Editing Sacrum (section)

==Structure==
The sacrum is a complex structure providing support for the spine and accommodation for the [[spinal nerve]]s. It also articulates with the hip bones. The sacrum has a base, an apex, and three surfaces – a pelvic, dorsal and a lateral surface. The '''base of the sacrum''', which is broad and expanded, is directed upward and forward. On either side of the base is a large projection known as an '''ala of sacrum''' and these alae (wings) articulate with the [[sacroiliac joint]]s. The alae support the [[psoas major muscle]]s and the [[lumbosacral trunk]] which connects the [[lumbar plexus]] with the [[sacral plexus]]. In the articulated pelvis, the alae are continuous with the [[iliac fossa]]. Each ala is slightly concave from side to side, and convex from the back and gives attachment to a few of the fibers of the [[iliacus muscle]]. The posterior quarter of the ala represents the transverse process, and its anterior three-quarters the costal process of the first sacral segment. Each ala also serves as part of the border of the [[pelvic brim]]. The alae also form the base of the lumbosacral triangle. The [[iliolumbar ligament]] and lumbosacral ligaments are attached to the ala.

In the middle of the base is a large oval articular surface, the upper surface of the body of the first sacral vertebra, which is connected with the under surface of the body of the last [[lumbar vertebra]] by an intervertebral [[fibrocartilage]]. Behind this is the large triangular orifice of the sacral canal, which is completed by the [[lamina of the vertebral arch|lamina]] and [[spinous process]] of the first sacral vertebra. The superior articular processes project from it on either side; they are oval, concave, directed backward and medialward, like the superior articular processes of a lumbar vertebra. They are attached to the body of the first sacral vertebra and to each ala, by short thick pedicles; on the upper surface of each pedicle is a vertebral notch, which forms the lower part of the foramen between the last lumbar and first sacral vertebrae.

The apex is directed downward and presents an oval facet for articulation with the [[coccyx]]. The sacral canal as a continuation of the [[vertebral canal]] runs throughout the greater part of the sacrum. The '''sacral angle''' is the angle formed by the true conjugate with the two pieces of sacrum.{{clarify|date=June 2015}} Normally, it is greater than 60 degrees. A sacral angle of lesser degree suggests funneling of the pelvis.{{clarify|date=June 2015}}

===Promontory===
{{anchor|Promontorium}}The sacral promontory marks part of the border of the [[pelvic inlet]], and comprises the [[iliopectineal line]] and the [[linea terminalis]].<ref name="isbn1-57947-669-4">{{cite book |author=Kirschner, Celeste G. |title=Netter's Atlas Of Human Anatomy For CPT Coding |publisher=American medical association |location=Chicago |year=2005 |isbn=1-57947-669-4 |page=274}}</ref> The sacral promontory articulates with the last lumbar vertebra to form the [[sacrovertebral angle]], an angle of 30 degrees from the horizontal plane that provides a useful marker for a [[sling (implant)|sling implant]] procedure.

===Surfaces===
[[File:Gray95.png|thumb|left|180px|Pelvic surface]]
[[File:Gray96.png|thumb|right|200px|Dorsal surface]]
The '''pelvic surface of the sacrum''' is concave from the top, and curved slightly from side to side. Its middle part is crossed by four transverse ridges, which correspond to the original planes of separation between the five sacral vertebrae. The body of the first segment is large and has the form of a lumbar vertebra; the bodies of the next bones get progressively smaller, are flattened from the back, and curved to shape themselves to the sacrum, being concave in front and convex behind. At each end of the transverse ridges, are the four '''anterior sacral foramina''', diminishing in size in line with the smaller vertebral bodies. The foramina give exit to the anterior divisions of the [[sacral nerves]] and entrance to the lateral sacral arteries. Each part at the sides of the foramina is traversed by four broad, shallow grooves, which lodge the anterior divisions of the sacral nerves. They are separated by prominent ridges of bone which give origin to the [[piriformis muscle]]. If a sagittal section be made through the center of the sacrum, the bodies are seen to be united at their circumferences by bone, wide intervals being left centrally, which, in the fresh state, are filled by the [[intervertebral disc]]s.

The '''dorsal surface of the sacrum''' is convex and narrower than the pelvic surface. In the middle line is the '''median sacral crest''', surmounted by three or four [[tubercle (anatomy)|tubercles]]—the rudimentary [[spinous process]]es of the upper three or four sacral vertebrae. On either side of the median sacral crest is a shallow sacral groove, which gives origin to the [[multifidus muscle]]. The floor of the groove is formed by the united [[vertebral lamina|laminae]] of the corresponding vertebrae. The laminae of the fifth sacral vertebra, and sometimes those of the fourth, do not meet at the back, resulting in a fissure known as the '''sacral hiatus'''  in the posterior wall of the '''sacral canal'''. The sacral canal is a continuation of the [[spinal canal]] and runs throughout the greater part of the sacrum. Above the sacral hiatus, it is triangular in form. The canal lodges the [[sacral nerves]], via the anterior and posterior sacral foramina.

On the lateral aspect of the sacral groove is a linear series of tubercles produced by the fusion of the articular processes which together form the indistinct '''medial sacral crest'''. The articular processes of the first sacral vertebra are large and oval-shaped. Their facets are concave from side to side, face to the back and middle, and articulate with the facets on the inferior processes of the fifth lumbar vertebra.

The tubercles of the inferior articular processes of the fifth sacral vertebra, known as the sacral cornua, are projected downward and are connected to the cornua of the coccyx. At the side of the articular processes are the four '''posterior sacral foramina'''; they are smaller in size and less regular in form than those at the front, and transmit the posterior divisions of the sacral nerves. On the side of the posterior sacral foramina is a series of tubercles, the transverse processes of the sacral vertebrae, and these form the '''lateral sacral crest'''. The transverse tubercles of the first sacral vertebra are large and very distinct; they, together with the transverse tubercles of the second vertebra, give attachment to the horizontal parts of the posterior sacroiliac ligaments; those of the third vertebra give attachment to the oblique fasciculi of the posterior sacroiliac ligaments; and those of the fourth and fifth to the sacrotuberous ligaments.
[[File:Sobo 1909 18.png|thumb|right|180px|Lateral surface]]

The '''lateral surface of the sacrum''' is broad above, but narrows into a thin edge below. The upper half presents in front an ear-shaped surface, the auricular surface, covered with cartilage in the immature state, for articulation with the ilium. Behind it is a rough surface, the '''sacral tuberosity''', on which are three deep and uneven impressions, for the attachment of the posterior sacroiliac ligament. The lower half is thin, and ends in a projection called the inferior lateral angle. Medial to this angle is a notch, which is converted into a foramen by the transverse process of the first piece of the coccyx, and this transmits the anterior division of the fifth sacral nerve. The thin lower half of the lateral surface gives attachment to the [[sacrotuberous ligament|sacrotuberous]] and [[sacrospinous ligament]]s, to some fibers of the [[Gluteus maximus muscle|gluteus maximus]] at the back and to the [[Coccygeus muscle|coccygeus]] in the front.

=== Articulations ===

The sacrum articulates with four bones:
* the last [[lumbar vertebra]] above
* the [[coccyx|coccyx (tailbone)]] below
* the [[Ilium (bone)|ilium]] portion of the [[hip bone]] on either side

Rotation of the sacrum superiorly and anteriorly whilst the coccyx moves posteriorly relative to the ilium is sometimes called "nutation" (from the Latin term [[wikt:nutatio|nutatio]] which means "nodding") and the reverse, postero-inferior motion of the sacrum relative to the ilium whilst the coccyx moves anteriorly, "counter-nutation".<ref>{{cite web|author=Joseph D. Kurnik, DC|url=http://www.chiroweb.com/archives/14/26/18.html|title=The AS Ilium Fixation, Nutation, and Respect|date=16 December 1996 }}</ref>
In upright vertebrates, the sacrum is capable of slight independent movement along the [[sagittal plane]]. On bending backward the top (base) of the sacrum moves forward relative to the ilium; on bending forward the top moves back.<ref>Maitland, J (2001). ''Spinal Manipulation Made Simple.'' Berkeley: North Atlantic Books, p.&nbsp;72.</ref>

The ''sacrum'' refers to all of the parts combined. Its parts are called ''sacral vertebrae'' when referred individually.

=== Variations ===
In some cases, the sacrum will consist of six pieces or be reduced in number to four.<ref>{{Cite book|url=https://archive.org/details/anatomyhumanbod00lewigoog|quote=sacrum will consist of six pieces.|title=Anatomy of the Human Body|last=Gray|first=Henry|date=1918|publisher=Lea & Febiger|pages=[https://archive.org/details/anatomyhumanbod00lewigoog/page/n108 111]|language=en}}</ref> The bodies of the first and second vertebrae may fail to unite.

===Development===
The [[somite]]s that give rise to the [[vertebral column]] begin to develop from head to tail along the length of the notochord. At day 20 of [[human embryogenesis|embryogenesis]], the first four pairs of somites appear in the future occipital bone region. Developing at the rate of three or four a day, the next eight pairs form in the cervical region to develop into the cervical vertebrae; the next twelve pairs will form the thoracic vertebrae; the next five pairs the lumbar vertebrae and by about day 29, the sacral somites will appear to develop into the sacral vertebrae; finally on day 30, the last three pairs will form the coccyx.<ref>Larsen, W.J. Human Embryology.2001.Churchill Livingstone Pages 63–64 {{ISBN|0-443-06583-7}}</ref>