Last revised by Chen Bing Marcus Sng on 15 Mar 2022

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 lower acetabulum.

The ischium has a body and ramus. Above, it forms the inferoposterior part of the acetabulum, and below, its ramus ascends anteriomedially at an acute angle to meet the descending pubic ramus, completing the obturator foramen.

The body of the ischium can be divided into:

  • a femoral surface which faces downwards, forwards, and laterally bounded by the margin of obturator foramen and forms the lateral limit of the ischial tuberosity
  • a dorsal surface which is continuous above with the iliac gluteal surface and below forms the large ischial tuberosity, which is palpable, usually 5 cm off midline and 5 cm above the gluteal skin fold; the posterior border forms the greater and lesser sciatic notches separated by the conspicuous ischial spine
  • a pelvic surface which is flat and smooth, facing the pelvic cavity; below 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 upper 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 upper and lower areas, upper subdivided by an oblique bony ridge separating the various muscle attachments.

The ischium contributes to two-fifths of the acetabular articlar surface. The ramus fuses with the inferior ramus of the pubis to form the ischiopubic ramus, hence is not a joint.

Muscle insertions on the ischium:

Muscle origins from the ischium:

  • 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 which forms the capsular reinforcement of hip joint.
  • greater sciatic foramen: bounded anterosuperiorly by the greater sciatic notch, posteriorly by the sacrotuberous ligamentand 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

One ossification center appears in the body of ischium at the prenatal 8th week, which fuses with the ossifying pubis to become a continuous ramus in the 7th or 8th year of life. 

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Cases and figures

  • Figure 1
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  • Figure 2a: muscle attachments (Gray's illustration)
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  • Figure 2b: muscle attachments (Gray's illustration)
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  • Figure 3: ossification centers (Gray's illustration)
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