Osteonecrosis is a process which leads to ischemic necrosis of bone and bone marrow as a result of direct or indirect vascular injury. In this process organic or inorganic matrix of the bone usually are not involved. Ischemic process may occur as a result of traumatic vascular injury (fractures and dislocations), intravascular coagulation and extravascular compression (increased intraosseous pressure, increased fat tissue in bone marrow and hypertrophy). Establishing the pathogenesis of osteonecrosis is important for the early and appropriate treatment. Bone and bone marrow cells are very sensitive to ischemia. Histologic findings of dead osteocyts develop in 24-72 hours. Fibrovascular tissue limits necrotic bone. During the repair process dead bone is removed by osteoclastic resorbtion and replaced by new bone tissue (creeping substitution). Separation and collapse of the subchondral bone are poor prognostic factors, leading to joint destruction. For this reason, early diagnosis is very important.