crescent sign

{{Short description|Pattern seen in radiologic examinations}}

{{distinguish|text=the air crescent sign}}

File:Crescent sign annotated.jpg

In radiology, the crescent sign is a finding on conventional radiographs that is associated with avascular necrosis.{{Cite journal | last1 = Pappas | first1 = J. N. | title = The musculoskeletal crescent sign | journal = Radiology | volume = 217 | issue = 1 | pages = 213–214 | year = 2000 | pmid = 11012446 | doi=10.1148/radiology.217.1.r00oc22213}}{{Cite journal | last1 = Kenzora | first1 = J. E. | last2 = Glimcher | first2 = M. J. | title = Pathogenesis of idiopathic osteonecrosis: The ubiquitous crescent sign | journal = The Orthopedic Clinics of North America | volume = 16 | issue = 4 | pages = 681–696 | year = 1985 | doi = 10.1016/S0030-5898(20)30435-1 | pmid = 4058896}}{{Cite journal | last1 = Norman | first1 = A. | last2 = Bullough | first2 = P. | title = The Radiolucent Crescent Line--An Early Diagnostic Sign of Avascular Necrosis of the Femoral Head | journal = Bulletin of the Hospital for Joint Diseases | volume = 24 | pages = 99–104 | year = 1963 | pmid = 14048829}} It usually occurs later in the disease, in stage III of the four-stage Ficat classification system. It appears as a curved subchondral radiolucent line that is often found on the proximal femoral or humeral head. Usually, this sign indicates a high likelihood of collapse of the affected bone. The crescent sign may be best seen in an abducted (frog-legged) position.{{Cite web | title = Rheumatology Image Bank: Avascular Necrosis, Crescent Sign: Ficat Stage III, Hip | url = http://images.rheumatology.org/viewphoto.php?albumId=75678&imageId=2861821 | accessdate = 26 June 2012 }}

The crescent sign is caused by the necrotic and repair processes that occur during avascular necrosis. Osteosclerosis occurs at a margin where new bone is placed over dead trabeculae. When the trabeculae experience stress leading to microfractures and collapse, the crescent sign appears.

The crescent sign may be seen with other bone diseases, such as shear fractures.