[Avascular necrosis of the femoral head].

Avascular necrosis of the femoral head (AVN) describes an ischemia-induced necrosis of the subchondral bone of the femoral head. Early diagnosis is crucial to initiate appropriate treatment and achieve the longest possible preservation of the native joint. Pathophysiologically, AVN results from impaired blood supply to the subchondral bone, promoted by various risk factors or underlying pathologies. The most relevant include prolonged or high-dose corticosteroid therapy and chronic alcohol abuse. Magnetic resonance imaging (MRI) is regarded as the key modality for early detection, enabling identification of characteristic changes before radiographic abnormalities become apparent. The earliest MRI finding is a band-like, T1-hypointense line demarcating the necrotic area from surrounding viable bone. The characteristic double-line sign-comprising an outer hypointense and an inner hyperintense rim-reflects reparative processes at the necrosis margin. Bone marrow edema occurs only after the development of a subchondral fracture and thus indicates an advanced stage, rather than, as previously assumed, an early manifestation of AVN. Differential diagnosis should primarily include transient bone marrow edema syndrome and subchondral insufficiency fracture. The Association Research Circulation Osseous (ARCO) classification provides a multimodal, internationally established system for standardized staging. It distinguishes four clinically relevant stages: isolated MRI detection (stage I), radiographic changes without fracture (stage II), subchondral fracture (stage III), and secondary osteoarthritis (stage IV). This system allows reproducible stage-based assessment, supports therapeutic planning, and facilitates interdisciplinary communication. Consequently, radiologic imaging plays a pivotal role in diagnosis, disease monitoring, and treatment decision-making for patients with AVN.