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Title: Sacrum  
Author: World Heritage Encyclopedia
Language: English
Subject: Pelvis, Sacral canal, Coccyx, Buttock augmentation, Multifidus muscle
Collection: Bones of the Thorax, Bones of the Vertebral Column, Irregular Bones, Sacrum
Publisher: World Heritage Encyclopedia


Sacrum, pelvic surface
Image of a male pelvis (sacrum is in center)
Latin Os sacrum
MeSH A02.835.232.834.717
FMA 16202
Anatomical terms of bone

In tetrapod anatomy, the sacrum ( or ; plural: sacra or sacrums;[1] Latin os sacrum[2]) is a large, triangular bone at the base of the spine and at the upper, back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. The sacrum consists of fused vertebrae – the number of which can vary between the species – dogs have three for example and humans and horses have five.

In humans, 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. Usually, it begins as five unfused vertebrae (numbered S1 to S5) which begin to fuse between the ages of 16 and 18 years and have usually completely fused into a single bone by the age of 34 years.

The sacrum has three different surfaces which are shaped to accommodate various structures. It articulates with four other bones. It is curved upon itself and placed obliquely (tilted forward). It is concave, facing forward. The base projects forward as the sacral promontory internally, which is the uppermost portion of the sacrum. 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 the alae (wings), and articulate with the ilium at the L-shaped sacroiliac joints.

The sacral vertebrae develop by the end of the first month of embryonic development, the higher vertebrae having developed first. There are congenital disorders that develop in the early stages of the fetus.


  • Name 1
  • Structure 2
    • Promontory 2.1
    • Surfaces 2.2
    • Articulations 2.3
    • Variations 2.4
      • Sexual dimorphism 2.4.1
    • Development 2.5
  • Clinical significance 3
    • Congenital disorders 3.1
    • Cancer 3.2
    • In osteopathic medicine 3.3
  • Other animals 4
  • Additional images 5
  • See also 6
  • References 7
  • External links 8


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.[3][3][4][5][6][7][8] Prior to the adoption of sacrum, the bone was also called holy bone in English,[9] paralleling German heiliges Bein or Heiligenbein (alongside Kreuzbein[10]) and Dutch heiligbeen.[9][11][12]

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

  • Anatomy photo:43:os-0401 at the SUNY Downstate Medical Center - "The Female Pelvis: Articulated bones of pelvis"
  • Anatomy photo:43:st-0401 at the SUNY Downstate Medical Center - "The Female Pelvis: Bones"

External links

  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. ^ a b c Hyrtl, J. (1880). Onomatologia Anatomica. Geschichte und Kritik der anatomischen Sprache der Gegenwart. Wien: Wilhelm Braumüller. K.K. Hof- und Unversitätsbuchhändler.
  4. ^ 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.
  5. ^ a b Anderson, D.M. (2000). Dorland’s illustrated medical dictionary (29th edition). Philadelphia/London/Toronto/Montreal/Sydney/Tokyo: W.B. Saunders Company.
  6. ^ 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.
  7. ^ Federative Committee on Anatomical Terminology (FCAT) (1998). Terminologia Anatomica. Stuttgart: Thieme
  8. ^ Lewis, C.T. & Short, C. (1879). A Latin dictionary founded on Andrews' edition of Freund's Latin dictionary. Oxford: Clarendon Press.
  9. ^ a b c d Schreger, C.H.Th. (1805). Synonymia anatomica. Synonymik der anatomischen Nomenclatur. Fürth: im Bureau für Literatur.
  10. ^ "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.
  11. ^ a b c 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.
  12. ^ Everdingen, J.J.E. van, Eerenbeemt, A.M.M. van den (2012). Pinkhof Geneeskundig woordenboek (12de druk). Houten: Bohn Stafleu Van Loghum.
  13. ^ Online Etymology Dictionary
  14. ^ a b 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.
  15. ^ used by Antimachus; see Henry George Liddell, Robert Scott, A Greek-English Lexicon
  16. ^ Kraus, L.A. (1844). Kritisch-etymologisches medicinisches Lexikon (Dritte Auflage). Göttingen: Verlag der Deuerlich- und Dieterichschen Buchhandlung.
  17. ^ Kirschner, Celeste G. (2005). Netter's Atlas Of Human Anatomy For CPT Coding. Chicago: American medical association. p. 274.  
  18. ^ Joseph D. Kurnik, DC. "The AS Ilium Fixation, Nutation, and Respect". 
  19. ^ Maitland, J (2001). Spinal Manipulation Made Simple. Berkeley: North Atlantic Books, p. 72.
  20. ^
  21. ^
  22. ^ Larsen, W.J. Human Embryology.2001.Churchill Livingstone Pages 63-64 ISBN 0-443-06583-7
  23. ^ Sonek JD, Gabbe SG, Landon MB, Stempel LE, Foley MR, Shubert-Moell K (March 1990). "Antenatal diagnosis of sacral agenesis syndrome in a pregnancy complicated by diabetes mellitus". Am. J. Obstet. Gynecol. 162 (3): 806–8.  
  24. ^
  25. ^ Wedel, F.P. "D.O." (PDF). A.T. Still University School of Osteopathic Medicine. Retrieved 18 February 2014. 
  26. ^ King, Christine, BVSc, MACVSc, and Mansmann, Richard, VMD, PhD. "Equine Lameness." Equine Research, Inc. 1997.

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


See also

Additional images

In dogs the sacrum is formed by three fused vertebrae. The sacrum in the horse is made up of five fused vertebrae.[26]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.

Other animals

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.[25]

In osteopathic medicine

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.[24]


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.[23] 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.

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.[5] The most common sites for spina bifida malformations are the lumbar and sacral areas.

Congenital disorders

Clinical significance

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.[22]


In the male the curvature is more evenly distributed over the whole length of the bone, and is altogether larger than in the female.

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.

The sacrum is noticeably sexually dimorphic (differently shaped in males and females).

Sexual dimorphism

The sacrum also varies considerably with respect to its degree of curvature.

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.

In some cases the sacrum will consist of six pieces[20] or be reduced in number to four.[21] The bodies of the first and second vertebrae may fail to unite.


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

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."[18] 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.[19]

The sacrum articulates with four bones:


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.

Lateral surface

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.

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 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.

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.

Dorsal surface
Pelvic surface


The sacral promontory marks part of the border of the pelvic inlet, and comprises the iliopectineal line and the linea terminalis.[17] 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.


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. Normally it is greater than 60 degrees. A sacral angle of lesser degree suggests funneling of the pelvis.


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".[15][16] The Latin word is found in the alternative Latin name of the sacrum, ossa clunium, as it were "bones of the buttocks".[9] Due to the fact that the os sacrum is broad and thick at its upper end,[11] the sacrum is alternatively called os latum, "broad bone".[9][14]

[14][3].vertebra magna 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 os sacrum As the Greek adjective ἱερός may also mean "strong", it has also been suggested that [11] Others attribute the adjective ἱερόν to the ancient belief that this specific bone would be indestructible.[13]

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