Aneurysmal Bone Cyst-cloaked Codman's Tumor in the Patella: Case Report of an Easily Misdiagnosed Entity.

Introduction: Chondroblastomas (CB) are rare benign cartilaginous tumors (1%) arising from secondary ossification centers in epiphysis of long bones, commonly the proximal humerus. They commonly occur in the 2nd decade of life and have a male predilection. The occurrence of these tumors in the patella is extremely rare (<2%). Case Report: A 16-year-old male presented with knee pain since 2 months, which aggravated with activities. Radiographic evaluation showed an osteolytic lesion along the superolateral pole of patella, which was hypointense on T1 and hyperintense on T2 along with multiple fluid-fluid levels, suggestive of aneurysmal bone cyst (ABC). Intralesional curettage was performed along with bone grafting from the ipsilateral iliac crest. Histopathological examination showed presence of chondroblasts along with abundant osteoclast-like giant cells, suggestive of a CB with a secondary ABC. The patient was able to return to his day-to-day activities after 4 weeks. Conclusion: CB with secondary ABC represents a diagnostic challenge due to atypical imaging features that mimic primary ABC. One must maintain high suspicion for this entity when evaluating lytic patellar lesions. Adequate tissue sampling and histopathological examination are mandatory for accurate diagnosis, as radiological findings alone are insufficient. Appropriate surgical management with intralesional curettage and bone grafting results in excellent functional recovery and low recurrence rates.