A descriptive term for a broad radiopaque band which corresponds to diffuse homogeneous sclerosis, covering up to 2⁄3 of the artiular surface of the head of the femur or humerus in avascular or ischaemic necrosis. It is usually caused by compromised epiphyseal blood supply, and classically seen in posterior hip dislocation
Differential diagnosis The sign is nonspecific and may occur in femoral head ischaemia due to anaemia, haemophilia, endocrinopathy, congenital disease, or malignancy
After the initial structural failure causing a sclerotic “snowcap”, a thin subchondral fracture “rim” appears; this part of the femoral head collapses, giving rise to a step-like defect.
Synonym Snow capping