Summary Unicameral Bone Cysts, also known as simple 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 Incidence 3% of all biopsied bone tumors Demographics age usually found in patients <20 years of age (85%) peak age of diagnosis between ages 3 and 14 years average age of diagnosis is approximately 9 years sex male:female ratio - 2 to 1 Location primarily found in long bones (95%) typically intramedullary and initially found in the metaphysics of long bones adjacent to the physis most commonly found in the proximal humerus of young patients (50-60%) and the proximal femur (30%) 50% of proximal femur lesions occur in patients older than age 17 can be found in other locations including the distal tibia, ilium, calcaneus, spine (posterior elements), and occasionally metacarpals, phalanges, or metaphysis of distal radius lesions in the iliac wing, ribs, talus, and radius primarily affect adults Risk factors no known risk factors Etiology Pathophysiology reactive bone lesion that arises in the metaphysis adjacent to physis and progresses toward the diaphysis with bone growth forms due to venous stasis in cancellous bone leading to bone resorption increased pressure and inflammatory proteins within cyst fluid increased levels of lysosomal enzymes versus serum levels resolution of the cyst typically occurs after losing connection with the physis Genetics no known genetic associations Classification Classification is important as it impacts treatment active if the cyst is adjacent (~1cm) to the physis latent if normal bone separates cyst from physis Presentation History often diagnosed incidentally on x-ray or after a pathologic fracture occurs Symptoms most asymptomatic (~80%) unless fracture occurs (usually with minor trauma) presents with localized pain from a pathologic fracture in ~50-75% of cases most common cause of pathologic fracture in children swelling Physical exam localized tenderness and swelling in the setting of pathologic fracture Imaging Radiographs recommended views orthogonal views of the involved bone findings a central, lytic, well-demarcated metaphyseal lesion with a sclerotic rim (2-3% cross physis) smooth endosteal scalloping without periosteal reaction unilocular, cystic expansion with symmetric thinning of cortices can become multiloculated after repeat fractures "fallen leaf" sign (pathologic fracture with fallen cortical fragment in the base of empty cyst is pathognomonic for UBC) "rising bubble" sign (gas bubble seen in most non-dependent part of the cyst cavity is pathognomic for UBC) trabeculated appearance after multiple fractures CT indications typically used when cyst is present in complex areas that are difficult to evaluate with plain radiographs. spine and pelvis used to evaluate cyst wall thickness, presence of occult fracture and risk of pathologic fracture findings similar to radiographs showing a central lytic metaphyseal lesion with a well-demarcated sclerotic rim MRI indications may be used when malignancy is unable to be excluded as a diagnosis findings T1: dark more predictive of fracture risk than xrays T2: bright T1 with contrast: classic rim enhancement of a cystic lesion fluid-fluid levels can be seen if fibrous septations are present or after fractures periosteal reaction, signal heterogeneity, and soft tissue edema present in the setting of fracture Bone scan indications no significant diagnostic role findings focal cold spot with surrounding uptake in the periphery "doughnut" sign diffuse increase in uptake in the setting of fracture 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) ABC lesions typically have a transverse diameter wider than the epiphysis UBC lesions have a transverse diameter is not greater than the epiphysis can be differentiated from UBC on MRI due to the appearance of double-density fluid levels Fibrous dysplasia occurs in similar age demographic and location within the diaphysis and metaphysis distinguished from UBC lesions by the ground glass appearance on xray Telangiectatic osteosarcoma can initially appear similar to UBC due to lytic appearance on plain radiographs can be differentiated from UBC due to telangiectasia osteosarcoma having expansile growth, matrix mineralization, periosteal reaction (codman triangle), and presence of soft tissue mass elevated white blood cell count, platelet counts, LDH, and alkaline phosphatase are seen in telangiectasia osteosarcoma, but normal in UBC 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 Diagnosis typically made based on history, physical examination, and plain radiographs alone advanced imaging and biopsy may be useful in diagnosis when unable to exclude malignancy Treatment Nonoperative observation indications small, asymptomatic lesions in the upper extremity modalities repeat radiographs of the lesion 6 months after the initial presentation if found incidentally 42% success rate of conservative management alone immobilization alone indications proximal humerus lesions with non-displaced or mildly displaced pathologic fracture (15% of lesions fill in with native bone after acute fracture) non-weightbearing extremity modalities immobilization for 4-6x weeks proximal humerus fracture - sling 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 displaced pathologic humerus fractures latent cysts in the proximal femur that are a structural concern and at risk for fracture and osteonecrosis contraindications avoid in active lesions as communication with physis may lead to growth arrest outcomes proximal femoral lesions with a pathologic fracture have a high rate of refracture and malunion when treated nonoperatively therefore, internal fixation is recommended highest success rate (88%) with curettage with elastic stable intramedullary nailing and bone grafting cyst healing after surgery is assessed by pain, cyst opacification, and cortical thickening complete healing >95% opacification, presence of cortical thickening, and no pain partial healing >80-95% opacification, with or without cortical thickening incomplete healing <80% opacification of the cyst and no cortical thickening Complications Recurrence incidence 10 to 30% post-treatment recurrence rate all UBC should be followed with serial radiographs to evaluate for resolution, persistence, or recurrence of the cyst regardless of treatment treatment complete surgical resection of the lesion treatment with intralesional corticosteroid, demineralized bone matrix, and autologous bone marrow injection may decrease recurrence Femoral Neck Varus Malunion / Osteonecrosis / Growth Arrest incidence <10% of patients with UBC risk factor pathologic fracture through unicameral bone cyst within the femoral neck treatment curettage and bone grafting with hip screw and proximal femoral plate Limb Length Discrepancy and Axial Deviation risk factors active cyst crossing physis or involving the epiphysis operative intervention during the active phase due to disruption of the physis Prognosis Overall, favorable prognosis with the majority of lesions being clinically insignificant 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