Ischium

The ischium (plural ischia) is among the three bones of the innominate bone: ilium, ischium, and pubis. It is the inferoposterior part of the innominate bone and contributes to the inferior acetabulum.

Gross anatomy

The ischium has a body and ramus. Superiorly, it forms the inferoposterior part of the acetabulum, and inferiorly, its ramus ascends anteromedially at an acute angle to meet the descending or inferior pubic ramus, completing the obturator foramen.

Osteology

The body of the ischium can be divided into:

• a femoral surface that faces inferiorly, anteriorly, and laterally bounded by the margin of the obturator foramen and forms the lateral limit of the ischial tuberosity

• a dorsal surface that is continuous superiorly with the iliac gluteal surface and inferiorly forms the large ischial tuberosity, which is palpable, usually 5 cm lateral to midline and 5 cm superior to the gluteal skin fold

• the posterior border contains the inferior portion of the greater sciatic notch and the lesser sciatic notch, which are separated by the conspicuous ischial spine ⁷

• a pelvic surface which is flat and smooth, facing the pelvic cavity; inferior to this is a part of the lateral wall of the ischiorectal fossa

The ramus of ischium contains anterior and posterior surfaces, the posterior surface is divided into the perineal and pelvic areas. The superior border completes the obturator foramen and the lower border, together with the medial border of the inferior pubic ramus, bounds the subpubic angle and pubic arch.

The ischial tuberosity is divided transversely into superior and inferior areas, superior part subdivided by an oblique bony ridge separating the various muscle attachments.

Articulations

The ischium contributes to two-fifths of the acetabular articular surface. The ramus forms a temporary cartilaginous synchondrosis joint with the inferior pubic ramus (ischiopubic synchondrosis) during childhood until it fuses to form the ischiopubic or conjoint ramus.

Attachments

Musculotendinous

Muscle insertions on the ischium:

• coccygeus and levator ani muscles insert at the ischial spine.

Muscle origins from the ischium:

• superior gemellus from the ischial spine

• inferior gemellus from the superior border of ischial tuberosity

• semimembranosus from the superolateral impression of ischial tuberosity

• biceps femoris long head and semitendinosus from the inferomedial impression of ischial tuberosity

• quadratus femoris originates between the origins of inferior gemellus and adductor magnus

• adductor magnus starts from the ischiopubic ramus and the inferolateral aspect of the ischial tuberosity

• gracilis muscle (a smaller part) originates from the inferior part of ramus and adjoining conjoined ischiopubic ramus

• deep and superficial transverse perineal muscles originate from the body of ischium

• obturator internus and obturator externus muscles have part origins from the superior part of the ramus

• piriformis muscle arises from the superior aspect of the greater sciatic notch.

Ligamentous

• sacrospinous ligament attaches to the ischial spine

• sacrotuberous ligament attaches to the posterior iliac spine and medial ischial tuberosity and part of it extends as the falciform ligament along the inferior ramus of ischium

• ischiofemoral ligament forms the capsular reinforcement of hip joint.

Relations

• greater sciatic foramen: bounded anterosuperiorly by the greater sciatic notch, posteriorly by the sacrotuberous ligament and inferiorly by the sacrospinous ligament and the ischial spine

• lesser sciatic foramen: bounded anteriorly by the ischial body, superiorly by the ischial spine and the sacrospinous ligament and posteriorly by the sacrotuberous ligament

Arterial supply

• artery of the ischium, a collateral of the pudendal artery

• branches of the obturator artery

Development

Ossification

One ossification center appears in the body of ischium at the prenatal 8th week with ossification continuing inferiorly and anteriorly to form the ischial ramus. At birth the ischial ramus and inferior pubic ramus are separated by a hyaline cartilaginous joint called the ischiopubic synchondrosis. This synchondrosis fuses to become the ischiopubic ramus typically in the 7th to 10th year of life with fusion occurring earlier in females compared to males ⁵. 

A single secondary ossification center ossifies posteroinferior to the body of ischium to form the ischial tuberosity with ossification commencing at 10-13 years and fusion occurring typically at 16-20 years with fusion occurring earlier in females ⁶. In a small number of cases the ischial spine may also develop from a secondary center.