Dorsal defect of the patella: concept of its origin and relationship with bipartite and multipartite patella.

In a series of 2286 single radiographic examinations of the knee in 1985, 6 dorsal defects of the patella (DDP) were detected. The diagnosis was made if a round lucent lesion of the dorsal superolateral surface of the patella was found abutting against articular cartilage. In four of our patients, an association with a multipartite patella (MP) was found. Biopsy of one lesion showed dense connective tissue and areas of bone necrosis. In one patient, the pattern of reossification of the lesion could be demonstrated. Our observations provide further evidence that the DDP is a stress-induced anomaly of ossification rather than a post-traumatic subarticular cyst of the patella, a diagnosis sometimes suggested by the clinical context. The initial lesion is probably a traction lesion at the insertion of the vastus lateralis muscle rather than ulceration of articular cartilage. We suggest a possible relationship between dysfunction of the quadriceps mechanism, patellar subluxation, and the genesis of the DDP.