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Ischial tuberosity

The ischial tuberosity, commonly referred to as the “sit bone,” is a large, roughened bony prominence on the posterior-inferior part of the ischium. It serves as a major weight-bearing structure when sitting and as a crucial site of attachment for muscles and ligaments of the posterior thigh and pelvis.

The tuberosity has two distinct regions:

  • Upper portion (quadrilateral area): gives attachment to the semimembranosus and part of the biceps femoris (long head) and semitendinosus.

  • Lower portion (triangular area): provides attachment for the adductor magnus and sacrotuberous ligament.

During sitting, body weight is transmitted through the ischial tuberosities. Its robust structure supports multiple powerful muscles that contribute to hip extension, knee flexion, and pelvic stability.

Clinically, the ischial tuberosity is important in hamstring avulsion injuries, ischial bursitis, fractures, osteomyelitis, and tumors. It is also a surface landmark used in orthopedic and surgical procedures.

Synonyms

  • Sit bone

  • Tuber ischiadicum

  • Ischiatic tuberosity

Function

  • Serves as the origin of hamstring muscles (semimembranosus, semitendinosus, long head of biceps femoris)

  • Provides attachment for adductor magnus and sacrotuberous ligament

  • Acts as a weight-bearing structure when sitting

  • Plays a critical role in hip extension, knee flexion, and pelvic stability

Nerve Supply (related muscles)

  • Sciatic nerve (L4–S3): supplies hamstrings originating at the ischial tuberosity

  • Posterior cutaneous nerve of thigh provides sensory innervation to overlying skin

Arterial Supply

  • Inferior gluteal artery

  • Medial circumflex femoral artery

  • Contributions from perforating branches of the profunda femoris artery

Venous Drainage

  • Inferior gluteal vein → internal iliac vein

  • Profunda femoris vein → femoral vein

MRI Appearance

T1-weighted images:

  • Bone cortex: hypointense (dark rim)

  • Bone marrow: intermediate signal

  • Shows muscle origins clearly at the tuberosity

T2-weighted images:

  • Bone cortex: hypointense

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

  • Detects marrow edema or early stress changes

PD Fat-Saturated (Proton Density FS):

  • Cortex: dark hypointense

  • Marrow edema: bright hyperintense signal

  • Excellent for evaluating hamstring origin tears, apophyseal injuries, and enthesopathy

STIR:

  • Suppresses fat, highlighting fractures, bursitis, infection, or bone marrow edema as hyperintense regions

  • Sensitive for early injury detection

T1 Post-Gadolinium (with fat saturation):

  • Bone cortex: does not enhance

  • Soft tissue inflammation, bursitis, neoplasm, or infection: bright enhancement

  • Helps detect ischial bursitis, neoplastic infiltration, or osteomyelitis

3D T2-weighted Imaging:

  • Bone cortex: sharply hypointense outline

  • Provides detailed multiplanar reconstructions for hamstring tendon attachment evaluation

  • Useful for surgical planning in hamstring avulsion repair

CT Appearance

Non-contrast CT:

  • Bone cortex: hyperdense and clearly delineated

  • Bone marrow: lower density relative to cortex

  • Excellent for fractures, cortical irregularities, apophyseal avulsions, or chronic enthesopathy

CT Post-Contrast:

  • Bone cortex: unchanged

  • Soft tissue structures: enhance with inflammation or neoplasia

  • Useful in diagnosing infection, tumor extension, or complex trauma

  • 3D reconstructions assist with orthopedic planning and pelvic surgery

Clinical Significance

  • Hamstring avulsion fractures: common in athletes and adolescents at the apophysis.

  • Ischial bursitis (weaver’s bottom): painful inflammation overlying the tuberosity.

  • Stress fractures: seen in athletes or after repetitive loading.

  • Oncology: primary bone tumors or metastatic disease may involve the tuberosity.

  • Surgical relevance: landmark for approaches to hip, pelvis, and sciatic nerve.

CT VRT 3D image

Ischial tuberosity 3D CT VRT anatomy image 1

CT VRT 3D image

Ischial tuberosity 3D CT VRT anatomy image

CT image

Ischial tuberosity ct axial image

MRI image

Ischial tuberosity MRI axial anatomy image-img-00000-00000