(Radiology. 2000;217:213-214.)
© RSNA, 2000
The Musculoskeletal Crescent Sign1
John N. Pappas, MD
1 From the Department of Radiology, Duke University Medical Center, Box 3808, Erwin Rd, Durham, NC 27710. Received January 8, 1999; revision requested February 18; final revision received May 5; accepted May 6. Address correspondence to the author (e-mail: pappa003@acpub.duke.edu).
Index terms: Femur, necrosis, 443.44 • Signs in Imaging
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APPEARANCE
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The crescent sign is seen on conventional radiographs and is recognized as a curvilinear subchondral radiolucent line (Fig 1a). It is typically seen along the anterolateral aspect of the proximal femoral head, which is optimally depicted on the frog-leg radiographic view.
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Figure 1a. (a) Conventional radiograph of the right femur in the frog-leg position obtained with the patient’s thigh abducted and flexed shows subchondral area of hyperlucency (arrows) in the anterolateral aspect of the proximal femoral head. (Courtesy of Clyde A. Helms, MD, Department of Radiology, Duke University Medical Center, Durham, NC.) (b) Specimen radiograph of a coronally sectioned femoral head segment reveals a subchondral fracture (arrows), which manifests as the crescent sign. Note the fragmentation and compaction of the subchondral cancellous trabeculae, which weakens the articulating surface. (Courtesy of Edward F. DiCarlo, MD, Department of Pathology, Hospital for Special Surgery, New York, NY.)
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Figure 1b. (a) Conventional radiograph of the right femur in the frog-leg position obtained with the patient’s thigh abducted and flexed shows subchondral area of hyperlucency (arrows) in the anterolateral aspect of the proximal femoral head. (Courtesy of Clyde A. Helms, MD, Department of Radiology, Duke University Medical Center, Durham, NC.) (b) Specimen radiograph of a coronally sectioned femoral head segment reveals a subchondral fracture (arrows), which manifests as the crescent sign. Note the fragmentation and compaction of the subchondral cancellous trabeculae, which weakens the articulating surface. (Courtesy of Edward F. DiCarlo, MD, Department of Pathology, Hospital for Special Surgery, New York, NY.)
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EXPLANATION
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The crescent sign is associated with avascular necrosis (AVN). Inadequate perfusion in the articular ends of bones leads to the ongoing processes of osteonecrosis and repair (1). Repair begins at the interface between necrotic and viable bone. Reactive new bone is laid down over dead trabeculae, which produces a sclerotic margin. An advancing front of fibrosis, hyperemia, inflammation, and bone resorption extends into the necrotic segment of bone as repair is attempted. Mechanical failure of trabecular bone at this interface results in progressive microfracture and collapse of the adjacent dead subchondral cancellous trabeculae, which leads to the development of a subchondral radiolucent area along the fracture line, or the crescent sign (Fig 1b) (2). The appearance of this sign is typically followed by collapse of the femoral head on its articulating surface, which ultimately leads to cartilaginous destruction and superimposed hip joint arthritis (3).
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DISCUSSION
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A number of predisposing conditions are associated with the development of AVN. These include hemoglobinopathies (sickle-cell disease), corticosteroid use and Cushing disease, traumatic disruption of the blood supply (intracapsular femoral neck fracture), chronic renal disease, alcoholism, pregnancy, high-dose irradiation, dysbaric disorders, and Gaucher disease (2,4).
AVN may be divided into four clinical and radiographic stages based on the Ficat classification system (5). In stage I, the conventional radiograph is normal, but magnetic resonance (MR) imaging or radionuclide imaging findings will confirm AVN. In stage II, cystic and sclerotic changes are seen on conventional radiographs. In stage III, the crescent sign is seen. In stage IV, there is flattening of the femoral head, narrowing of the hip joint, and, ultimately, severe joint destruction.
AVN most commonly involves the proximal femur and, to a lesser extent, the proximal humerus (Fig 2). AVN also occurs in the distal femur and talus and, less frequently, in the carpal lunate, metatarsal head, and tarsal navicular bone (6).
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Figure 2. Anteroposterior radiograph of the left humeral head in a patient being treated with high-dose corticosteroids shows multiple subchondral areas of hyperlucency (arrowheads) that are indicative of stage III AVN.
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Regardless of the anatomic site of AVN, the presence of a crescent sign helps in the classification of disease stage and in the identification of treatment strategy. Because of the generally poor prognosis of late-stage AVN, there is interest in early-detection methods. MR imaging has been proved to be effective in the detection of even asymptomatic AVN because of its ability to depict bone marrow abnormalities (7–10).
Alternatively, the ability of radionuclide bone scanning to depict regions of bone deposition and mineralization enables early detection of AVN well before conventional radiographs demonstrate increased sclerosis (1,8). The whole-body bone scan is also capable of demonstrating multiple sites of involvement.
Risk stratification with MR and radionuclide imaging has spared patients unnecessary surgery and has been used to determine the appropriate timing of head-preserving interventions such as core drilling, bone grafting, and osteotomy (11).
Although the onset of symptoms in AVN may be sudden, especially after renal transplantation and in sickle-cell disease, its onset is more often insidious. Subtle symptoms may precede radiologic changes by several months to a few years (12). Usually, subjective pain and clinical joint dysfunction prompt radiographic evaluation once damage has already occurred. Radiographs obtained at this time will often show the typical crescent sign or flattening of the femoral head, suggesting collapse of a sizable segment of the joint surface.
Although the crescent sign is most often seen in AVN, it does occur with other bone diseases (6). For instance, osteochondral shear fractures of the femoral head, as seen in patients with trauma, have also been known to show a crescent sign; however, shear fractures can usually be easily differentiated by means of clinical history.
In conclusion, the crescent sign is seen most often in AVN and occurs later in the course of the disease. The crescent sign is depicted on conventional radiographs as a subchondral radiolucent line typically involving the proximal femoral or humeral head. Once this radiographic sign is detected, further collapse is likely. With the availability of MR imaging and radionuclide scanning, earlier diagnosis and subsequent intervention are possible and ultimately result in a better prognosis.
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FOOTNOTES
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A trainee (resident or fellow) wishing to submit a manuscript for Signs in Imaging should first write to the Editor for approval of the sign to be prepared, to avoid duplicate preparation of the same sign.
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REFERENCES
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Bullough PG, Vigorita VJ. Bullough and Vigorita’s orthopaedic pathology 3rd ed. St Louis, Mo: Mosby–Year Book, 1996.
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Norman A, Bullough PG. The radiolucent crescent line: an early diagnostic sign of avascular necrosis of the femoral head. Bull Hosp Jt Dis 1963; 24:99-104.[Medline]
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Resnick D, Niwayama G. Diagnosis of bone and joint disorders 2nd ed. Philadelphia, Pa: Saunders, 1988; 3188-3237.
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Helms CA. Fundamentals of skeletal radiology 2nd ed. Philadelphia, Pa: Saunders, 1995; 61, 233.
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Ficat RP. Idiopathic bone necrosis of the femoral head: early diagnosis and treatment. J Bone Joint Surg Br 1985; 67:3-9.
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Kenzora JE, Glimcher M. Pathogenesis of idiopathic osteonecrosis: the ubiquitous crescent sign. Orthop Clin North Am 1985; 16:681-695.[Medline]
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Gillespy T, Genant HK, Helms CA. Magnetic resonance imaging of osteonecrosis. Radiol Clin North Am 1986; 24:193-208.[Medline]
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Mitchell DG, Rao VM, Dalinka MK, et al. Femoral head avascular necrosis: correlation of MR imaging, radiographic staging, radionuclide imaging, and clinical findings. Radiology 1987; 162:709-715.[Abstract/Free Full Text]
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Mitchell MD, Kundel HL, Steinberg ME, Kressel HY, Alavi A, Axel L. Avascular necrosis of the hip: comparison of MR, CT, and scintigraphy. AJR Am J Roentgenol 1986; 147:67-71.[Abstract/Free Full Text]
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Wassenaar RP, Verburg H, Taconis WK, Van Der Eijken JW. Avascular osteonecrosis of the femoral head treated with a vascularized iliac bone graft: preliminary results and follow-up with radiography and MR imaging. RadioGraphics 1996; 16:585-594.[Abstract]
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