Sacrum

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For other uses, see Sacrum (disambiguation).

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Sacrum
Gray95.png
Sacrum, pelvic surface
Gray241.png
Image of a male pelvis (sacrum is in center)
Details
Latin Os sacrum
Identifiers
MeSH A02.835.232.834.717
Dorlands
/Elsevier		 o_07/12598664
TA Lua error in Module:Wikidata at line 744: attempt to index field 'wikibase' (a nil value).
TH {{#property:P1694}}
TE {{#property:P1693}}
FMA 16202
Anatomical terms of bone
[[[d:Lua error in Module:Wikidata at line 863: attempt to index field 'wikibase' (a nil value).|edit on Wikidata]]]


The sacrum (/ˈsækrəm/ or /ˈseɪkrəm/; plural: sacra or sacrums;[1] Latin os sacrum[2]) in human anatomy is a large, triangular bone at the base of the spine, that forms by the fusing of sacral vertebrae S1–S5, between 18 and 30 years of age.[3]

The sacrum articulates (forms a joint with) with four other bones. It is situated at the upper, back part of the pelvic cavity, where it is anatomically inserted between the two hip bones (ilium). The two lateral projections of the sacrum are called the alae (wings), and articulate with the ilium at the L-shaped sacroiliac joints. The upper part of the sacrum connects with the last lumbar vertebra, and its lower part with the coccyx (tailbone) via the sacral and coccygeal cornu.

The sacrum has three different surfaces which are shaped to accommodate surrounding pelvic structures. Overall it is concave (curved upon itself). The base of the sacrum (the broadest and uppermost part) is tilted forward as the sacral promontory internally. The central part is curved outward toward the posterior, allowing greater room for the pelvic cavity.

In all other quadrupedal vertebrates, the pelvic vertebrae undergo a similar developmental process to form a sacrum in the adult, even while the bony tail (caudal vertebrae) remain unfused. The number of sacral vertebrae varies slightly. A horse will fuse S1–S5, but a dog will fuse S1–S3. For example, the rat fuses four pelvic vertebrae between their lumbar and the caudal vertebrae of their tail.[4] The Stegosaurus dinosaur had a greatly enlarged neural canal in the sacrum, characterized as a "posterior brain case".[5]

Name

English sacrum was introduced as a technical term in anatomy in the mid-18th century, as a shortening of the Late Latin name os sacrum "sacred bone", itself a translation of Greek ἱερόν ὀστέον, the term found in the writings of Galen.[6][6][7][8][9][10][11] Prior to the adoption of sacrum, the bone was also called holy bone in English,[12] paralleling German heiliges Bein or Heiligenbein (alongside Kreuzbein[13]) and Dutch heiligbeen.[12][14][15]

The origin of Galen's term is unclear. Supposedly the sacrum was the part of an animal offered in sacrifice (since the sacrum is the seat of the organs of procreation).[16] Others attribute the adjective ἱερόν to the ancient belief that this specific bone would be indestructible.[14] As the Greek adjective ἱερός may also mean "strong", it has also been suggested that os sacrum is a mistranslation of a term intended to mean "the strong bone". This is supported by the alternative Greek name μέγας σπόνδυλος by the Greeks, translating to "large vertebra", translated into Latin as vertebra magna.[6][17]

In Classical Greek the bone was known as κλόνις (Latinized clonis); this term is cognate to Latin clunis "buttock", Sanskrit śróṇis "haunch" and Lithuanian šlaunis "hip, thigh".[18][19] The Latin word is found in the alternative Latin name of the sacrum, ossa clunium, as it were "bones of the buttocks".[12] Due to the fact that the os sacrum is broad and thick at its upper end,[14] the sacrum is alternatively called os latum, "broad bone".[12][17]

Structure

The sacrum is a complex structure providing support for the spine and accommodation for the spinal nerves. 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. (In anatomy, the broadest part of a structure is known as the base). A projection on either side of the sacrum is known as the ala (plural alae). 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.[clarification needed] Normally it is greater than 60 degrees. A sacral angle of lesser degree suggests funneling of the pelvis.[clarification needed]

Promontory

The sacral promontory marks part of the border of the pelvic inlet, and comprises the iliopectineal line and the linea terminalis.[20] 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 procedure.

Surfaces

Pelvic surface
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 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 discs.

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 tubercles; the rudimentary spinous processes 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 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.

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 as inferior articular processes of the fifth sacral vertebra, known as the sacral cornu, are projected downward, and are connected to the cornu 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.

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 and sacrospinous ligaments, to some fibers of the gluteus maximus at the back and to the coccygeus in the front.

Articulations

The sacrum articulates with four bones:

Rotation of the sacrum superiorly and anteriorly whilst the coccyx moves posteriorly relative to the ilium is sometimes called "nutation" (from the Latin term nutatio which means "nodding") and the reverse, postero-inferior motion of the sacrum relative to the ilium whilst the coccyx moves anteriorly, "counter-nutation."[21] 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.[22]

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[23] or be reduced in number to four.[24] The bodies of the first and second vertebrae may fail to unite.

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.

Sexual dimorphism

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.

Development

The somites that give rise to the vertebral column begin to develop from head to tail along the length of the notochord. At day 20 of 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.[25]

Clinical significance

Congenital disorders

The congenital disorder, spina bifida, occurs as a result of a defective embryonic neural tube, characterised by the incomplete closure of vertebral arch or of the incomplete closure of the surface of the vertebral canal.[8] The most common sites for spina bifida malformations are the lumbar and sacral areas.

Another congenital disorder is that of caudal regression syndrome also known as sacral agenesis. This is characterised by an abnormal underdevelopment in the embryo (occurring by the seventh week) of the lower spine.[26] Sometimes part of the coccyx is absent, or the lower vertebrae can be absent, or on occasion a small part of the spine is missing with no outward sign.

Cancer

The sacrum is one of the main sites for the development of the sarcomas known as chondrosarcomas that are derived from the remnants of the embryonic notochord.[27]

In osteopathic medicine

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 it 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.[28]

Other animals

In dogs the sacrum is formed by three fused vertebrae. The sacrum in the horse is made up of five fused vertebrae.[29]In birds the sacral vertebrae are fused with the lumbar and some caudal and thoracic vertebrae to form a single structure called the synsacrum. In the frog the ilium is elongated and forms a mobile joint with the sacrum that acts as an additional limb to give more power to its leaps.

Additional images

  • Image of a female pelvis seen anteriorly, sacrum at centre.

  • Lateral surfaces of sacrum and coccyx.

  • Base of sacrum.

  • Median sagittal section of the sacrum.

  • Left levator ani from within.

  • The posterior divisions of the sacral nerves.

  • Behind view.

  • Cut side view.

  • Lumbar and sacral plexus. Deep dissection. Anterior view.

  • Sacrum. Pelvic surface.

  • Sacrum. Dorsal surface.

See also

References

This article incorporates text in the public domain from the 20th edition of Gray's Anatomy (1918)

  1. ↑ Oxford Dictionaries and Webster's New College Dictionary (2010) admit the plural sacrums alongside sacra; The American Heritage Dictionary, Collins Dictionary and Webster's Revised Unabridged Dictionary (1913) give sacra as the only plural.
  2. ↑ "sacred bone", translation of Greek ἱερόν ὀστέον, supposedly so called because of its role in ancient animal sacrifice.
  3. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  4. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  5. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  6. ↑ 6.0 6.1 6.2 Hyrtl, J. (1880). Onomatologia Anatomica. Geschichte und Kritik der anatomischen Sprache der Gegenwart. Wien: Wilhelm Braumüller. K.K. Hof- und Unversitätsbuchhändler.
  7. ↑ Liddell, H.G. & Scott, R. (1940). A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with the assistance of. Roderick McKenzie. Oxford: Clarendon Press.
  8. ↑ 8.0 8.1 Anderson, D.M. (2000). Dorland’s illustrated medical dictionary (29th edition). Philadelphia/London/Toronto/Montreal/Sydney/Tokyo: W.B. Saunders Company.
  9. ↑ His, W. (1895). Die anatomische Nomenclatur. Nomina Anatomica. Der von der Anatomischen Gesellschaft auf ihrer IX. Versammlung in Basel angenommenen Namen. Leipzig: Verlag Veit & Comp.
  10. ↑ Federative Committee on Anatomical Terminology (FCAT) (1998). Terminologia Anatomica. Stuttgart: Thieme
  11. ↑ Lewis, C.T. & Short, C. (1879). A Latin dictionary founded on Andrews' edition of Freund's Latin dictionary. Oxford: Clarendon Press.
  12. ↑ 12.0 12.1 12.2 12.3 Schreger, C.H.Th. (1805). Synonymia anatomica. Synonymik der anatomischen Nomenclatur. Fürth: im Bureau für Literatur.
  13. ↑ "cross bone", also of unclear origin. According to Grimm, Deutsches Wörterbuch ("Kreuz", meaning 8a), Kreuz "cross" is used of the sacral area of the spine, but also of the spine as a whole, with usage examples from the 17th-century (Christian Weise, Isaacs Opferung, 1682, 3.11). Notabilia Venatoris by Hermann Friedrich von Göchhausen (1710) and Teutscher Jäger by Johann Friedrich von Flemming (1719, p. 94) also give kreuz as hunting terminology for a specific bone of the stag.
  14. ↑ 14.0 14.1 14.2 Foster, F.D. (1891-1893). An illustrated medical dictionary. Being a dictionary of the technical terms used by writers on medicine and the collateral sciences, in the Latin, English, French, and German languages. New York: D. Appleton and Company.
  15. ↑ Everdingen, J.J.E. van, Eerenbeemt, A.M.M. van den (2012). Pinkhof Geneeskundig woordenboek (12de druk). Houten: Bohn Stafleu Van Loghum.
  16. ↑ Online Etymology Dictionary
  17. ↑ 17.0 17.1 Hyrtl, J. (1875). Lehrbuch der Anatomie des Menschen. Mit Rücksicht auf physiologische Begründung und praktische Anwendung. (Dreizehnte Auflage). Wien: Wilhelm Braumüller K.K. Hof- und Universitätsbuchhändler.
  18. ↑ used by Antimachus; see Henry George Liddell, Robert Scott, A Greek-English Lexicon
  19. ↑ Kraus, L.A. (1844). Kritisch-etymologisches medicinisches Lexikon (Dritte Auflage). Göttingen: Verlag der Deuerlich- und Dieterichschen Buchhandlung.
  20. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  21. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  22. ↑ Maitland, J (2001). Spinal Manipulation Made Simple. Berkeley: North Atlantic Books, p. 72.
  23. ↑ http://www.webcitation.org/query?url=http://www.geocities.com/akramjfr/sacralization.html&date=2009-10-25+12:10:24
  24. ↑ http://www.webcitation.org/query?url=http://www.geocities.com/akramjfr/lumbarization.html&date=2009-10-25+12:10:08
  25. ↑ Larsen, W.J. Human Embryology.2001.Churchill Livingstone Pages 63-64 ISBN 0-443-06583-7
  26. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  27. ↑ http://www.chordomafoundation.org/understanding-chordoma/
  28. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  29. ↑ King, Christine, BVSc, MACVSc, and Mansmann, Richard, VMD, PhD. "Equine Lameness." Equine Research, Inc. 1997.

External links

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