Definition
The ischium (Os ischii) is a flat bone of the human pelvis. It borders on the pubic bone (Os pubis) and the ilium (Os ilium) and forms together with these the so-called hip bone (Os coxae). Together with the sacrum, this bone closes the complete pelvic ring and thus forms the basis of the bony pelvis.
Anatomy
The ischium can be divided into 3 parts, the ischium body and an upper and lower ischium branch, which merge into each other. The body of the ischium forms the largest part of the hip joint socket and its rear boundary. At its back it has two characteristic indentations (Incisura ischiadica major/minor).
The larger of the two ends in a sharp bone spine, which serves as a starting point for the sacrum-ischiatic spine ligament (Lig. sacrospinale). This is part of the sacro-iliac joint and contributes to its stabilization.
Together with another ligament, it also forms the boundary of the small ischial hole at the small notch, which is an important point of passage for conduction pathways from the small pelvis to the thigh and the groin region. Together with the upper branch of the pubic bone, the upper ischial branch (ramus superior ossis ischii) forms the upper boundary of the hip hole (foramen obturatum) through which several conduits run from the pelvis to the leg. A prominent bone edge demarcates the outer side of the branch from the rear side and serves as the point of origin of various thigh muscles.
The inner side is part of the bony wall of the small pelvis and also has a sharp edge in its lower part. This forms the starting point for a further retaining ligament of the sacro-iliac joint. It also serves as a point of origin for a part of the pelvic floor muscles.
The lower ischial branch has an uneven surface on its outside from which the Musculus obturatorius externus originates. The inner side is part of the bony front wall of the pelvis and, with its bony edges, forms the starting point for connective tissue parts of the pelvic floor. On its rear side, the two branches of the ischium run out together into the large ischial tuberosity (Tuber ischiadicum).
