Unicameral bone cyst

This is a case of a 9 year old girl who presented with hip pain. Frontal and lateral radiographs of the hip demonstrates a bubbly, expansile lytic lesion within the proximal femur. There is no evidence of a matrix. The lesion is well defined, with a so called narrow zone of transition. No pathologic fracture is demonstrated radiographically. There is no periosteal reaction or a soft tissue component. Due to the pain the patient was experiencing, she want on to surgical biopsy and curretage. Pathology demonstrates a simple bone cyst, also known as a unicameral bone cyst. The radiographic appearance correlates well with the pathology. Additional differential consideration given the patient age and appearance of the lesion would include aneurysmal bone cyst as well as monostotic fibrous dysplasia. When pathologic fracture of a simple bone cyst occurs, a "fallen fragment sign" can be seen. This sign has high specificity but poor sensitivity.