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Other Terms: Pelvis, Sacral bone, Sacrum [sacral vertebrae I - V], Os sacrum [vertebrae sacrales I - V]
The sacrum is a large triangular shaped mass that forms from the fusion of five vertebrocostal segments. The base of the triangle is superior and tapers to a flattened apex inferiorly. It is concave anteriorly and convex posteriorly. The lateral margins of the triangle are widest superiorly where the bone articulates with the two ilia. Forming the large basal portion of the vertebral column, the bone wedges between the two os coxae to form the pelvic skeleton. Its ventral surface, smoother than the rough dorsal surface, forms the posterior wall of the pelvis. Encased within this triangular mass of bone is a hollow sacral canal. This canal opens through foramina onto the ventral and dorsal surfaces of the bone. It forms a large oval articular surface superiorly with the fifth lumbar vertebra and a smaller oval facet at its apex for articulation with the coccyx. The bone is similar to other vertebrae in basic construction. It has an outer shell of compact bone with an inner core of spongy bone.
Sacrum is a Latin word meaning sacred. The origin of this term is uncertain, debated, and interesting. Various proposals for its origin exist. Some think the name arose because the bone played a key part in ancient pagan sacrificial rites. Others say that because it is the last bone of the body to decay, the rest of the body will reorganize around it on resurrection day. Possibly it was called sacred because it protects the genital organs that were held sacred in ancient times.
The sacrum articulates with four bones: the fifth lumbar vertebra, the two os coxae, and the coccyx. The body of the upper sacral segment forms a large oval surface that forms a cartilaginous symphysial joint with the inferior surface of the fifth lumbar vertebra's body. Similarly, the inferior surface of the fifth sacral segment forms a small oval surface that articulates with the superior surface of the coccyx via a cartilaginous symphyseal joint. Superolaterally, the sacrum forms wide auricular surfaces that form a synovial joint with corresponding surfaces on the ilia.
The sacrum ossifies from centers that are similar to other vertebrocostal elements, except these separate centers all eventually ossify into one element. Each of the five vertebral bodies has a primary center and secondary epiphyses. The primary center appears in the superior segment during the eighth embryonic week. Primary centers appear in the next two segments during the third month. The centers in the final two segments do not begin to ossify until the sixth to eighth month. Bilateral centers for the vertebral arches appear during the sixth to eighth month near the base of the arch and before birth costal centers make their appearance laterally. Progressing from caudal to cranial, the vertebral arch centers join with the bodies from the second year to the sixth year. Around the sixteenth year, the secondary epiphysial plates form above and below each body segment. Between the eighteenth and twentieth year secondary epiphyses form for the auricular surface and the lower lateral margin of the bone. In the early years of life the individual segments are separated by intervertebral discs. Progressing in a caudal to cranial direction, the discs ossify from ages eighteen to twenty-five.
Os sacrum [vertebrae sacrales I - V]