System used to describe bone necrosis after acute injury in talus. Stage 1, normal radiograph with abnormality evident on MRI or bone scan (usu. due to repeat mild trauma); stage 2, partial separation of subchondral fragment (single event); stage 2a, formation of cyst (repeat mild trauma); stage 3, total fragment separation that maintains anatomic position (single event); stage 4, total fragment separation with loss of anatomic position (single event).