Articular cartilage lesions at different stages of steroid-induced osteonecrosis of the femoral head: characteristics and mechanisms of crescent sign formation

• Articular cartilage lesions in steroid-induced osteonecrosis of the femoral head (SIONFH) are primarily localized to the deep cartilage and calcified cartilage, with severity correlating with the degree of collapse. • Subchondral bone necrosis alters the local microenvironment and elastic modulus, leading to calcified cartilage sclerosis and subsequent brittle fracture at the necrosis-sclerosis junction, initiating the crescent sign. • Granulation tissue invasion from subchondral bone trabecular spaces stimulates chondrocyte terminal differentiation, apoptosis, and matrix degradation, impairing cartilage repair and leading to bone-cartilage separation. • The crescent sign on imaging corresponds to the pathological separation and defect between articular cartilage and subchondral bone, resulting from progressive bone and cartilage fracture and failed integration.