View source for Ischium

{{short description|Lower and back region of the hip bone}}
{{More citations needed|date=April 2009}}
<!-- {{distinguish|Ischia}}  << There are no anatomical or even biological denotations of the term "Ischia". It is mostly a geographical term which is also not easily confused with "Ischium". So this template can probably be dispensed with ... -->
{{Infobox bone
| Name          = Ischium of pelvis
| Latin         = os ischii
| Image         = Pelvic girdle illustration.svg
| Caption       = Pelvic girdle
| Image2        = Gray341.png
| Caption2      = Left hip-joint, opened by removing the floor of the acetabulum from within the pelvis. Ischium labeled at bottom left
| Origins       = [[Superior gemellus muscle|Superior gemellus]]
| Articulations =
}}

The '''ischium''' ({{IPAc-en|ˈ|ɪ|s|k|i|.|ə|m}};<ref>{{cite web |title=ischium |url=http://www.thefreedictionary.com/ischium |website=[[The Free Dictionary]]}}</ref> {{plural form}}: '''ischia''') is a paired bone forming the lower and back part of the [[hip bone]].

Situated below the [[ilium (bone)|ilium]] and behind the [[pubis (bone)|pubis]], it is one of three regions whose fusion creates the [[coxal bone]]. The superior portion of this region forms approximately one-third of the [[acetabulum]].

==Structure==
The ischium is made up of three parts–the body, the superior ramus and the inferior ramus. The body contains a prominent [[ischial spine|spine]], which serves as the origin for the [[superior gemellus muscle]]. The indentation inferior to the spine is the [[lesser sciatic notch]]. Continuing down the posterior side, the [[ischial tuberosity]] is a thick, rough-surfaced prominence below the lesser sciatic notch. This is the portion that supports weight while sitting (especially noticeable on a hard surface) and can be felt simply by sitting on the fingers. It serves as the origin for the [[inferior gemellus muscle]] and the [[hamstring]]s. The superior ramus is a partial origin for the [[internal obturator muscle|internal obturator]] and the [[external obturator muscle]]s. The '''inferior ramus''' serves partially as origin for part of the [[adductor magnus muscle]] and the [[gracilis muscle]]. The inferior ischial ramus joins the inferior ramus of the [[pubis (bone)|pubis]] anteriorly and is the strongest of the hip (coxal) bones.

===Body===
The body enters into and constitutes a little more than two-fifths of the [[acetabulum]]. Its external surface forms part of the lunate surface of the acetabulum and a portion of the [[acetabular fossa]]. Its internal surface is part of the wall of the [[lesser pelvis]]; it gives origin to some fibers of the internal obturator. No muscles [[Insertion (anatomy)|insert]] on the body.

Its anterior border projects as the posterior obturator tubercle. From its posterior border, there extends backward a thin and pointed triangular eminence, more or less elongated in different subjects, the [[ischial spine]], origin of the [[gemellus superior muscle]].

Above the spine is a large notch, the [[greater sciatic notch]]; Below the spine is a smaller notch, the [[lesser sciatic notch]].

===Superior ramus===
The superior ramus of the ischium (descending ramus) projects downward and backward from the body and presents for examination three surfaces: external, internal, and posterior.

The external surface is quadrilateral in shape. It is bounded above by a groove that lodges the tendon of the external obturator; below, it is continuous with the inferior ramus; in front it is limited by the posterior margin of the [[obturator foramen]]; behind, a prominent margin separates it from the posterior surface. In front of this margin, the surface gives origin to the [[quadratus femoris]], and anterior to this to some of the fibers of origin of the external obturator; the lower part of the surface gives origin to part of the [[adductor magnus]].

The internal surface forms part of the bony wall of the lesser [[pelvis]]. In front, it is limited by the posterior margin of the [[obturator foramen]]. Below, it is bounded by a sharp ridge that provides attachment to a [[falciform]] prolongation of the [[sacrotuberous ligament]], and, more anteriorly, gives origin to the [[superficial transverse perineal muscle|transverse perineal]] and [[ischiocavernosus]] muscles.

Posteriorly the ramus forms a large swelling, the [[tuberosity of the ischium]], where the [[hamstring]]s originate.

===Inferior ramus===
The inferior ramus of the ischium (ascending ramus) is the thin, flattened part of the ischium, which ascends from the superior ramus, and joins the inferior ramus of the [[pubis (bone)|pubis]]—the junction being indicated in the adult by a raised line.

The outer surface is uneven for the origin of the [[obturator externus]] and some of the fibers of the [[adductor magnus]]; its inner surface forms part of the anterior wall of the pelvis.

Its medial border is thick, rough, slightly everted, forms part of the outlet of the pelvis, and presents two ridges and an intervening space.

The ridges are continuous with similar ones on the inferior ramus of the pubis: to the outer is attached the deep layer of the superficial perineal fascia (fascia of Colles), and to the inner the inferior fascia of the urogenital diaphragm.

Tracing these two ridges downward, they join with each other just behind the point of origin of the [[transverse perineal muscles]]. Here, the two layers of fascia are continuous behind the posterior border of the muscle.

To the intervening space, just in front of the point of junction of the ridges, the transverse perineal attaches, and in front of this is a portion of the [[ischiocavernosus]], and the [[crus penis]] in the male, or the [[crus clitoridis]] in the female.

Its lateral border is thin and sharp, and forms part of the medial margin of the [[obturator foramen]].

==Clinical significance==
Clinically, an [[avulsion fracture]] of the [[ischial tuberosity]] may occur.<ref>{{cite journal|last1=Wootton|first1=JR|last2=Cross|first2=MJ|last3=Holt|first3=KW|title=Avulsion of the ischial apophysis. The case for open reduction and internal fixation.|journal=The Journal of Bone and Joint Surgery. British Volume|date=July 1990|volume=72|issue=4|pages=625–7|doi=10.1302/0301-620X.72B4.2380217|pmid=2380217|doi-access=}}</ref>

Avulsion fractures of the hip bone (avulsion or tearing away of the ischial tuberosity) may occur in adolescents and young adults during sports that require sudden acceleration or deceleration forces, such as sprinting or kicking in football, soccer, jumping hurdles, basketball, and martial arts. These fractures occur at [[tubercle (anatomy)|tubercles]]  (bony projections that lack secondary [[ossification center]]s). Avulsion fractures occur where muscles are attached: anterior superior and inferior iliac spines, ischial tuberosities, and ischiopubic rami. A small part of bone with a piece of a tendon or ligament attached is avulsed (torn away).<ref>{{cite book|last1=Dalley|first1=Keith L. Moore, Anne M.R. Agur, Arthur F.|title=Clinically oriented anatomy|date=2010|publisher=Lippincott Williams & Wilkins, Wolters Kluwer|location=Philadelphia [etc.]|isbn=978-1605476520|edition=6th ed., [International ed.].}}</ref>

[[Ischial bursitis]] (also known as ''weaver's bottom'') is inflammation of the [[synovial bursa]] located between the [[gluteus maximus muscle]] and the ischial tuberosity,<ref>{{cite book|last=Fauci|first=Anthony|title=Harrison's Rheumatology, Second Edition|year=2010|publisher=McGraw-Hill Professional Publishing; Digital Edition|isbn=9780071741460|pages=271}}</ref> and is usually caused by prolonged sitting on a hard surface.

==History==
Adoption of  ''ischium'' into English-language medical literature dates back to c. 1640; the Latin term derives from Greek [[:wikt:ἰσχίον|ἰσχίον]] ''iskhion'' meaning "hip joint". The division of the ''acetabulum'' into ''ischium'' (ἰσχίον) and ''ilium'' (λαγών, ''os lagonicum'') is due to [[Galen]], ''De ossibus''. Galen, however, omits mention of the ''pubis'' as a separate bone.<ref>''De ossibus'' chapter 20, ed. Kühn, vol. 2, p. 772, cited after Charles Singer, C. Rabin, ''A Prelude to Modern Science: Being a Discussion of the History, Sources and Circumstances of the 'Tabulae Anatomicae Sex' of Vesalius'', Cambridge University Press, 1946 (reprinted 2012), [https://books.google.com/books?id=Sdu01WgjHZMC&pg=PA35 p. 35].</ref>

==Other animals==

===Dinosaurs===
The [[clade]] [[Dinosaur]]ia is divided into the [[Saurischia]] and [[Ornithischia]] based on [[pelvis|hip structure]], including importantly that of the ischium.<ref>Seeley, H.G. (1888). "''On the classification of the fossil animals commonly named Dinosauria.''" Proceedings of the Royal Society of London, '''43''': 165-171.</ref> In the majority of dinosaurs, the ischium extends down from the ilium and towards the tail of the animal.  The '''[[Acetabulum#Perforate Acetabulum|acetabulum]]''', which can be thought of as a "hip-socket", is a cup-shaped opening on each side of the pelvic girdle formed where the [[Ischium#In dinosaurs|ischium]], [[Ilium bone#In dinosaurs|ilium]], and [[pubis (bone)#Dinosaurs|pubis]] all meet, and into which the head of the femur inserts.  The orientation and position of the acetabulum is one of the main morphological traits that caused dinosaurs to walk in an upright posture with their legs directly underneath their bodies.<ref>Martin, A.J. (2006). Introduction to the Study of Dinosaurs. Second Edition. Oxford, Blackwell Publishing. pg. 299-300. {{ISBN|1-4051-3413-5}}.</ref>
<gallery>
Ornithischia pelvis structure.svg|Ornithischian pelvic structure (left side)
Saurischia pelvis structure.svg|Saurischian pelvic structure (left side).
</gallery>

==Additional images==
<gallery>
Gray235.png|Right hip bone. External surface.
Gray236.png|Right hip bone. Internal surface.
Gray237.png|Plan of ossification of the hip bone.
Gray241.png
Gray436.png|The obturator externus.
Skeletal pelvis-pubis.svg|Pelvis
</gallery>

==See also==
{{Anatomy-terms}}

==References==
{{Gray's}}
{{Reflist}}
* Saladin, Kenneth S. Anatomy and Physiology The Unity of Form and Function. 5th ed. McGraw-Hill Science Engineering, 2009. Print.

==External links==
{{Commons}}
* {{SUNYAnatomyLabs|44|st|07|22}} - "The Male Peniel: Hip Bone"
* {{ViennaCrossSection|pelvis/pelvis-e12-15}}

{{Pelvis}}
{{Portal bar|Anatomy}}
{{Authority control}}

[[Category:Ischium| ]]
[[Category:Bones of the pelvis]]