The sacrum situates at the upper, back part of thepelvic cavity, between the twowings of the pelvis. It forms joints with four other bones. The two projections at the sides of the sacrum are called the alae (wings), and articulate with theilium at the L-shapedsacroiliac joints. The upper part of the sacrum connects with the lastlumbar vertebra (L5), and its lower part with thecoccyx (tailbone) via the sacral and coccygeal cornua.
The sacrum has three different surfaces which are shaped to accommodate surrounding pelvic structures. Overall, it isconcave (curved upon itself). Thebase of the sacrum, the broadest and uppermost part, is tilted forward as the sacral promontory internally. The central part is curved outward toward theposterior, allowing greater room for thepelvic cavity.
In all otherquadrupedalvertebrates, 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. For instance, the S1–S5 vertebrae of a horse will fuse, the S1–S3 of a dog will fuse, and four pelvic vertebrae of a rat will fuse between the lumbar and the caudal vertebrae of its tail.[3]
The sacrum is a complex structure providing support for the spine and accommodation for thespinal 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. Thebase 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 anala of sacrum and these alae (wings) articulate with thesacroiliac joints. The alae support thepsoas major muscles and thelumbosacral trunk which connects thelumbar plexus with thesacral plexus. In the articulated pelvis, the alae are continuous with theiliac 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 theiliacus 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 thepelvic brim. The alae also form the base of the lumbosacral triangle. Theiliolumbar 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 lastlumbar vertebra by an intervertebralfibrocartilage. Behind this is the large triangular orifice of the sacral canal, which is completed by thelamina andspinous 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 thecoccyx. The sacral canal as a continuation of thevertebral canal runs throughout the greater part of the sacrum. Thesacral 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]
The sacral promontory marks part of the border of thepelvic inlet, and comprises theiliopectineal line and thelinea terminalis.[5] The sacral promontory articulates with the last lumbar vertebra to form thesacrovertebral angle, an angle of 30 degrees from the horizontal plane that provides a useful marker for asling implant procedure.
Thepelvic 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 fouranterior sacral foramina, diminishing in size in line with the smaller vertebral bodies. The foramina give exit to the anterior divisions of thesacral 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 thepiriformis 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 theintervertebral discs.
Thedorsal surface of the sacrum is convex and narrower than the pelvic surface. In the middle line is themedian sacral crest, surmounted by three or fourtubercles—the rudimentaryspinous 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 themultifidus muscle. The floor of the groove is formed by the unitedlaminae 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 thesacral hiatus in the posterior wall of thesacral canal. The sacral canal is a continuation of thespinal canal and runs throughout the greater part of the sacrum. Above the sacral hiatus, it is triangular in form. The canal lodges thesacral 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 indistinctmedial 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 fourposterior 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 thelateral 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
Thelateral 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, thesacral 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 thesacrotuberous andsacrospinous ligaments, to some fibers of thegluteus maximus at the back and to thecoccygeus in the front.
Rotation of the sacrum superiorly and anteriorly whilst the coccyx moves posteriorly relative to the ilium is sometimes called "nutation" (from the Latin termnutatio which means "nodding") and the reverse, postero-inferior motion of the sacrum relative to the ilium whilst the coccyx moves anteriorly, "counter-nutation".[6]In upright vertebrates, the sacrum is capable of slight independent movement along thesagittal plane. On bending backward the top (base) of the sacrum moves forward relative to the ilium; on bending forward the top moves back.[7]
Thesacrum refers to all of the parts combined. Its parts are calledsacral vertebrae when referred individually.
In some cases, the sacrum will consist of six pieces or be reduced in number to four.[8] The bodies of the first and second vertebrae may fail to unite.
Thesomites that give rise to thevertebral column begin to develop from head to tail along the length of the notochord. At day 20 ofembryogenesis, 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.[9]
Thecongenital disorder,spina bifida, occurs as a result of a defectiveembryonicneural tube, characterised by the incomplete closure of vertebral arch or of the incomplete closure of the surface of the vertebral canal.[10] The most common sites for spina bifida malformations are the lumbar and sacral areas.
Another congenital disorder is that ofcaudal regression syndrome also known assacral agenesis. This is characterised by an abnormal underdevelopment in theembryo (occurring by the seventh week) of the lower spine.[11] 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.
Sacral fractures are relatively uncommon; however, they are often associated with neurological deficits. In the presence of neurological signs, they are mostly treated withsurgical fixation.[12]
Indogs, the sacrum is formed by three fused vertebrae. The sacrum in thehorse is made up of five fused vertebrae.[14] Inbirds, the sacral vertebrae are fused with the lumbar and some caudal and thoracic vertebrae to form a single structure called thesynsacrum. In thefrog, 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.
Englishsacrum was introduced as a technical term in anatomy in the mid-18th century, as a shortening of theLate Latin nameos sacrum "sacred bone", itself a translation ofGreek ἱερόν ὀστέον, the term found in the writings ofGalen.[15][16][10][17][18][19] Prior to the adoption ofsacrum, the bone was also calledholy bone in English,[20] paralleling Germanheiliges Bein orHeiligenbein (alongsideKreuzbein[21]) and Dutchheiligbeen.[20][22][23]
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 theorgans of procreation).[24] Others attribute the adjective ἱερόν to the ancient belief that this specific bone would be indestructible.[22] As the Greek adjective ἱερός may also mean "strong", it has also been suggested thatos 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 asvertebra magna.[15][25]
InClassical Greek the bone was known as κλόνις (Latinizedclonis); this term is cognate to Latinclunis "buttock",Sanskritśróṇis "haunch" and Lithuanianšlaunis "hip, thigh".[26][27] The Latin word is found in the alternative Latin name of the sacrum,ossa clunium, as it were "bones of the buttocks".[20] Due to the fact that the os sacrum is broad and thick at its upper end,[22] the sacrum is alternatively calledos latum, "broad bone".[20][25]
^Kilincer, Cumhur; et al. (2009)."Sacrum anatomy".Scientific spine. Trakya Üniversitesi Rektörlüğü, Yerleşkesi, 22030 Edirne, Turkey: Self. Retrieved8 November 2015.{{cite web}}: CS1 maint: location (link)
^"Skeletal system"(PDF). Dept. of Biology. Gambier, Ohio: Kenyon College. Retrieved9 November 2015.
^Galton, P.M.; Upchurch, P. (2004). "Stegosauria". In Weishampel, D.B.; Dodson, P.; Osmólska, H. (eds.).The Dinosauria (2nd ed.). University of California Press. p. 361.ISBN0-520-24209-2.
^Kirschner, Celeste G. (2005).Netter's Atlas Of Human Anatomy For CPT Coding. Chicago: American medical association. p. 274.ISBN1-57947-669-4.
^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.doi:10.1016/0002-9378(90)91015-5.PMID2180307.
^Mirghasemi, Alireza; Mohamadi, Amin; Ara, Ali Majles; Gabaran, Narges Rahimi; Sadat, Mir Mostafa (November 2009). "Completely displaced S-1/S-2 growth plate fracture in an adolescent: case report and review of literature".Journal of Orthopaedic Trauma.23 (10):734–738.doi:10.1097/BOT.0b013e3181a23d8b.ISSN1531-2291.PMID19858983.S2CID6651435.
^King, Christine, BVSc, MACVSc, and Mansmann, Richard, VMD, PhD. "Equine Lameness." Equine Research, Inc. 1997.
^abHyrtl, J. (1880).Onomatologia Anatomica. Wien: Wilhelm Braumüller. K.K. Hof- und Universitätsbuchhändler.Geschichte und Kritik der anatomischen Sprache der Gegenwart
^Liddell, H.G.; Scott, R. (1940). Jones, Sir Henry Stuart; McKenzie, Roderick (eds.).A Greek-English Lexicon. Oxford: Clarendon Press.
^His, W. (1895).Die anatomische Nomenclatur. Nomina Anatomica. Leipzig: Verlag Veit & Comp.Der von der Anatomischen Gesellschaft auf ihrer IX. Versammlung in Basel angenommenen Namen
^Federative Committee on Anatomical Terminology (FCAT) (1998).Terminologia Anatomica. Stuttgart: Thieme.
^Lewis, C.T.; Short, C. (1879).A Latin Dictionary. Oxford: Clarendon Press.founded on Andrews' edition of Freund's Latin dictionary
^abcdSchreger, C.H.Th. (1805).Synonymia Anatomica. Fürth: im Bureau für Literatur.Synonymik der anatomischen Nomenclatur
^"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) andTeutscher Jäger by Johann Friedrich von Flemming (1719, p. 94) also givekreuz as hunting terminology for a specific bone of the stag.
^abcFoster, F.D. (1891–1893).An Illustrated Medical Dictionary. New York: D. Appleton and Company.Being a dictionary of the technical terms used by writers on medicine and the collateral sciences, in the Latin, English, French, and German languages.
^Everdingen, J.J.E. van, Eerenbeemt; A.M.M. van den (2012).Pinkhof Geneeskundig woordenboek (12de druk ed.). Houten: Bohn Stafleu Van Loghum.{{cite encyclopedia}}: CS1 maint: multiple names: authors list (link)
^abHyrtl, J. (1875).Lehrbuch der Anatomie des Menschen. Mit Rücksicht auf physiologische Begründung und praktische Anwendung (Dreizehnte Auflage ed.). Wien: Wilhelm Braumüller K.K. Hof- und Universitätsbuchhändler.
^used byAntimachus; seeLiddell, Henry George; Scott, Robert."klo-nis".A Greek-English Lexicon.
^Kraus, L.A. (1844).Kritisch-etymologisches medicinisches Lexikon (Dritte Auflage ed.). Göttingen: Verlag der Deuerlich- und Dieterichschen Buchhandlung.
