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Updated: Jan 31 2024

Unicameral Bone Cyst

Images
https://upload.orthobullets.com/topic/8035/images/Case A - femur - xray - parsons_moved.png
https://upload.orthobullets.com/topic/8035/images/Case A - femur - T1- parsons_moved.png
https://upload.orthobullets.com/topic/8035/images/Case C - inactive - parsons_moved.png
https://upload.orthobullets.com/topic/8035/images/Case B - fallen fragment - parsons_moved.jpg
https://upload.orthobullets.com/topic/8035/images/Histology A - Parsons_moved.jpg
  • 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 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
        • 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
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