Dorsal defect of the patella (DDP): a characteristic radiographic lesion.

The dorsal defect of the patella (DDP) is a benign lesion with specific radiographic features. Seven cases of DDP are reviewed; all 7 lesions were identical in location and radiographic appearance. The characteristic lesion is round and lytic, with well-defined margins; it is located in the superolateral aspect of the patella adjacent to the subchondral bone. Arthrography reveals intact cartilage. DDP may occur in both sexes and may be bilateral. It is often found incidentally on radiographs ordered for acute trauma. Its etiology is not known. Differential diagnosis is also discussed.