The sacrum is the keystone of the pelvis; Discuss.

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Jubin Joseph

The sacrum is the keystone of the pelvis; Discuss.

The pelvis provides a strong and stable connection between the trunk and lower extremities. It consists of four bones: the two hip bones (forming the lateral and anterior walls), and the sacrum and the coccyx (which are part of the vertebral column and form the back wall). The two hip bones articulate anteriorly through the pubic symphysis and posteriorly with the sacroiliac joints forming a strong basin-shaped structure that contains and protects the lower parts of the intestinal and urinary tracts and the internal organs of reproduction.

The sacrum is the strongest bone of the pelvis and consists of 5 vertebrae fused together to form a single wedge-shaped bone, which has forward concavity. The upper border articulates with the fifth lumbar vertebrae. Obstetricians count the fifth lumbar as a part of the pelvis, since it is bound to the innominates by ilio-lumbar ligaments, which extend from the tips of its transverse processes to the crests of the ilia.  These ilio-lumbar ligaments tend to compel the fifth lumbar vertebra to act somewhat as though it were a portion of the solid pelvis.

The sacrum is an important part of the pelvis as it is the origin of some very important ligaments. The sacrotuberous ligament is strong and extends from the lateral part of the sacrum and the coccyx and the posterior inferior iliac spine to the ischial tuberosity. Also there is the sacrospinous ligament, which is strong and triangular in shape. It is attached by its base to the lateral part of the sacrum and the coccyx and by its apex to the spine of the ischium. Both the sarcotuberous and the sacrospinous ligaments prevent the lower end of the sacrum and the coccyx from being rotated upward at the sacroiliac joint by the weight of the body. The two ligaments also convert the greater and lesser sciatic notches into foramina, the greater and lesser sciatic foramina.

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The sacrum also plays an significant role in the innervation of the pelvis. The pelvis is mainly innervated buy the sacral and coccygeal nerves and by the pelvic part of the autonomic nervous system. The piriformis muscle pads the posterior wall of the pelvis and forms a bed for the sacral and coccygeal nerve plexuses. On the anterior surface of the piriformis muscle is the sacral plexus. This is formed by the lumbosacral trunk, the ventral rami of the first three sacral nerves and the descending part of the fourth sacral nerve. The main nerves of the sacral plexus lie ...

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