PMC:3945237 / 15377-18284 JSONTXT

{"project":"2_test","denotations":[{"id":"24778812-2261723-24198595","span":{"begin":248,"end":250},"obj":"2261723"}],"text":"Computed tomography (CT)\nThe high spatial resolution and contrast resolution of CT allow analysis of morphologic features. The sensitivity of CT in detecting early AVN is 55%, which is similar to the sensitivity of planar nuclear medicine imaging [20,21]. CT is more appropriate in evaluating the extent of involvement, such as subchondral lucencies and sclerosis during the reparative stage, before the onset of femoral head collapse and superimposed degenerative disease.\nCT scans do not demonstrate the early vascular and marrow abnormalities that result in osteonecrosis [21].Osteoporosis is the first visible sign. Later, the central bony asterisk is distorted, appearing as clumping and fusion of the peripheral asterisk rays. Clumping appears as spots or as hyperdense \"roads\" of various width (figure 9). This represents changes in the sclerotic interface between necrotic and viable bone and is analogous to the line of low signal surrounding the necrotic bone seen on MRI images. Early signs are caused by microfractures resulting from reduced mechanical load of dead bone trabeculae, altering the shape of the asterisk. Signs also are related to new bone formation on the dead trabeculae. The lucent cystic region, representing the reparative zone, may be appreciated.\nFigu.9 Axial CT scan of a patient with avascular necrosis of the femoral head shows clumping and distortion of the central trabeculae representing the asterisk sign (arrowhead) and an adjacent low-density region (arrow) is representing the reparative zone Unless the asterisk sign is appreciated, articular surface abnormalities may be interpreted as degenerative joint disease. The lucency within the reparative zone may be confused with malignancy, infection, insufficiency fracture, or plasma cell myeloma.\nAlthough CT may delineate subtle alterations of bone density when plain radiograph findings are normal, MRI and SPECT scintigraphy are much more sensitive for evaluating early manifestations of the disease, such as bone marrow edema. CT scans are insensitive for detecting stage 0 and 1 AVN, but are excellent for detecting femoral head collapse, early degenerative joint disease (DJD), and the presence of loose bodies.\nDespite many investigations into AVN, many issues remain unresolved. The pathogenesis in most cases is only speculative and may involve intravascular factors such as microemboli or extravascular factors such as increased interosseous pressure. MRI has emerged as the diagnostic test of choice for suspected early lesions, and radiographs should be used to diagnose and follow advanced lesions. Bone scanning can be useful for early diagnosis and CT scanning or tomography may help plan surgical procedures. The role of the functional exploration of bone is controversial.\nIn conclusion, diagnosing AVN as early as possible is imperative for a greater chance of success of conservative treatment."}