Bilateral symmetrical cysts in the upper tibiae in a skeletally mature patient: might they be simple bone cysts?

A 55-year-old Japanese woman presented with right knee pain of 1-month duration. Radiological studies revealed bilateral mild osteoarthritic changes in the medial knee joint compartment and symmetrical cysts in the upper tibial metaphyses, extending to the epiphyses. Intraosseous ganglion was considered the most probable diagnosis. However, intraoperatively, serous fluid-filled cavities were recognized; these were curetted and filled with hydroxyapatite granules. Histopathological examination of the cyst wall revealed thin fibrous tissue formed of collagen fibers without a lining cell layer, with scattered lymphocytes, histiocytes, irregular masses of fibrin-like material, and periosteal osteocartilagenous callus formation; a picture compatible with simple bone cysts. Bilateral symmetrical cysts of the upper tibial metaphyses extending to the epiphyses are extremely rare. A literature review revealed that the age incidence, and bony locations of multiple and epiphyseal simple bone cysts are atypical in relation to the classic metaphyseal simple bone cysts. Also, multiple and epiphyseal simple bone cysts have a better prognosis than the classic metaphyseal ones. Four clinicoanatomic varieties of simple bone cysts are recognized; classic metaphyseal, nontubular, epiphyseal, and multiple.