This disease entity is observed in various joints. Most often affected are the femoral condyles, the trochlea tali, and the distal humerus. The etiology is not yet clear, but repetitive microtraumata, focal ischemia, and disturbances of ossification are discussed. Typically, children, adolescents, and young adults are affected. The potential of healing, and thus the likelihood of a favorable outcome, is greater in younger patients. The stability of the fragment and the status of the overlaying cartilage are essential factors in choosing therapy. Radiography shows the dissected element separated by a rim of sclerosis from the surrounding bone.
In MRI, the signal of the fragment may be heterogeneous and may show several small lowsignal- intensity foci corresponding to bony elements. These findings are not related to instability. Demarcation towards the surrounding bone shows low signal intensity in T1-weighted sequences. If there is linear high signal intensity in the interface between fragment and surrounding bone in T2-weighted sequences it has to be determined whether this pattern corresponds to loosening of the fragment, i.e., fluid entering the interface, or whether this pattern represents granulation tissue, i.e., repair.
Continuity of the high signal intensity from underneath the fragment through the bone and a full-thickness cartilage defect to the joint space is indicative of loosening (DeSmet 1996). Therefore, it is strongly suggested that the presence of a high-signal-intensity line longer than 5 mm underneath the fragment on T2- weighted sequences be interpreted as a predictor of instability only when there is a concomitant breach through the cartilage. Direct MR arthrography may be helpful. Large cyst-like areas underneath the fragment are suggestive of loosening as well.
Intravenous gadolinium may help in such cases, clearly demonstrating vascularized granulation tissue by post-injection enhancement indicating an ongoing repair process.