Crescent sign in radiology refers to a thin, curvilinear lucent line parallel and inverted to the cortical margin of a bone. It represents the margin of a disk of articular cartilage and underlying subchondral bone, that has fractured. This is a medical sign

Medical sign

A medical sign is an objective indication of some medical fact or characteristic that may be detected by a physician during a physical examination of a patient....

 of osteochondritis dissecans

Osteochondritis dissecans

Osteochondritis dissecans , often abbreviated to OCD or OD, is a joint disorder in which cracks form in the articular cartilage and the underlying subchondral bone. OCD is caused by blood deprivation in the subchondral bone. This loss of blood flow causes the subchondral bone to die in a process...

, a consequence of avascular necrosis

Avascular necrosis

Avascular necrosis is a disease where there is cellular death of bone components due to interruption of the blood supply. Without blood, the bone tissue dies and the bone collapses...

.

This sign commonly occurs on the femur

Femur

The femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...

, both in the hip joint on the femoral head, and in the knee joint. Interruption of the blood supply to the bone leads to ischemic necrosis

Necrosis

Necrosis is the premature death of cells in living tissue. Necrosis is caused by factors external to the cell or tissue, such as infection, toxins, or trauma. This is in contrast to apoptosis, which is a naturally occurring cause of cellular death...

 of the marrow and bone that it supplies. Eventually, the bone infarcts and insufficiency fracture

Fracture

A fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...

s may ensue. Fractures that occur in the subchondral bone may be recognized by a crescentic lucent zone that separates the fragment from the remainder of the bone. Usually indicates stage III osteonecrosis per Enneking's criteria.

The treatment is immobilization, to prevent the joint collapse.