Ischium

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Not to be confused with Ischia.
Ischium of pelvis
Pelvic girdle illustration.svg
Pelvic girdle
Gray341.png
Left hip-joint, opened by removing the floor of the acetabulum from within the pelvis. (Ischium labeled at bottom left.)
Details
Latin os ischii
Origins Superior gemellus
Identifiers
Gray's p.234
MeSH A02.835.232.611.548
Dorlands
/Elsevier
o_07/12598437
TA A02.5.01.201
FMA FMA:16592
Anatomical terms of bone

The ischium forms the lower and back part of the hip bone (os coxae).

Situated below the ilium and behind the pubis, it is one of these three bones whose fusion creates the coxa. The superior portion of this bone forms approximately one third of the acetabulum.

Structure[edit]

It is divisible into three portions:

The ischial ramus joins the inferior ramus of the pubis anteriorly. It is the strongest of the coxal bones.

Body[edit]

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 Obturator internus. Two muscles; the deep[1] and superficial transverse perinei muscles, originates from the body of ischium. No muscles do 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, the ischial spine, more or less elongated in different subjects.

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

Superior ramus[edit]

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 which lodges the tendon of the Obturator externus; 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 Obturator externus; 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 which gives attachment to a falciform prolongation of the sacrotuberous ligament, and, more anteriorly, gives origin to the Transversus perinæi and Ischiocavernosus.

Posteriorly the ramus forms a large swelling, the tuberosity of the ischium

Inferior ramus[edit]

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—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.

If these two ridges be traced downward, they will be found to join with each other just behind the point of origin of the transversus perinæi; 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 transversus perinæi is attached, and in front of this a portion of the crus penis vel clitoridis and the ischiocavernosus.

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

Clinical significance[edit]

Clinically, avulsion fracture of the ischial apophysis may occur (Wootton 1990).[2]

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 apophyses (bony projections that lack secondary ossification centers). 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) (Moore 2006).

History[edit]

The word ischium dates back to c. 1640 B.C.E., from Greek ἰσχίον iskhion meaning "hip joint", in plural, "the hips", and most likely comes from ισχύς iskhys "loin", which is of unknown origin.[3]

Other animals[edit]

Dinosaurs[edit]

The clade Dinosauria is divided into the Saurischia and Ornithischia based on hip structure, including importantly that of the ischium.[4] In the majority of dinosaurs, the ischium extends down from the ilium and towards the tail of the animal. The 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, ilium, and 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.[5]

Additional images[edit]

References[edit]

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

  1. ^ http://www.prohealthsys.com/anatomy/pelvic_floor_muscles.php
  2. ^ Avulsion of the ischial apophysis J. R. Wootton, M. J. Cross, K. W. G. Holt - Journal of Bone and Joint Surgery, British Volume. Volume 72-B, No. 4. Date: 1990-07. Retrieved: 2010-05-16.
  3. ^ http://www.etymonline.com/index.php?term=ischium
  4. ^ Seeley, H.G. (1888). "On the classification of the fossil animals commonly named Dinosauria." Proceedings of the Royal Society of London, 43: 165-171.
  5. ^ Martin, A.J. (2006). Introduction to the Study of Dinosaurs. Second Edition. Oxford, Blackwell Publishing. pg. 299-300. ISBN 1–4051–3413–5.
  • Moore, Keith L., Arthur F. Dalley, and A. M. R. Agur. Clinically Oriented Anatomy. Philadelphia: Lippincott Williams & Wilkins, 2006. Print.
  • Saladin, Kenneth S. Anatomy and Physiology The Unity of Form and Function. 5th ed. McGraw-Hill Science Engineering, 2009. Print.

See also[edit]

This article uses anatomical terminology; for an overview, see anatomical terminology.

External links[edit]