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True pelvisThe true pelvis is cylindrical in shape and has an inlet, a wall, and an outlet. The inlet is open while the pelvic floor closes the outlet and separates the pelvic cavity, above, from the perineum, below.Pelvic InletCircular, heart shaped, opening between the abdominal cavity and the pelvic cavity through which structures traverse between the abdomen and pelvic cavity. It is completely surrounded by bones and joints.The sacral promontory protrudes into the inlet forming its posterior margin in the midline.

Pelvic InletOn either side of the promontory, the margin is formed by the alae of the sacrum. The margin of the pelvic inlet then crosses the sacro-iliac joint and continues along the linea terminalis (i.e. the arcuate line, the pecten pubis or pectineal line, and the pubic crest) to the pubic symphysis.

Pelvic inletStructures pass between the pelvic cavity and the abdomen through the pelvic inlet. During childbirth, the fetus passes through the pelvic inlet from the abdomen, into which the uterus has expanded during pregnancy; and then passes through the pelvic outlet.

Pelvic WallsThe walls of the pelvic cavity consist of the sacrum, the coccyx, the pelvic bones inferior to the linea terminalis, two ligaments, and two muscles.

LigamentsSacrospinous and Sacrotuberous ligaments are important architectural elements of the walls because they link each pelvic bone to the sacrum and coccyx.They stabilize the sacrum on the pelvic bones by resisting the upward tilting of the inferior aspect of the sacrum The Sacrospinous ligamenttriangular shapedits apex attached to the ischial spine and its base attached to the related margins of the sacrum and the coccyx.

The sacrotuberous ligamentis also triangular in shape and is superficial to the sacrospinous ligament. Its base has a broad attachment that extends from the posterior superior iliac spine of the pelvic bone, along the dorsal aspect and the lateral margin of the sacrum, and onto the dorsolateral surface of the coccyx.Laterally, the apex of the ligament is attached to the medial margin of the ischial tuberosity.ForamenThese ligaments convert the greater and lesser sciatic notches of the pelvic bone into foramina.The greater sciatic foramen lies superior to the sacrospinous ligament and the ischial spine. The lesser sciatic foramen lies inferior to the ischial spine and sacrospinous ligament between the sacrospinous and sacrotuberous ligaments.

Muscles Obturator internusis a flat, fan-shaped muscle that originates from the deep surface of the obturator membrane and from associated regions of the pelvic bone that surround the obturator foramen.The muscle fibers converge to form a tendon that leaves the pelvic cavity through the lesser sciatic foramen, makes a 90 bend around the ischium between the ischial spine and ischial tuberosity, passes posteriorly, and crosses the hip joint to insert on the greater trochanter of the femur. Piriformisis triangular in shape and originates in the bridges of bone between the four anterior sacral foramina. It passes laterally through the greater sciatic foramen, crosses the posterosuperior aspect of the hip joint, and inserts on the greater trochanter of the femur above the insertion of the obturator internus muscle A large part of the posterolateral wall of the pelvic cavity is formed by the piriformisIn addition, this muscle separates the greater sciatic foramen into two regions, one above the muscle and one below. Vessels and nerves coursing between the pelvic cavity and the gluteal region pass through these two regions.

Pelvic outletThe diamond shaped pelvic outlet is formed by both bone and ligaments It is limited anteriorly in the midline by the pubic symphysis. On each side, the inferior margin of the pelvic bone projects posteriorly and laterally from the pubic symphysis to end in the ischial tuberosity. Pelvic outletTogether, these elements construct the pubic arch, which forms the margin of the anterior half of the pelvic outlet. The sacrotuberous ligament continues this margin posteriorly from the ischial tuberosity to the coccyx and sacrum.The pubic symphysis, ischial tuberosities, and coccyx can all be palpated.

Terminal parts of the urinary and gastrointestinal tracts and the vagina pass through the pelvic outlet. The area enclosed by the boundaries of the pelvic outlet and below the pelvic floor is the perineum. Clinical SignificanceChanges with PregnancyDuring pregnancy, the symphysis pubis and the ligaments of the sacroiliac and sacrococcygeal joints undergo softening in response to hormones, thus increasing the mobility and increasing the potential size of the pelvis during childbirth.The hormones responsible are estrogen and progesterone produced by the ovary and the placenta. An additional hormone, called relaxin, produced by these organs can also have a relaxing effect on the pelvic ligaments.

Clinical SignificancePelvic measurements in obstetrics The diagonal conjugate between the promontory and the midpoint of the pubic symphysis (11 cm) The maximum transverse diameter of the inlet (11.5 cm)The bispinous diameter the distance between ischial spines (9 cm) The sagittal outlet the distance between the tip of the coccyx and the inferior margin of the pubic symphysis (10 cm) Clinical pelvimetry Can be obtained using plain radiographs and computed tomography scans. Both techniques use ionizing radiation and even a low dose carries a potential risk to the mother and fetus. magnetic resonance imaging carries no radiation risk for the fetus or mother