housed in the pelvic girdle, which is made up of the bones, joints, and muscles just inferior and posterior to the abdomen and above the lower extremities. The pelvic cavity is the interior portion of the pelvis. It’s bounded superiorly by the pelvic inlet, which is the opening to the abdomen. Inferiorly, it’s bounded by an opening called the pelvic outlet. It’s bounded anteriorly by a pelvic joint called the pubic symphysis and posteriorly by the coccyx.
The pelvic inlet is formed by the following structures:
Superior part of the pubic symphysis
Posterior border of the pubic crest (between the pubic tubercle and the pubic symphysis joint)
Superior ramus of the pubic bone
Arcuate line of the ilium (a line that resembles a ridge on the internal surface of the ilium)
Anterior portion of the sacral ala (large triangular portions on either side of the sacral base)
Sacral promontory (most superior and anterior part of the sacrum)
The pelvic outlet is formed by these structures:
Inferior portion of the pubic symphysis
Inferior rami of the pubic bones and ischial tuberosities of the ischial bones
Sacrotuberous ligaments (running from the sacrum to the ischial tuberosities)
Tip of the coccyx (tailbone)
Bones and joints
The pelvic girdle is formed by the strong bones of the hips and sacrum and the joints that hold them together.
The left and right pelvic bones (called the ossa coxae) create a basin shape and form the pelvic walls, which include the posterior wall, an anterior wall, and two lateral pelvic walls. They articulate with the sacrum and the femurs of the thighs.
The pelvic bones are the ilium, the ischium, and the pubis. These three bones are joined together at the acetabulum, which forms the socket for the hip joint.
Ilium: The flat upper part of the pelvic bone. The large fan-shaped portion is called the ala, and the body is at the bottom of the fan. It has an iliac crest running between the anterior and posterior superior iliac spines. The iliac fossa is the interior concave (bowl-shaped) part formed by the ala.
The iliac crest is generally easy to palpate. Press your fingers into the patient’s side near the waist and move your fingers inferiorly until you feel the large bony crest — that’s the iliac crest.
Ischium: This bone has two portions: the body and the ramus. The body forms part of the acetabulum, and the ramus forms the posterior portion of the obturator foramen, which is a large opening. The ischium also has a projection near the area where the body and ramus merge. This projection is called the ischial spine. The ischial tuberosity is a large protuberance on the posteroinferior aspect of the ischium.
Pubis: The pubis is the most anterior portion of the pelvic bone. Its superior pubic ramus forms part of the acetabulum, and its inferior pubic ramus completes the obturator foramen (the opening created by the ischium and pubis through which nerves and blood vessels travel). The body of the pubis has a pubic crest, which turns into the pubic tubercle.
The inferior ramus of the pubis and the ischial ramus meet to form the ischiopubic ramus. The pubic arch is formed where the right and left ischiopubic rami meet at the pubic symphysis. The angle formed by the joining of the rami is called the subpubic angle.
The joints of the pelvis include the pubic symphysis; the sacroiliac joints. The pubic symphysis is a cartilaginous joint formed between the two hip bones. It’s located at the front of the pelvis where the left and right pubic bones meet. This joint has a fibrocartilaginous disc between the bones and is surrounded by ligaments. It’s a very sturdy joint that has almost no movement; however, hormones that circulate through a pregnant woman’s body soften the ligaments to allow some movement so that the baby is able to move through the pelvic outlet during delivery.
Muscles and fascia
The pelvis may not move as much as other parts of the body, but it does have several muscles. The pelvic diaphragm is the combination of the following two muscles and their fascial coverings:
Coccygeus muscles: These muscles originate at the ischial spines and insert at the bottom of the sacrum and coccyx. They help support the pelvic organs.
Levator ani muscle: This muscle is attached to the pubic bones, the ischial spines, and the tendinous arch of levator ani. The levator ani has three parts: puborectalis, pubococcygeus, and iliococcygeus. Together these three parts support pelvic organs and resist any increase in abdominal pressure (like when you sneeze or cough) to maintain fecal and urinary continence.
Parietal layer: Lining the pelvic walls, this layer is a continuation of the abdominal. The parietal pelvic fascia covers muscles of the pelvis.
Visceral layer: This layer covers and helps support the pelvic organs.