Secondary aneurysmal bone cyst in fibrous dysplasia of the proximal femur.

Fibrous dysplasia of bone is a congenital skeletal disorder characterized by proliferation of spindle cells inside an intramedullary collagenous stroma and by formation of metaplastic bone. An aneurysmal bone cyst is a lesion of unknown etiology. Macroscopically it appears like a blood-filled cavity that expands the affected bone. Histologically, fibrous septa are bordered by macrophages, giant cells, inflammatory cells, and areas of bone production. Hemorrhagic areas can rise into fibrous dysplasia. The lesion appears as an aggressive and destructive process that is difficult to diagnose properly. A 29-year-old woman presented with a 2-year history of left inguinal and lumbar pain. She reported increased pain after physical activity. Magnetic resonance imaging of the left femur (without intravenous gadolinium) showed a mass extending 14 cm along the femoral metaphysis. This lesion involved cortical bone that was broken in the posteromedial site. Computed tomography confirmed the osteostructural lytic alteration that "blows up" the proximal femur metaphysis. A trochar biopsy allowed the diagnosis of aneurysmal bone cyst on fibrous dysplasia, excluding malignancy. The lesion was treated with curettage and local adjuvants and filled with bone grafts. Internal fixation with a sliding screw plate was also performed. Radiographs at 1 year postoperatively showed good consolidation of the grafts and absence of local recurrence. The patient achieved active and passive mobilization of the left hip with no pain.