What causes Snow-capping in the epiphysis (often femoral or humeral head) on plain radiography?
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Dense sclerosis of the epiphysis representing the healing phase of avascular necrosis (osteonecrosis), a condition caused by interruption of blood supply leading to bone cell death, revascularization, and repair; associated with risk factors including trauma, glucocorticoid use, sickle cell disease, alcohol abuse, and systemic lupus erythematosus.
Why is it called so?
The sign is named for the appearance of dense sclerosis capping the superior aspect of the femoral or humeral head on plain radiographs, resembling a snow-capped mountain peak. It is a descriptive teaching term rather than a formal diagnostic criterion.
Pathophysiology
Interruption of vascular supply causes ischemic necrosis of osteocytes within the epiphysis; during the reparative healing phase, reactive bone formation at the necrotic interface produces dense sclerosis visible as a radiopaque cap prior to potential subchondral collapse.
Alternative names: Snowcap sign, snowcapping
Other associated named signs: Crescent sign, double-line sign — both also seen in the radiographic/MRI evaluation of avascular necrosis.
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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