Two cases of dorsal defect of the patella with arthroscopically visible cartilage surface perforations.

Dorsal defect of the patella is an approximately 1-cm, circular, radiolucent, well-marginated benign lesion invariably located in the superlateral aspect of the patella. Only approximately 50 cases have been described in the world literature, of which < 10 cases have been studied histologically. We report two patients with bilateral, virtually symmetrical, dorsal defects of the patella and describe the arthroscopic and histologic findings. Case 1 was a 14-year-old girl and case 2 was a 35-year-old man. Cartilage perforations were seen via arthroscopy on the cartilage surface of both knees. The pathophysiology of this disorder remains to be clarified, although van Holsbeeck et al. proposed that vastus lateralis traction injury in the ossification process of the patella may be involved, as in the case of bipartite patella. The present arthroscopic and histological findings support this hypothesis.