Corpus: Ischium

The os ischii or ischium is a part of the bony pelvis. Together with the ilium (os ilium) and the pubis (os pubis), it forms the coxae (os coxae).

The os ischii consists of a body (corpus ossis ischii) and an upper and lower ischial branch (ramus superior ossis ischii and ramus inferior ossis ischii).

Note: Some authors define only one ischial branch, which then corresponds to the ramus superior ossis ischii and the ramus inferior ossis ischii discussed here.

Corpus ossi ischii[Bearbeiten]

The body of the ischial bone forms the posterior part of the acetabulum. The following structures are found on its posterior edge (posterior margin):

• the caudal part of the incisura ischiadica major
• a sharp, flat bone spine, the spina ischiadica
• the incisura ischiadica minor

Ramus superior ossis ischii[Bearbeiten]

The ramus superior ossis ischii (upper ischial branch) forms the cranial boundary of the foramen obturatum together with the ramus superior ossis pubis (upper pubic branch). It runs inferiorly and posteriorly from the ischial body. It has three sides: an external facies, an internal facies and a posterior facies.

The facies externa has a roughly quadrangular shape. At its upper end there is a sulcus for the tendon of the external obturator muscle. The lower part merges into the inferior ramus. It is bordered anteriorly by the posterior edge of the obturator foramen. Posteriorly, a prominent bony edge separates it from the posterior facies. The origin of the quadratus femoris muscle lies in front of this edge. Further anteriorly, some fibres of the external obturator muscle originate. The adductor magnus muscle originates at the lower end of the surface.

The facies interna of the upper ischial branch forms part of the bone wall of the pelvis. It is also bounded anteriorly by the posterior edge of the obturate foramen. Below, it merges into the inferior ramus and is bordered by a sharp bony edge that serves as the base for the crescent-shaped extension of the sacrotuberous ligament. The transversus perinei superficialis muscle and the ischiocavernosus muscle also originate in this area.

A large bony tubercle, the ischial tuberosity (tuber ischiadicum), rises on the posterior facies.

Ramus inferior ossis ischii[Bearbeiten]

The flattened ramus inferior ossis ischii emerges from the ramus superior and, together with the ramus inferior ossis pubis (lower pubic branch), forms the caudal boundary of the foramen obturatum.

The facies externa is uneven and serves as the origin of the musculus obturatorius externus and some fibres of the musculus adductor magnus. The internal facies forms part of the anterior wall of the pelvis. Its medial edge is thick and has two bony edges that continue on the lower pubic ramus and a depression between them.

The outer edge serves as an attachment for the superficial perineal fascia (Fascia perinei superficialis), the inner edge as an attachment for the inner fascia of the urogenital diaphragm. The origin of the transversus perinei profundus muscle lies between them.

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