Secondary aneurysmal bone cyst of the patella.

Aneurysmal bone cyst accounts for 1% of primary bone tumors and is one of the benign tumor-like lesions. Patellar involvement is quite rare. Its development on the basis of any previous lesion such as chondroblastoma is called secondary aneurysmal bone cyst. A 26-year-old male patient presented with right knee pain of three-year history. Physical examination showed a firm, immobile swelling at the anterior aspect of the right knee, without increase in temperature or redness of the skin. There was no pain on palpation and joint range of motion was normal. Radiological studies were suggestive of an aneurysmal bone cyst. At surgery, the cystic lesion was removed via curettage and the residual cavity was filled with an autogenous bone graft taken from the iliac wing. The histopathologic diagnosis was secondary aneurysmal bone cyst in association with chondroblastoma. During a 1.5-year follow-up, the patient had no complaint and no recurrence was observed.