|Sacrum, pelvic surface|
|Image of pelvis (sacrum is in center)|
|Gray's||subject #24 106|
In humans, the sacrum (plural: sacrums or sacra) is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx (tailbone). It consists of usually five initially unfused vertebrae which begin to fuse between ages 16–18 and are usually completely fused into a single bone by age 34.
It is curved upon itself and placed obliquely (that is, tilted forward). It is kyphotic—that is, concave facing forward. The base projects forward as the sacral promontory internally, and articulates with the last lumbar vertebra to form the prominent sacrovertebral angle. The central part is curved outward toward the posterior, allowing greater room for the pelvic cavity. The two lateral projections of the sacrum are called ala (wings), and articulate with the ilium at the L-shaped sacroiliac joints.
The name is derived from the Latin (os) sacrum, (sacer, sacra, sacrum, "sacred"), a translation of the Greek hieron (osteon), meaning sacred or strong bone. It is called so either because supposedly sacrum was the part of an animal offered in sacrifice (since the sacrum is the seat of the organs of procreation) or because of the belief that the soul of the man resides there . In Slavic languages and in German this bone is called the 'cross bone' (Kreuzbein), in Dutch 'holy bone' (Heiligbeen).
The sacrum articulates with four bones:
Rotation of the sacrum infero-posteriorly a few degrees vis-à-vis the ilia is sometimes called "nutation" (from the Latin term nutatio which means "nodding") and the reverse, antero-superiorly motion, "counter-nutation." In upright vertebrates, the sacrum is capable of slight independent movement along the sagittal plane. When you bend backward the top (base) of the sacrum moves forward relative to the ilium; when you bend forward the top moves back.
The sacrum is called so when referred to all of the parts combined. Its parts are called sacral vertebrae when referred individually.
Sacral Diagnosis is a common issue in Osteopathic Manipulative Medicine. There are many types of sacral diagnoses, such as torsion and shear. To diagnose a sacral torsion, the axis of rotation is found with the axis named after its superior pole. If the opposite side of the pole is rotated anteriorly, it is rotated towards the pole, in which case is is called either a right-on-right (R on R) or left-on-left (L on L) torsion. The first letter in the diagnosis pertains to the direction of rotation of the superior portion of the sacrum opposite the side of the superior axis pole, and the last letter pertains to the pole.
The sacrum is noticeably sexually dimorphic (differently shaped in males and females).
In the female the sacrum is shorter and wider than in the male; the lower half forms a greater angle with the upper; the upper half is nearly straight, the lower half presenting the greatest amount of curvature. The bone is also directed more obliquely backward; this increases the size of the pelvic cavity and renders the sacrovertebral angle more prominent.
In the male the curvature is more evenly distributed over the whole length of the bone, and is altogether larger than in the female.
Sometimes the uppermost transverse tubercles are not joined to the rest of the ala on one or both sides, or the sacral canal may be open throughout a considerable part of its length, in consequence of the imperfect development of the laminae and spinous processes.
The sacrum also varies considerably with respect to its degree of curvature.
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