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.
Classification
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stage 0
plain radiograph: normal
MRI: normal
clinically: no symptoms
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stage I
plain radiograph: normal
MRI or bone scan: abnormal
clinically: no symptoms or presence symptoms
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stage II
plain radiograph: trabecular bone changes without changes in subchondral bone; preserved joint space
MRI: abnormal, diagnostic appearance
clinically: symptoms present
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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
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stage IV
features of osteoarthritis, with distorted femoral head shape, acetabular changes and narrowed joint space
clinically: symptoms present
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