ARCO classification of femoral head osteonecrosis

Last revised by Arlene Campos on 21 May 2024

The ARCO classification (Association Research Circulation Osseous classification) is one of the staging systems used to assess femoral head osteonecrosis. It was created in 1994 and periodically revised. The revision from 2019 2 includes using radiographs and MRIs.


  • stage 0

    • plain radiograph: normal

    • MRI: normal

    • clinically: no symptoms

  • stage I

    • ​plain radiograph: normal

    • MRI or bone scan: abnormal

    • clinically: no symptoms or presence symptoms

  • stage II

    • plain radiograph: trabecular bone changes without changes in subchondral bone; preserved joint space

    • MRI: abnormal, diagnostic appearance

    • clinically: symptoms present

  • stage III

    • ​presence of trabecular bone changes and subchondral fracture (crescent sign or subchondral bone collapse); preserved joint space

    • clinically: symptoms present

    • further subdivided with regard to the depth of femoral head depression: IIIA - depression ≤2 mm, IIIB - depression >2 mm

  • stage IV

    • features of osteoarthritis, with distorted femoral head shape, acetabular changes and narrowed joint space

    • clinically: symptoms present

See also

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