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Ischium bone

The ischium is one of the three bones that fuse to form the hip bone (os coxae), along with the ilium and pubis. It constitutes the posteroinferior part of the acetabulum and pelvis. The ischium provides weight-bearing support when sitting, serves as a strong anchor point for ligaments and muscles of the hip and thigh, and contributes to pelvic stability.

It consists of three main anatomical parts:

  • Body of ischium: contributes to the posteroinferior acetabulum, articulating with the head of the femur.

  • Ischial ramus: fuses anteriorly with the inferior pubic ramus to form the ischiopubic ramus, part of the obturator foramen border.

  • Ischial tuberosity: large, roughened prominence that serves as the main weight-bearing area in sitting and origin for hamstring muscles.

  • Ischial spine: pointed projection between the greater and lesser sciatic notches; attachment for the sacrospinous ligament and landmark in obstetrics.

Attachments

  • Muscles:

    • Ischial tuberosity: origins of hamstrings (semimembranosus, semitendinosus, biceps femoris long head) and adductor magnus (hamstring part).

    • Ischial ramus: origins of obturator internus, adductor magnus, and quadratus femoris.

    • Ischial spine: attachment for superior gemellus.

  • Ligaments:

    • Sacrotuberous ligament attaches to ischial tuberosity.

    • Sacrospinous ligament attaches to ischial spine.

Relations

  • Forms posteroinferior boundary of acetabulum.

  • Contributes to greater and lesser sciatic foramina.

  • Provides major support surface when sitting.

Synonyms

  • Os ischii

  • Inferior pelvic bone

  • Ischial portion of os coxae

Function

  • Supports body weight when sitting (ischial tuberosities).

  • Provides origin and attachment for posterior thigh, pelvic floor, and hip muscles.

  • Contributes to acetabular socket stability.

  • Serves as an obstetric landmark (ischial spines help assess pelvic dimensions during childbirth).

Nerve Supply (related structures)

  • Muscles originating from ischium: innervated mainly by sciatic nerve branches (tibial and common peroneal divisions) and obturator nerve.

Arterial Supply

  • Obturator artery

  • Inferior gluteal artery

  • Internal pudendal artery

Venous Drainage

  • Corresponding veins drain into the internal iliac vein.

MRI Appearance

T1-weighted images:

  • Cortex: low signal intensity (dark).

  • Marrow: intermediate signal depending on fat content.

  • Provides excellent detail of ischial tuberosity, spine, and acetabular contribution.

T2-weighted images:

  • Cortex: hypointense.

  • Marrow: intermediate to bright depending on water/fat composition.

  • Detects bone marrow edema, fractures, or cysts.

PD Fat-Saturated:

  • Cortex: dark hypointense.

  • Bone marrow edema: bright hyperintense, highlighting occult fractures or enthesopathies.

  • Very sensitive for hamstring origin pathology at ischial tuberosity.

STIR:

  • Suppresses fat and highlights marrow edema, trauma, or infection.

  • Useful for stress fractures, bursitis, or osteomyelitis.

T1 Post-Gadolinium (with fat saturation):

  • Normal bone: cortex remains dark, marrow enhances slightly.

  • Pathological conditions (osteomyelitis, tumors, enthesitis) show intense enhancement.

  • Enhances peritendinous inflammation at hamstring origin or bursae.

3D T2-weighted Imaging:

  • Provides isotropic voxels with multiplanar reconstruction of ischium morphology.

  • Useful for surgical planning in acetabular fractures, pelvic reconstructions, and tumor resections.

CT Appearance

Non-contrast CT:

  • Cortex: hyperdense, sharply defined.

  • Marrow: lower density.

  • Excellent for identifying fractures, cortical irregularities, acetabular involvement, or bony outgrowths.

CT Post-Contrast:

  • Bone cortex unchanged, hyperdense.

  • Enhancement in adjacent soft tissues indicates abscess, neoplasm, or inflammatory changes.

  • 3D reconstructions useful in trauma, pelvic ring fractures, and pre-surgical planning.

Clinical Significance

  • Ischial tuberosity avulsion fractures: common in athletes due to hamstring pull.

  • Ischial bursitis ("weaver’s bottom"): inflammation from prolonged sitting.

  • Ischial spine prominence: used as obstetric landmark to assess pelvic outlet dimensions.

  • Acetabular involvement: ischium contributes to acetabular fractures, FAI, and dysplasia.

  • Oncology: site for pelvic tumors and secondary metastases.

CT VRT 3D image

ischium bone 3d image

CT image

ischium bone ct axial image

MRI image

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