summary Unicameral Bone Cysts are non-neoplastic, serous fluid-filled bone lesions most commonly found in the proximal humerus. The condition typically presents in patients < 20 years of age with a pathological fracture through the lesion. Diagnosis is made with radiographs showing a central, lytic, well-demarcated metaphyseal lesions with cystic expansion and a characteristic "fallen leaf" sign. Treatment is usually immobilization for proximal humerus lesions with pathologic fracture. Surgical curettage and bone grafting is indicated for pathologic fractures that have a high rate of refracture and malunion. Epidemiology Demographics age usually found in patients <20 years of age Anatomic location location usually found in the proximal humerus of young patients can be found in other locations including proximal femur, distal tibia, ilium, calcaneus, and occasionally metacarpals, phalanges, or distal radius arises in the metaphysis adjacent to physis and progresses toward the diaphysis with bone growth Classification Classification is important as it impacts treatment active if cyst is adjacent to the physis latent if normal bone separates cyst from physis Presentation Symptoms most asymptomatic unless fracture occurs (usually with minor trauma) presents with pain from a pathologic fracture in ~50% Imaging Radiographs central, lytic, well-demarcated metaphyseal lesion (2-3% cross physis) cystic expansion with symmetric thinning of cortices "fallen leaf" sign (pathologic fracture with fallen cortical fragment in base of empty cyst is pathognomonic) trabeculated appearance after multiple fractures MRI very dark on T1 very bright on T2 gadolinium shows classic rim enhancement of a cystic lesion Bone scan is variable, but usually warm Labs Specific laboratory tests usually not required Histology Characteristic findings cyst with thin fibrous lining containing fibrous tissue, giant cells, and hemosiderin pigment chronic inflammatory cells may be found in small numbers cementum spherules (calcified eosinophilic fibrinous material) in 10% uniform population of spindle cells without nuclear atypia Biopsy usually indicated for questionable diagnosis Differential ABC is more expansive than UBC (UBC lesion usually not wider than physis) Telangiectatic osteosarcoma Differential of Unicameral Bone Cyst "Bubbly" lytic lesion on xray Treatment is USUALLY Aspiration and Injection Treatment is OCCASIONALLY curettage and bone grafting. UBC o o o ABC o NOF o o Enchondroma o Treatment Nonoperative immobilization alone indications proximal humerus lesions with pathologic fracture (15% of lesions fill in with native bone after acute fracture) aspiration/methylprednisolone acetate injection indications active cysts (communicates with physis) in the proximal humerus technique usually requires several injections, especially in very young children bone marrow injections have recently been reported to be effective Operative curettage and bone grafting +/- internal fixation based on tumor location indications symptomatic latent cysts that have not responded to steroid injections latent cysts in the proximal femur that are a structural concern and at risk for fracture and osteonecrosis proximal femoral lesions with a pathologic fracture have a high rate of refracture and malunion when treated nonoperatively therefore, internal fixation is recommended contraindications avoid in active lesions as communication with physis may lead to growth arrest Prognosis As a patient approaches skeletal maturity, a UBC will often decrease in size and may heal after growth is complete Fracture healing usually does not lead to cyst resolution Requires close follow up while in active phase due to recurrence and risk of fracture or growth arrest Treatment with Intralesional corticosteroid, demineralized bone matrix, and autologous bone marrow injection may decrease recurrence