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Table 1 Classification of avascular osteonecrosis (AVN) as performed by the Association Research Circulation Osseous (ARCO). Diagnostic findings, localisation and extent of AVN are considered. AVN-associated pain usually occurs in late ARCO stages III and IV but can also be found in earlier stages. BME = bone marrow oedema; nps = no pathological signs [2730]

From: Rationale for prostaglandin I2in bone marrow oedema – from theory to application

ARCO stages
  0 I II III IV
Diagnostic techniques and findings X-ray nps nps Sclerosis, osteolysis, focal osteoporosis Crescent sign, flattening of the articular surface (subchondral fracture) Collapse, joint space narrowing (osteoarthritis)
CT nps nps Asterix sign, sclerosis Subchondral fracture Collapse, joint space narrowing (osteoarthritis)
Bone scan nps Cold spot Cold in hot spot Hot in hot spot Hot spot
MRI nps BME Osteonecrosis, reactive interface Subchondral fracture Collapse, joint space narrowing (osteoarthritis)
Subclassification No - medial No
   - central  
   - lateral  
Quantification No % area involvement: Length of crescent: % of surface collapse and dome depression No
   Minimal A: <15% A: <15% (A, B, C)  
   Moderate B: 15 to 30% B: 15 to 30%   
   Extensive C: >30% C: >30%