Sacrum

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Sacrum
Gray95.png
Sacrum, pelvic surface
Gray241.png
Image of a male pelvis (sacrum is in center)
Latin Os sacrum
Gray's p.106
MeSH Sacrum
TA A02.2.05.001
FMA FMA:16202
Anatomical terms of bone

In humans, the sacrum (/ˈsækrəm/ or /ˈskrəm/; plural: sacrums or sacra) 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. Its upper part connects with the last lumbar vertebra, and its lower part with the coccyx (tailbone). Usually, it begins as five unfused vertebrae which begin to fuse between the ages of 16–18 years and have usually completely fused into a single bone by the age of 34 years.

It is curved upon itself and placed obliquely (tilted forward). It 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 the ala (wings), and articulate with the ilium at the L-shaped sacroiliac joints.

Structure[edit]

The sacrum consists of several parts:

  • The pelvic surface of the sacrum is concave from above downward, and slightly so from side to side.
  • The dorsal surface of the sacrum is convex and narrower than the pelvic.
  • The lateral surface of the sacrum is broad above, but narrowed into a thin edge below.
  • The base of the sacrum, which is broad and expanded, is directed upward and forward.
  • The apex (apex oss. sacri) is directed downward, and presents an oval facet for articulation with the coccyx.
  • The vertebral canal (canalis sacralis; sacral canal) runs throughout the greater part of the bone; above, it is triangular in form; below, its posterior wall is incomplete, from the non-development of the laminae and spinous processes. It lodges the sacral nerves, and its walls are perforated by the anterior and posterior sacral foramina through which these nerves emerge.

Articulations[edit]

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."[1] 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.[2]

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

Variations[edit]

In some cases the sacrum will consist of six pieces[3] or be reduced in number to four.[4] 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[edit]

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[edit]

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.[5]

Clinical significance[edit]

In oncology[edit]

The sacrum is one of the main sites for the development of the sarcomas known as chordomas (chordosarcomas) that are derived from the remnants of the embryonic notochord.[6]

In osteopathic medicine[edit]

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.[7]

History[edit]

Etymology[edit]

The 'English' name sacrum [8] is derived from the full Latin expression os sacrum,[9][10] a translation of Ancient Greek ἱερόν ὀστέον,[11] attested in the writings of Greek physician Galen.[11][12] Both os as ὀστέον mean bone [12][13] and sacrum as ἱερόν mean holy.[12][13] Formerly the os sacrum was called holy bone [14] in English. In other languages similar expressions, like heiliges Bein [14] or Heiligenbein [15] (German) and heilig been [16] (Dutch) exist.

There are a few probable etymological explanations for ἱερόν ὀστέον. Supposedly the sacrum was the part of an animal offered in sacrifice (since the sacrum is the seat of the organs of procreation).[17] Alternatively, it was believed that that the soul of the man resided in the sacrum.[citation needed] Others[15] attribute the adjective ἱερόν to the ancient belief that this specific bone would be indestructable. Another source mentions that the os sacrum, being the largest of the vertebrae, was also called μέγας σπόνδυλος[18] by the Greeks, with μέγας, big [12] and σπόνδυλος, vertebra.[12] In certain instances ἱερός was considered as a synonym of μέγας[18] in Ancient Greek, hence the transformation from μέγας σπόνδυλος to ἱερόν ὀστέον. Latin vertebra magna,[11] with magna, big,[13] is a translation of μέγας σπόνδυλος.

Besides the aforementioned expressions, the Ancient Greeks, as attested in the poetry of Greek poet Antimachus,[12] used κλόνις,[12][19] Latinized as clonis.[19] to refer to the sacrum. Κλόνις is cognate to Latin clunis [12][19] ("buttock"[13]). The latter can be found in the genitive plural (='"of the buttocks") in the synonymous Latin expression for the ossa sacra, ossa clunium.[14] Due to the fact that the os sacrum is broad and thick at its upper end,[15] the os sacrum is alternatively called os latum,[14][18] from latum, broad.[13]

Additional images[edit]

See also[edit]

References[edit]

  1. ^ Joseph D. Kurnik, DC. "The AS Ilium Fixation, Nutation, and Respect". 
  2. ^ Maitland, J (2001). Spinal Manipulation Made Simple. Berkeley: North Atlantic Books, p. 72.
  3. ^ http://www.webcitation.org/query?url=http://www.geocities.com/akramjfr/sacralization.html&date=2009-10-25+12:10:24
  4. ^ http://www.webcitation.org/query?url=http://www.geocities.com/akramjfr/lumbarization.html&date=2009-10-25+12:10:08
  5. ^ Larsen, W.J. Human Embryology.2001.Churchill Livingstone Pages 63-64 ISBN 0-443-06583-7
  6. ^ http://www.chordomafoundation.org/understanding-chordoma/
  7. ^ Wedel, F.P. "D.O.". A.T. Still University School of Osteopathic Medicine. Retrieved 18 February 2014. 
  8. ^ Anderson, D.M. (2000). Dorland’s illustrated medical dictionary (29nd 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. ^ a b c Hyrtl, J. (1880). Onomatologia Anatomica. Geschichte und Kritik der anatomischen Sprache der Gegenwart. Wien: Wilhelm Braumüller. K.K. Hof- und Unversitätsbuchhändler.
  12. ^ a b c d e f g h 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.
  13. ^ a b c d e Lewis, C.T. & Short, C. (1879). A Latin dictionary founded on Andrews' edition of Freund's Latin dictionary. Oxford: Clarendon Press.
  14. ^ a b c d Schreger, C.H.Th.(1805). Synonymia anatomica. Synonymik der anatomischen Nomenclatur. Fürth: im Bureau für Literatur.
  15. ^ 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.
  16. ^ Everdingen, J.J.E. van, Eerenbeemt, A.M.M. van den (2012). Pinkhof Geneeskundig woordenboek (12de druk). Houten: Bohn Stafleu Van Loghum.
  17. ^ Online Etymology Dictionary
  18. ^ a b c 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.
  19. ^ a b c Kraus, L.A. (1844). Kritisch-etymologisches medicinisches Lexikon (Dritte Auflage). Göttingen: Verlag der Deuerlich- und Dieterichschen Buchhandlung.

External links[edit]

This article incorporates text from a public domain edition of Gray's Anatomy.