Secondary Aneurysmal Bone Cystic Change of the Chondroblastoma, Mistaken for a Primary Aneurysmal Bone Cyst in the Patella
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 osteolytic lesion with a sclerotic margin in the lateral aspect of the patella, and there was no pathologic fracture or periosteal reaction (). 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 a partially multilobulated and septated mass in the lateral aspect of the patella. A fluid-fluid level was noted in this mass with T1-low and T2-high signal intensity. The margins of the lesion were well defined and a cortical break was not seen in the anterior and posterior cortex of the patella (). Based on the imaging studies, the preoperative diagnosis was foremost aneurysmal bone cyst or simple bone cyst.\nAt surgery, there was no abnormality in the soft tissue around the patella. The lesion was aspirated using a needle and about 2 mL bloody fluids were discharged from the cavity. The lesion was a cystic cavity and was surrounded by a gray or brownish tissue. Multiple fragments of hemorrhagic soft tissue admixed with recognizable bone fragments were revealed. Curettage was completely performed and cauterization of the inner wall of the cavity was thoroughly done. After massive saline irrigation, the cavity was filled with autogenous iliac bone chips.\nMicroscopically, sheets of chondroblasts were intermingled with fine eosinophilic chondroid matrix. The chondroblasts showed oval shaped nuclei with moderate eosinophilic cytoplasm. Several multinucleated giant cells were scattered. Occasional mitotic figures were also identified. The indented nuclear grooves were characteristic nuclear features. Blood-filled cystic cavities were present in the chondroblastoma, consistent with secondary aneurysmal bone cyst ().\nThe final diagnosis was chondroblastoma with a secondary aneurysmal bone cyst. Post-operatively, the patient was ambulating without a limp and had a normal, pain-free range of motion. At the 3-month follow-up, she had returned to her usual daily and recreational activities. Radiologically, the grafted bones were well incorporated and there were no other abnormal symptoms and no recurrent evidence in the patella at the post-operative 1.5-year follow-up ().