Subchondral insufficiency fractures are non-traumatic fractures that occur immediately below the cartilage of a joint. Patients with subchondral insufficiency fracture characteristically have unremarkable plain radiographs, while MRI examination may reveal extensive bone marrow edema and subchondral bone collapse. Subchondral insufficiency fractures of the knee is also known as spontaneous osteonecrosis of the knee (SONK).