Crescent sign is seen in which condition?

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The crescent sign reflects a subchondral fracture beneath the articular surface caused by avascular necrosis. When blood supply to a bone region is lost, the bone becomes necrotic and weak; as it collapses under load, a thin radiolucent crescent forms just under the cartilage, marking the edge between dead and surviving bone. This radiographic cue is classic for avascular necrosis, typically of the femoral head, and signals impending or early collapse of the joint surface. The other conditions don’t produce this distinct subchondral crescent: osteoarthritis shows joint space narrowing and osteophytes with subchondral sclerosis instead; gout causes punch-out erosions with overhanging edges; osteomyelitis presents with bone destruction, sequestrum, and periosteal reaction rather than a localized subchondral crescent.