History: hip pain, trauma
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Hip X-ray. curvilinear lucency in the subchondral right femoral head
Answer: crescent sign of Avascular necrosis (Stage 3)
Although intrinsically straight forward, the FICAT classification system is confusing because of the “Preclinical” Stage 0, and “PreRadiographic” stage 1. I’ve attached it below. The crescent sign puts you in Stage 3 (collapse) with end stage osteoarthritis putting you in Stage 4.