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THE PELVIS

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The pelvis is made up of the two innominate bones (which occupy the front and sides) and the sacrum and coccyx (which are behind). The bones articulate through four joints. The sacroiliac joint is the most important, linking the sacrum to the iliac part of the innominate bones. The symphysis of the pubis joins the two pubic bones. The sacrococcygeal joint attaches the sacrum to the coccyx.

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The false pelvis lies above the true pelvis, superior to the linea terminalis. Its only obstetric function is to support the enlarged uterus during pregnancy. Its boundaries are:

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  1. Posteriorly: lumbar vertebrae

  2. Laterally: iliac fossae

  3. Anteriorly: anterior abdominal wall

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The true pelvis (Fig. 5-1A) lies below the pelvic brim, or linea terminalis, and is the bony canal through which the fetus must pass. It is divided into three parts: (1) the inlet, (2) the pelvic cavity, and (3) the pelvic outlet.

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The inlet (pelvic brim) is bounded:

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  1. Anteriorly by the pubic crest and spine

  2. Laterally by the iliopectineal lines on the innominate bones

  3. Posteriorly by the anterior borders of the ala and promontory of the sacrum

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The pelvic cavity (Fig. 5-1B) is a curved canal.

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  1. The anterior wall is straight and shallow. The pubis is approximately 5 cm long

  2. The posterior wall is deep and concave. The sacrum is approximately 10 to 15 cm long

  3. The ischium and part of the body of the ilium are found laterally

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The pelvic outlet is diamond shaped. It is bounded:

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  1. Anteriorly by the arcuate pubic ligament and the pubic arch

  2. Laterally by the ischial tuberosity and the sacrotuberous ligament

  3. Posteriorly by the tip of the sacrum

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The pelvic inclination (Fig. 5-1C) is assessed when the woman is in the upright position. The plane of the pelvic brim makes an angle of about 60° with the horizontal. The anterior superior iliac spine is in the same vertical plane as the pubic spine.

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The axis of the birth canal (Fig. 5-1D) is the course taken by the presenting part as it passes through the pelvis. At first it moves downward and backward to the level of the ischial spines, which is the area of the bony attachment of the pelvic floor muscles. Here the direction changes and the presenting part proceeds downward and forward.

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The pelvic planes (Fig. 5-2) are imaginary flat surfaces passing across the pelvis at different levels. They are used for the purposes of description. The important ones are as follows:

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