Whereas the bone marrow ''edema'' pattern can be seen with chronic articular conditions such as osteoarthritis, the classic OCD pattern exhibits a well-demarcated, crescent- or oval-shaped lesion that is low signal on T1-weighted images and heterogeneous in appearance on T2-weighted images. These lesions undergo variable degrees of enhancement, which is indicative of an active reparative process. The demarcation zone between the subchondral lesion and healthy marrow is frequently bright on T2-weighted images, which can represent granulation tissue or fluid communicating with the articular surface. Low signal at the demarcation zone on T2-weighted images is ascribed to healing and suggests stability of the lesion. This is an important differentiator, because treatment is dependent on stability of the osteochondral fragment. MRI findings of fragment instability include : (1) a high signal intensity line on T2-weighted images of the demarcation zone measuring greater than 5 mm in length, or circumferentially surrounding the osteochondral fragment; (2) A round homogeneously bright area on T2-weighted images measuring at least 5 mm in diameter in the subchondral bone; or (3) a focal cartilage defect of greater than 5 mm. The most common sign of instability is high T2-weighted signal at the demarcation zone between healthy marrow and the osteochondral fragment. The reported accuracy of this finding in the literature has been variable [101-103], primarily because of the difficulty differentiating fluid from high T2-weighted signal intensity of granulation tissue. Specificity for identifying an unstable osteochondral fragment is improved if the hypertense demarcation zone is accompanied by a chondral defect . To improve specificity in diagnosis of an unstable fragment, intra-articular injection of gadolinium contrast has been used to demonstrate communication of the demarcation zone with the articular surface . Following intravenous gadolinium contrast administration, enhancement of the demarcation zone is indicative of reparative granulation tissue . Although in an animal model this findings was associated with unstable fragments , other authors have interpreted this finding as a sign of stability and healing . The presence of sub-chondral cysts greater than 5 mm in size is less common, but is indicative of unstable lesions. At surgery, cystic lesions in the talus contained loose granulation tissue rather than fluid . For knee examinations, current MRI techniques generally have sufficient contrast resolution to identify overlying cartilage defects; however, this finding can be equivocal in thinner cartilage of the talar dome. In follow-up studies, the presence of an overlying chondral defect was predictive of a poor outcome . Skeletally mature patients who have fused growth plates also are more likely to have poor outcomes.
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