The ischium is known as a part of the bony pelvis that consists of the ischial body and two ischial branches. The ischium provides an attachment point for many muscles and tendons. For this reason, it is sometimes affected by tendon and muscle diseases in addition to fractures.
What is the ischium?
The ischium of most mammals actually corresponds to two ischial bones, which together form the pelvic symphysis, contributing to the fibrocartilaginous connection of the pelvic halves. In humans, the ischium is also a part of the bony pelvis and one of numerous flat bones of the body. In the human organism, the ischium consists of the corpus ossi ischii, the ramus superior ossis ischii, and the ramus interferior ossis ischii. The body of the os ischii is part of the posterior portion of the acetabulum and bears the caudal portion of the incisura ischiadica major, the flat bony spine of the spina ischiadica, and the incisura ischiadica minor at the posterior margin (margo posterior). The anatomy of the human ischium does not necessarily correspond to the ischial anatomy of other mammals. For example, in quadrupeds, the spina ischiadica points upward and has a groin shape. In quadrupeds, it is still part of the ilium. Common to the ischium of most species is the caudal union of the ischial body and branches to form the ischial plate (tabula ossis ischii).
Anatomy and structure
Plate bones (ossa plana) are sheet-like bones of small thickness. Externally, they are composed of compact bone substance. Internally, they are composed of spongy bone substance that carries bone marrow in its interstices. The bony pelvis (osi ischii) of humans contains such flat bones. It forms the arcuate lower limit of the so-called obstructed hip hole (foramen obturatum). To the ischial tuberosity (tuber ischiadicum), the bony pelvis is thickened downward. The ischial tuberosity is a muscle origin and an important sitting point. It is overlaid by fat pads. Above the ischial tuberosity lies the ischial spine (spina ischiadica). It divides in the pelvic line to form the incisura ischiadica major or the large ischial tuberosity and the incisura ischiadica minor or small ischial tuberosity. Together with the ilium and pubis, the ischium forms the coxae. The Os ischii is a single body with upper and lower ischial branches. Some authors define the two branches as only one ischial branch. A variety of muscles attach to the human ischium.
Function and tasks
Plate bones are mostly used for mechanical protection of internal organs and provide space for the attachment of skeletal muscles due to their planar shape. The function of the human ischium, in addition to muscle attachment, is primarily to stabilize the bony pelvis. The superior ischial branch, together with the superior pubic branch, forms a cranial boundary for the foramen obturatum. This structure has three distinct sides: the so-called facies externa, facies interna, and facies posterior. The facies externa is quadrangular and carries the tendon of the obturatorius externus muscle in a sulcus at the upper end. At the lower end, it becomes the ramus inferior, providing stability. The anterior border of these structures is the foramen obturatum. A prominent bony edge separates the structure from the facies posterior. Near the edge, the quadratus femoris muscle takes its origin. Towards the anterior, fibers of the obturatorius externus muscle lie, and towards the inferior, the adductor magnus muscle originates. The facies interna forms a bone wall portion of the lesser pelvis on the superior ischial branch. At a sharp bony edge at the lower end, the sickle-shaped extension of the ligamentum sacrotuberale can be seen. The transversus perinei superficialis muscle and the ischiocavernosus muscle also arise from the ischium there. The ramus inferior ossis ischii goes is an oblate bone from the ramus superior. Together with the ramus inferior ossis pubis, it forms the caudal border of the foramen obturatum. The uneven facies externa is the origin of the musculus obturatorius externus and partially of the musculus adductor magnus. The facies interna, on the other hand, forms an anterior wall portion of the lesser pelvis. The outer edge of this structure is the origin of the superficial perineal fascia (fascia perinei superficialis). Attached to the inner edge of the structure is the internal fascia of the diaphragma urogenitale. Between them, the transversus perinei profundus muscle takes its origin.
Diseases
Like any other bone in the body, the ischium can be affected by fractures. A fracture corresponds to the complete severing of a bone by direct or indirect force. Iliac fractures are either primary or secondary fractures. In primary fractures, the bone fragments are no more than one millimeter apart. Secondary fractures have a fracture gap greater than one millimeter. An ischial bone fracture is usually preceded by falls or other accidents. A fatigue fracture of the ischium is also not uncommon after extreme stress. Since the anatomical structure of the ischium provides an attachment for a large number of tendons and muscles, however, muscle- and tendon-related pain is the main cause of ischial complaints. Tendonitis of the ischial tendons, for example, is common. Such usually occur due to friction, infection or overload and initially manifest themselves in only mild discomfort. As a result, tendonitis is often noticed extremely late and is then already characterized by crunching calcifications that make treatment much more difficult. Torn muscle fibers occur much less frequently with respect to the ischium than strains or muscle contusions. Muscle inflammation of the ischium is also rather rare, but nevertheless should not be completely disregarded.
