Chondroblastoma, Mistaken for a Primary Aneurysmal

rare in the patella. However, the clinical diagnosis was wrong; the final microscopic diagnosis was a chondroblastoma with a secondary aneurysmal bone cyst. We present this interesting case with a review of literatures. Case Report A 29-year-old woman presented to our clinic with a 3-month history of left knee pain. The patient had no significant previous medical history or trauma. Pain was aggravated after going up and down the stairs. On physical examination, she had localized tenderness in the lateral aspect of the patella. However, there was no evidence of a soft-tissue mass and overlying skin lesion. Further, no joint effusion was noted. She had full range of motion and the neurocirculatory status was normal. Laboratory studies indicated that all inflammatory markers were within the normal limits. Radiographic examination revealed a well-defined osteo-lytic lesion with a sclerotic margin in the lateral aspect of the pa-tella, and there was no pathologic fracture or periosteal reaction (Fig. 1A). A technechium-99m bone scan showed an increased uptake isolated to the lateral aspect of the left patella. Magnetic resonance imaging (MRI) was also conducted, which showed