Staging of Avascular Necrosis

Stage 0

• Patient is asymptomatic.
• Radiography findings are normal.
• Histology findings demonstrate osteonecrosis.

Stage I

• Patient may or may not be symptomatic.
• Radiography and CT scan findings are unremarkable.
• AVN is considered likely based on MRI and bone scan results (may be subclassified by extent of involvement [see below]).
• Histology findings are abnormal.

Stage II

• Patient is symptomatic.
• Plain radiography findings are abnormal and include osteopenia, osteosclerosis, or cysts.
• Subchondral radiolucency is absent.
• MRI findings are diagnostic.

Stage III

• Patient is symptomatic.
• Radiographic findings include subchondral lucency (crescent sign) and subchondral collapse.
• Shape of the femoral head is generally preserved on radiographs and CT scans.
• Subclassification depends on the extent of crescent, as follows:
  • Stage IIIa: Crescent is less than 15% of the articular surface.
  • Stage IIIb: Crescent is 15-30% of the articular surface.
  • Stage IIIc: Crescent is more than 30% of the articular surface.

Stage IV

• Flattening or collapse of femoral head is present.
• Joint space may be irregular.
• CT scanning is more sensitive than radiography.
• Subclassification depends on the extent of collapsed surface, as follows:
  • Stage IVa: Less than 15% of surface is collapsed.
  • Stage IVb: Approximately 15-30% of surface is collapsed.
  • Stage IVc: More than 30% of surface is collapsed.

Stage V

• Radiography findings include narrowing of the joint space, osteoarthritis with sclerosis of acetabulum, and marginal osteophytes.

Stage VI

• Findings include extensive destruction of the femoral head and joint.