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PELVIC BONES

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The pelvis is the bony basin in which the trunk terminates and through which the body weight is transmitted to the lower extremities. In females, it is adapted for childbearing. The pelvis consists of four bones: the two innominates, the sacrum, and the coccyx. These are united by four joints.

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Innominate Bones

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The innominate bones are placed laterally and anteriorly. Each is formed by the fusion of three bones—the ilium, ischium, and pubis—around the acetabulum.

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Ilium
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The ilium is the superior bone: It has a body (which is fused with the ischial body) and an ala. Points of note concerning the ilium include:

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  1. The anterior superior iliac spine gives attachment to the inguinal ligament

  2. The posterior superior iliac spine marks the level of the second sacral vertebra. Its presence is indicated by a dimple in the overlying skin

  3. The iliac crest extends from the anterior superior iliac spine to the posterior superior iliac spine

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Ischium
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The ischium consists of a body in which the superior and inferior rami merge.

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  1. The body forms part of the acetabulum

  2. The superior ramus is posterior and inferior to the body

  3. The inferior ramus fuses with the inferior ramus of the pubis

  4. The ischial spine separates the greater sciatic from the lesser sciatic notch. It is an important landmark. Part of the levator ani muscle is attached to it

  5. The ischial tuberosity is the inferior part of the ischium and is the bone on which humans sit

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Pubis
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The pubis consists of the body and two rami.

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  1. The body has a rough surface on its medial aspect. This is joined to the corresponding area on the opposite pubis to form the symphysis pubis. The levator ani muscles are attached to the pelvic aspect of the pubis

  2. The pubic crest is the superior border of the body

  3. The pubic tubercle, or spine, is the lateral end of the pubic crest. The inguinal ligament and conjoined tendon are attached here

  4. The superior ramus meets the body of the pubis at the pubic spine and the body of the ilium at the iliopectineal line, where it forms a part of the acetabulum

  5. The inferior ramus merges with the inferior ramus of the ischium

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Landmarks can be identified:

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  1. The iliopectineal line extends from the pubic tubercle back to the sacroiliac joint. It forms the greater part of the boundary of the pelvic inlet

  2. The greater sacrosciatic notch is between the posterior inferior iliac spine superiorly and the ischial spine inferiorly

  3. The lesser sacrosciatic notch is bounded by the ischial spine superiorly and the ischial tuberosity inferiorly

  4. The obturator foramen is delimited by the acetabulum, the ischial rami, and the pubic rami

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Sacrum

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