Answer: Unicameral bone cyst

Histology: Unicameral bone cyst is a developmental lesion that occurs either in the proximal humerus or proximal femur. Most lesions occur in the proximal humerus. They develop at the proximal epiphyseal plate over a period of years and involved in metaphysis. Then they cease to form a cystic cavity and the cyst migrates into the shaft of the bone as the humerus grows. Lesions are symmetrical lytic areas with mild cortical thinning. They are cystic cavities lined with fibrous membrane. Histologic study of the fibrous membrane shows blood, fibrous tissue admixed with spicules of reactive bone. Often there is a cementum-like amorphus material in the wall of the cyst. This cementum-like material is diagnostic of unicameral bone cyst.

Discussion: The lesion is not a aneurysmal bone cyst. There is no expansion of the bone and the multiple locules are not present. Indeed, this case showed a single locule. Aneurysmal bone cyst is a reactive lesion composed of multiple cystic spaces filled with blood. The present lesion is filled with serous material. Occasionally, unicameral bone cyst may fracture, and the fracture callus may be confused with the material from the aneurysmal bone cyst.

The lesion is not fibrous dysplasia. The radiographic image may be confused with fibrous dysplasia i.e. a long lesion and a long bone. However, the present lesion was cystic and the fibrous material was taken from the lining of the cystic cavity. It is extremely important to get a history from the surgeon that the lesion was cystic. Although fibrous dysplasia may have some areas of cystic change, it is predominantly a solid lesion.