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Updated: Jan 23 2022

Unicameral Bone Cyst

Images A - femur - xray - parsons_moved.png A - femur - T1- parsons_moved.png C - inactive - parsons_moved.png B - fallen fragment - parsons_moved.jpg A - Parsons_moved.jpg
  • 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
        • 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
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