The sacrum (plural: ''sacra'' or ''sacrums''), in
human anatomy, is a large, triangular
bone
A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, an ...
at the base of the
spine that forms by the fusing of the sacral vertebrae (S1S5) between ages 18 and 30.
The sacrum situates at the upper, back part of the
pelvic cavity
The pelvic cavity is a body cavity that is bounded by the bones of the pelvis. Its oblique roof is the pelvic inlet (the superior opening of the pelvis). Its lower boundary is the pelvic floor.
The pelvic cavity primarily contains the reprod ...
, between the two
wings 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 the
ilium
Ilium or Ileum may refer to:
Places and jurisdictions
* Ilion (Asia Minor), former name of Troy
* Ilium (Epirus), an ancient city in Epirus, Greece
* Ilium, ancient name of Cestria (Epirus), an ancient city in Epirus, Greece
* Ilium Building, a ...
at the L-shaped
sacroiliac joints. The upper part of the sacrum connects with the last
lumbar vertebra
The lumbar vertebrae are, in human anatomy, the five vertebrae between the rib cage and the pelvis. They are the largest segments of the vertebral column and are characterized by the absence of the foramen transversarium within the transverse ...
(L5), and its lower part with the
coccyx (tailbone) via the sacral and coccygeal cornua.
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
The pelvic cavity is a body cavity that is bounded by the bones of the pelvis. Its oblique roof is the pelvic inlet (the superior opening of the pelvis). Its lower boundary is the pelvic floor.
The pelvic cavity primarily contains the reprod ...
.
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. For instance, the S1S5 vertebrae of a horse will fuse, the S1S3 of a dog will fuse, and four pelvic vertebrae of a rat will fuse between the lumbar and the caudal vertebrae of its tail.
The ''
Stegosaurus''
dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
had a greatly enlarged neural canal in the sacrum, characterized as a "
posterior brain case".
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. On either side of the base is a large projection known as an ala of sacrum and these alae (wings) articulate with the
sacroiliac joints. The alae support the
psoas major muscle
The psoas major ( or ; from grc, ψόᾱ, psóā, muscles of the loins) is a long fusiform muscle located in the lateral lumbar region between the vertebral column and the brim of the lesser pelvis. It joins the iliacus muscle to form the il ...
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
The lumbar vertebrae are, in human anatomy, the five vertebrae between the rib cage and the pelvis. They are the largest segments of the vertebral column and are characterized by the absence of the foramen transversarium within the transverse ...
by an intervertebral
fibrocartilage
Fibrocartilage consists of a mixture of white fibrous tissue and cartilaginous tissue in various proportions. It owes its inflexibility and toughness to the former of these constituents, and its elasticity to the latter. It is the only type of ...
. Behind this is the large triangular orifice of the sacral canal, which is completed by the
lamina and
spinous process
The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...
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. Normally it is greater than 60 degrees. A sacral angle of lesser degree suggests funneling of the pelvis.
Promontory
The sacral promontory marks part of the border of the
pelvic inlet, and comprises the
iliopectineal line and the
linea terminalis.
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
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
The piriformis muscle () is a flat, pyramidally-shaped muscle in the gluteal region of the lower limbs. It is one of the six muscles in the lateral rotator group.
The piriformis muscle has its origin upon the front surface of the sacrum, and ...
. 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 process
The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...
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
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.
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 ligament
The sacrospinous ligament (small or anterior sacrosciatic ligament) is a thin, triangular ligament in the human pelvis. The base of the ligament is attached to the outer edge of the sacrum and coccyx, and the tip of the ligament attaches to th ...
s, to some fibers of the
gluteus maximus at the back and to the
coccygeus
The coccygeus muscle or ischiococcygeus is a muscle of the pelvic floor, located posterior to levator ani and anterior to the sacrospinous ligament.
Structure
The coccygeus muscle is posterior to levator ani and anterior to the sacrospinous lig ...
in the front.
Articulations
The sacrum articulates with four bones:
* the last
lumbar vertebra
The lumbar vertebrae are, in human anatomy, the five vertebrae between the rib cage and the pelvis. They are the largest segments of the vertebral column and are characterized by the absence of the foramen transversarium within the transverse ...
above
* the
coccyx (tailbone) below
* the
ilium
Ilium or Ileum may refer to:
Places and jurisdictions
* Ilion (Asia Minor), former name of Troy
* Ilium (Epirus), an ancient city in Epirus, Greece
* Ilium, ancient name of Cestria (Epirus), an ancient city in Epirus, Greece
* Ilium Building, a ...
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
nutatio which means "nodding") and the reverse, postero-inferior motion of the sacrum relative to the ilium whilst the coccyx moves anteriorly, "counter-nutation".
In upright vertebrates, the sacrum is capable of slight independent movement along the
sagittal plane
The sagittal plane (; also known as the longitudinal plane) is an anatomical plane that divides the body into right and left sections. It is perpendicular to the transverse and coronal planes. The plane may be in the center of the body and div ...
. On bending backward the top (base) of the sacrum moves forward relative to the ilium; on bending forward the top moves back.
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. The bodies of the first and second vertebrae may fail to unite.
Development
The
somites that give rise to the
vertebral column
The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordate ...
begin to develop from head to tail along the length of the notochord. At day 20 of
embryogenesis
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...
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.
Clinical significance
Congenital disorders
The
congenital disorder
A birth defect, also known as a congenital disorder, is an abnormal condition that is present at birth regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities c ...
,
spina bifida
Spina bifida (Latin for 'split spine'; SB) is a birth defect in which there is incomplete closing of the spine and the membranes around the spinal cord during early development in pregnancy. There are three main types: spina bifida occulta, m ...
, occurs as a result of a defective
embryonic neural tube
In the developing chordate (including vertebrates), the neural tube is the embryonic precursor to the central nervous system, which is made up of the brain and spinal cord. The neural groove gradually deepens as the neural fold become elevated, ...
, characterised by the incomplete closure of vertebral arch or of the incomplete closure of the surface of the vertebral canal.
[Anderson, D.M. (2000). ''Dorland’s illustrated medical dictionary'' (29th edition). Philadelphia/London/Toronto/Montreal/Sydney/Tokyo: W.B. Saunders Company.] 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
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...
(occurring by the seventh week) of the lower spine. 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.
Fracture
Sacral fractures are relatively uncommon; however, they are often associated with neurological deficits. In the presence of neurological signs, they are mostly treated with
surgical fixation.
Cancer
The sacrum is one of the main sites for the development of the
sarcoma
A sarcoma is a malignant tumor, a type of cancer that arises from transformed cells of mesenchymal (connective tissue) origin. Connective tissue is a broad term that includes bone, cartilage, fat, vascular, or hematopoietic tissues, and sa ...
s known as
chordomas that are derived from the remnants of the
embryonic notochord
In anatomy, the notochord is a flexible rod which is similar in structure to the stiffer cartilage. If a species has a notochord at any stage of its life cycle (along with 4 other features), it is, by definition, a chordate. The notochord cons ...
.
Other animals
In
dogs the sacrum is formed by three fused vertebrae. The sacrum in the
horse
The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million ...
is made up of five fused vertebrae. In
bird
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
s the sacral vertebrae are fused with the lumbar and some caudal and thoracic vertebrae to form a single structure called the
synsacrum
The synsacrum is a skeletal structure of birds and other dinosaurs, in which the sacrum is extended by incorporation of additional fused or partially fused caudal or lumbar vertebrae and it can only be seen in birds. Some posterior thoracic vert ...
. In the
frog
A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-frog" '' Triadobatrachus'' is ...
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.
History
English ''sacrum'' was introduced as a technical term in anatomy in the mid-18th century, as a shortening of the
Late Latin
Late Latin ( la, Latinitas serior) is the scholarly name for the form of Literary Latin of late antiquity.Roberts (1996), p. 537. English dictionary definitions of Late Latin date this period from the , and continuing into the 7th century in the ...
name ''os sacrum'' "sacred bone", itself a translation of
Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
ἱερόν ὀστέον, the term found in the writings of
Galen
Aelius Galenus or Claudius Galenus ( el, Κλαύδιος Γαληνός; September 129 – c. AD 216), often Anglicized as Galen () or Galen of Pergamon, was a Greek physician, surgeon and philosopher in the Roman Empire. Considered to be on ...
.
Prior to the adoption of ''sacrum'', the bone was also called ''holy bone'' in English,
paralleling German ''heiliges Bein'' or ''Heiligenbein'' (alongside ''Kreuzbein'') and Dutch ''heiligbeen''.
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).
Others attribute the adjective ἱερόν to the ancient belief that this specific bone would be indestructible.
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''.
In
Classical Greek
Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic peri ...
the bone was known as κλόνις (Latinized ''clonis''); this term is cognate to Latin ''clunis'' "buttock",
Sanskrit
Sanskrit (; attributively , ; nominalization, nominally , , ) is a classical language belonging to the Indo-Aryan languages, Indo-Aryan branch of the Indo-European languages. It arose in South Asia after its predecessor languages had Trans-cul ...
' "haunch" and Lithuanian ''šlaunis'' "hip, thigh".
[used by Antimachus; see ] The Latin word is found in the alternative Latin name of the sacrum, ''ossa clunium'', as it were "bones of the buttocks".
Due to the fact that the os sacrum is broad and thick at its upper end,
the sacrum is alternatively called ''os latum'', "broad bone".
Additional images
File:Gray242.png, Image of a female pelvis seen anteriorly, sacrum at centre.
File:Gray97.png, Lateral surfaces of sacrum and coccyx.
File:Gray98.png, Base of sacrum.
File:Gray99.png, Median sagittal section of the sacrum.
File:Gray404.png, Left levator ani from within.
File:Gray803.png, The posterior divisions of the sacral nerves.
File:Slide2CORO.JPG, Sacrum. Pelvic surface.
File:Slide4CORO.JPG, Sacrum. Dorsal surface.
File:Sacrum Anatomy by Jason Christian.webm, Sacrum Anatomy
See also
*
Bone terminology
*
Pelvimetry
*
Rump (croup)
References
External links
* – "The Female Pelvis: Bones"
*
{{Authority control
Bones of the thorax
Irregular bones
Bones of the vertebral column